United States

Environmental Protection

Agency

In coordination with the Office of Water/Office of Science and Technology, Washington, DC

Region III

Chesapeake Bay

Program Office

Region III

Water Protection

Division

EPA 903-R-03-002

April 2003

April 2003

Ambient Water Quality

Criteria for Dissolved

Oxygen, Water Clarity and

Chlorophyll a for the

Chesapeake Bay and Its

Tidal Tributaries
Ambient Water Quality Criteria 

for Dissolved Oxygen, Water Clarity 

and Chlorophyll a for the Chesapeake Bay 

and Its Tidal Tributaries

April 2003

U.S. Environmental Protection Agency

Region III

Chesapeake Bay Program Office

Annapolis, Maryland

and

Region III

Water Protection Division

Philadelphia, Pennsylvania

in coordination with

Office of Water

Office of Science and Technology

Washington, D.C.
▼vii

Foreword

Foreword

I

n order to achieve and maintain the water quality conditions necessary to protect

the aquatic living resources of the Chesapeake Bay and its tidal tributaries, the U.S.

Environmental Protection Agency (EPA) Region III has developed this guidance

document, entitled Ambient Water Quality Criteria for Dissolved Oxygen, Water

Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries (Regional

Criteria Guidance). This document presents the EPA’s regionally-based nutrient and

sediment enrichment criteria expressed as dissolved oxygen, water clarity and chlorophyll a criteria, applicable to the Chesapeake Bay and its tidal tributaries. EPA is

issuing this guidance pursuant to Section 117(b) of the Clean Water Act and in accordance with the water quality standards regulations (40 CFR Part 131).

This Regional Criteria Guidance provides EPA’s recommendations to the Chesapeake Bay states for use in establishing their water quality standards consistent with

Section 303(c) of the Clean Water Act. Under Section 303(c), states and authorized

tribes have the primary responsibility for adopting water quality standards as state or

tribal law or regulation. The standards must contain scientifically defensible water

quality criteria that are protective of designated and existing uses. EPA’s water

quality standards regulations suggest three possible sources for establishing protective criteria: 1) guidance for water quality criteria recommendations published under

the authority of Section 304(a) of the Clean Water Act, 2) Section 304(a) guidance

modified to reflect site-specific conditions, or 3) other scientifically defensible

methods (see 40 CFR 131.11). Section 117 of the Clean Water Act authorizes a

Chesapeake Bay programs office to publish information pertaining to the environmental quality of the Chesapeake Bay, as well as to coordinate Federal and state

efforts to improve the quality of the Bay. 

Quantified water quality criteria contained within state or tribal water quality standards are essential to a water quality-based approach to pollution control. Whether

expressed as numeric criteria or quantified translations of narrative criteria within

state or tribal water quality standards, quantified criteria serve as a critical basis for

assessing the attainment of designated uses and measuring progress toward meeting
▼viii

Foreword

the water quality goals of the Clean Water Act and the Chesapeake 2000 agreement.

This Regional Criteria Guidance presents scientifically defensible methods and

serves as guidance for the states to use in developing appropriate and protective

Section 303 criteria and standards for the Chesapeake Bay. EPA’s Regional Criteria

Guidance is not law or regulation; it is guidance that states in the Chesapeake Bay

watershed may consider in the development and/or modification of appropriate

criteria for their water quality standards.

REBECCA W. HANMER, Director

Region III Chesapeake Bay Program Office

JON M. CAPACASA, Acting Director

Region III Water Protection Division

GEOFFREY H. GRUBBS, Director

Office of Science and Technology
▼ix

Executive Summary

Executive Summary

I

n order to achieve and maintain water quality conditions necessary to protect

aquatic living resources of the Chesapeake Bay and its tidal tributaries, the U.S.

Environmental Protection Agency (EPA) Region III has developed guidance, entitled

Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries (Regional Criteria

Guidance). This final guidance is intended to assist the Chesapeake Bay states,

Maryland, Virginia and Delaware, and the District of Columbia, in adopting revised

water quality standards to address nutrient and sediment-based pollution in the

Chesapeake Bay and its tidal tributaries.

EPA Region III developed this guidance to promote the overall goals of the Clean

Water Act and specifically in accordance with the EPA National Strategy for the

Development of Regional Nutrient Criteria, announced in June 1998. This national

nutrient strategy laid out the EPA’s intentions to develop technical guidance manuals

for four types of waters (lakes and reservoirs, rivers and streams, estuaries and

coastal waters and wetlands) and to produce criteria for specific nutrient eco-regions

(www.epa.gov/ost/standards/nutrient.html). In addition, the EPA is committed to

working with states and tribes to develop more refined and localized nutrient and

nutrient enrichment-related criteria based on approaches described in the water body

guidance manuals. The Regional Criteria Guidance provides the regional nutrient

guidance applicable to the Chesapeake Bay and its tidal tributaries.

EPA Region III developed the Regional Criteria Guidance in accordance with

Section 117(b) of the Clean Water Act using the multi-stakeholder approach to

implementing the Chesapeake 2000 agreement. Chesapeake 2000 was signed on

June 28, 2000, by the governors of Maryland, Pennsylvania and Virginia, the mayor

of the District of Columbia, the chair of the Chesapeake Bay Commission and the

Administrator of the U.S. EPA. Subsequently, the governors of Delaware, New York

and West Virginia signed a Memorandum of Understanding committing to implement the Water Quality Protection and Restoration section of the agreement.

The water quality criteria and tidal-water designated uses presented in this document

are the product of a collaborative effort among the Chesapeake Bay Program

partners. They represent a scientific consensus based on the best available scientific
▼x

Executive Summary

findings and technical information defining the water quality conditions necessary to

protect Chesapeake Bay aquatic living resources from effects due to nutrient and

sediment over-enrichment. Various stakeholder groups have been involved in their

development, with contributions from staff of federal and state governments, local

agencies, scientific institutions, citizen conservation groups, business and industry.

In the Regional Criteria Guidance the EPA recommends and expects that the numerical criteria and refined designated uses will be considered by and appropriately

incorporated into the water quality standards of the Chesapeake Bay jurisdictions

with tidal waters—Maryland, Virginia, Delaware and the District of Columbia.

Using existing state authority and public process, each jurisdiction is expected to

consider and propose criteria and appropriate designated uses, subject to review and

approval by the EPA, that are consistent with the requirements of the Clean Water

Act. The EPA will consider the Regional Criteria Guidance in reviewing any state

submission regarding this issue. The guidance contained in this document is subject

to change with the synthesis and interpretation of future scientific findings. 

REFINED DESIGNATED USES: 

ESSENTIAL AQUATIC LIFE COMMUNITIES

EPA Region III has identified and described five habitats (or designated uses) that,

when adequately protected, will ensure the protection of the living resources of the

Chesapeake Bay and its tidal tributaries. Those five uses (see Figure 1) provide the

context in which EPA Region III derived adequately protective Chesapeake Bay

water quality criteria for dissolved oxygen, water clarity and chlorophyll a, which

are the subject of this Regional Criteria Guidance. Accurate delineation of where to

apply these tidal-water designated uses is critical to the Chesapeake Bay water

quality criteria. EPA Region III is publishing a Technical Support Document for the

Identification of Chesapeake Bay Designated Uses and Attainability, which provides

further information on the development and geographical extent of the designated

uses to which the criteria may apply.

The migratory fish spawning and nursery designated use protects migratory and

resident tidal freshwater fish during the late winter to late spring spawning and

nursery season in tidal freshwater to low-salinity habitats. Located primarily in the

upper reaches of many Bay tidal rivers and creeks and the upper mainstem Chesapeake Bay, this use will benefit several species including striped bass, perch, shad,

herring, sturgeon and largemouth bass.

The shallow-water bay grass designated use protects underwater bay grasses and the

many fish and crab species that depend on the vegetated shallow-water habitat

provided by underwater grass beds.

The open-water fish and shellfish designated use focuses on surface water habitats

in tidal creeks, rivers, embayments and the mainstem Chesapeake Bay, and protects

diverse populations of sport fish, including striped bass, bluefish, mackerel and sea

trout, as well as important bait fish such as menhaden and silversides.
▼xi

Executive Summary

The deep-water seasonal fish and shellfish designated use protects animals inhabiting

the deeper transitional water-column and bottom habitats between the well-mixed

surface waters and the very deep channels. This use protects many bottom-feeding fish,

crabs and oysters, and other important species such as the bay anchovy.

The deep-channel seasonal refuge designated use protects bottom sedimentdwelling worms and small clams that bottom-feeding fish and crabs consume

naturally. Low to occasional no dissolved oxygen conditions occur in this habitat

zone during the summer.

Deep-Channel

Seasonal Refuge Use

Deep-Channel

Seasonal Refuge Use

A. Cross-Section of Chesapeake Bay or Tidal Tributary

B. Oblique View of the Chesapeake Bay and its Tidal Tributaries

Shallow-Water

Bay Grass Use Open-Water

Deep-Water Fish and Shellfish Use

Seasonal Fish and

Shellfish Use

Deep-Water

Seasonal Fish and

Shellfish Use

Migratory Fish

Spawning and

Nursery Use

Shallow-Water

Bay Grass Use Open-Water

Fish and Shellfish Use

Figure 1. Conceptual illustration of the five Chesapeake Bay tidal water designated use zones.
▼xii

Executive Summary

The Chesapeake Bay watershed states with tidally influenced Bay waters—Maryland, Virginia, Delaware and the District of Columbia—are ultimately responsible

for defining and formally adopting a refined set of designated uses into their respective water quality standards.

DISSOLVED OXYGEN CRITERIA

Oxygen is one of the most essential environmental constituents supporting life. In

the Chesapeake Bay’s deeper waters, there is a natural tendency toward reduced

dissolved oxygen conditions because of the Bay’s physical morphology and estuarine circulation. The Chesapeake Bay’s highly productive shallow waters, coupled

with strong density stratification, long residence times (weeks to months), low tidal

energy and its tendency to retain, recycle and regenerate nutrients from the

surrounding watershed, all set the stage for low dissolved oxygen conditions.

Against this backdrop, EPA Region III has derived a set of dissolved oxygen criteria

to protect specific aquatic life communities (outlined above) and reflect the Chesapeake Bay’s natural processes that define distinct habitats (Figure 1).

The derivation of these criteria followed the EPA’s national guidelines; the EPA,

National Marine Fisheries Science and U.S. Fish and Wildlife Service’s joint

national endangered species consultation guidelines; and the risk-based approach

used in developing the EPA’s Virginian Province saltwater dissolved oxygen criteria

(for estuarine and coastal waters from Cape Cod, Massachusetts to Cape Hatteras,

North Carolina). The resulting criteria reflect the needs and habitats of Chesapeake

Bay estuarine living resources and are structured to protect five tidal-water designated uses (Figure 2). 

Criteria for the migratory fish spawning and nursery, shallow-water bay grass and

open-water fish and shellfish designated uses were set at levels to prevent impairment

of growth, and to protect the reproduction and survival of all organisms (Table 1).

Criteria for deep-water seasonal fish and shellfish designated use habitats during

seasons when the water column is significantly stratified were set at levels to protect

juvenile and adult fish, shellfish and the recruitment success of the bay anchovy.

Criteria for deep-channel seasonal refuge designated use habitats in summer were set

to protect the survival of bottom sediment-dwelling worms and clams.

WATER CLARITY CRITERIA

Underwater bay grass beds in the Chesapeake Bay create rich animal habitats that

support the growth of diverse fish and invertebrate populations. Underwater bay

grasses, also referred to as submerged aquatic vegetation or SAV, help improve tidal

water quality by retaining nutrients as plant material, stabilizing bottom sediments

(preventing their resuspension) and reducing shoreline erosion. The health and

survival of these underwater plant communities in the Chesapeake Bay and its tidal
▼xiii

Executive Summary

tributaries depend on suitable environmental conditions. The loss of underwater bay

grasses from the shallow waters of the Chesapeake Bay, which was first noted in the

early 1960s, is a widespread, well-documented problem. The primary causes of the

decline of these underwater bay grasses are nutrient over-enrichment and increased

suspended sediments in the water, and associated reductions in light availability

(Figure 3). Other factors such as climatic events and herbicide toxicity may also have

contributed to the loss of bay grasses. In order to restore these critical habitats and

food sources, enough light must penetrate the shallow waters to support the survival,

growth and repropagation of diverse, healthy underwater bay grass communities.

EPA Region III has identified Chesapeake Bay water clarity criteria to establish the

minimum level of light penetration required to support the survival, growth and

continued propagation of underwater bay grasses. Using a worldwide literature

synthesis, an evaluation of Chesapeake Bay-specific field study findings, as well as

model simulation and diagnostic tools, the EPA derived Chesapeake Bay-specific

water clarity criteria for low and higher salinity habitats (Table 2).

The water clarity criteria, applied only during the bay grass growing seasons, are

presented in terms of the percent ambient light at the water surface extending

through the water column and the equivalent Secchi depth by application depth. The

Migratory Spawning and

Nursery Habitats

Deep-Channel Habitats

Striped Bass: 5-6

White Perch: 5

Crabs: 3

American Shad: 5

Worms: 1

6

5

4

0

1

2

3

Yellow Perch: 5

Alewife: 3.6

Bay Anchovy: 3

Spot: 2

Hard Clams: 5

Deep-Water Habitats

Shallow-Water and

Open-Water Habitats

Figure 2. Dissolved oxygen (mg liter-1) concentrations required by different Chesapeake Bay species

and communities.
▼xiv

Executive Summary

Designated Use Criteria Concentration/Duration Protection Provided Temporal Application

7-day mean > 6 mg liter-1(tidal habitats with 0-0.5 ppt salinity)

Survival/growth of larval/juvenile tidal-fresh resident fish;protective of threatened/endangered species.

Instantaneous minimum > 5 mg liter-1 Survival and growth of larval/juvenile migratory fish;

protective of threatened/endangered species.

February 1 - May 31 Migratory fish

spawning

and

nursery use

Open-water fish and shellfish designated use criteria apply June 1 - January 31

Shallow-water bay

grass use

Open-water fish and shellfish designated use criteria apply Year-round

30-day mean > 5.5 mg liter-1(tidal habitats with 0-0.5 ppt salinity)

Growth of tidal-fresh juvenile and adult fish; protective ofthreatened/endangered species.

30-day mean > 5 mg liter-1(tidal habitats with >0.5 ppt salinity)

Growth of larval, juvenile and adult fish and shellfish;protective of threatened/endangered species.

7-day mean > 4 mg liter-1 Survival of open-water fish larvae.

Open-water fish and

shellfish use

Instantaneous minimum > 3.2 mg liter-1 Survival of threatened/endangered sturgeon species.1

Year-round

30-day mean > 3 mg liter-1 Survival and recruitment of bay anchovy eggs and larvae.1-day mean > 2.3 mg liter-1 Survival of open-water juvenile and adult fish.Instantaneous minimum > 1.7 mg liter-1 Survival of bay anchovy eggs and larvae.

June 1 - September 30 Deep-water

seasonal fish and

shellfish use

Open-water fish and shellfish designated-use criteria apply October 1 - May 31

Instantaneous minimum > 1 mg liter-1 Survival of bottom-dwelling worms and clams. June 1 - September 30

Deep-channel

seasonal refuge use Open-water fish and shellfish designated use criteria apply October 1 - May 31

1 At temperatures considered stressful to shortnose sturgeon (>29C), dissolved oxygen concentrations above an instantaneous minimum of 4.3 mg

liter-1 will protect survival of this listed sturgeon species.

Table 1. Chesapeake Bay dissolved oxygen criteria.

Deep-channel

seasonal refuge use
▼xv

Executive Summary

Table 2. Summary of Chesapeake Bay water clarity criteria for application to shallow-water bay grass

designated use habitats.

Light Transmission Light Attenuation

Surface

Reflection

Plankton

Chlorophyll a

Total

Suspended

Solids

Water-Column

Light Attenuation

(Kd)

Epiphyte

Light Attenuation

(Ke

)

•Water

•Particles

•Color

•Algae

•Detritus

•Sediments

Epiphytes

Light-at-Leaf

Light-through-Water

Underwater Bay Grasses

DIN

DIP

Grazers

Figure 3. Availability of light for underwater bay grasses is influenced by water-column 

and at-the-leaf surface light attenuation processes. DIN = dissolved inorganic nitrogen and 

DIP = dissolved inorganic phosphorus.

Water Clarity Criteria as Secchi Depth

Water Clarity Criteria Application Depths

0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0

 Salinity

Regime

Water Clarity

Criteria as

Percent Lightthrough-Water

Secchi Depth (meters) for above Criteria Application Depth

Temporal

 Application

Tidal-fresh 13 % 0.2 0.4 0.5 0.7 0.9 1.1 1.2 1.4 April 1 - October 31

Oligohaline 13 % 0.2 0.4 0.5 0.7 0.9 1.1 1.2 1.4 April 1 - October 31

Mesohaline 22 % 0.2 0.5 0.7 1.0 1.2 1.4 1.7 1.9 April 1 - October 31

Polyhaline 22 % 0.2 0.5 0.7 1.0 1.2 1.4 1.7 1.9 March 1 - May 31,

September 1 - November 30

1.1
recommended percent light-through-water criteria can be directly measured using a

Secchi disk or a light meter. A specific application depth is required in order to apply

and determine attainment of the water clarity criteria.

CHLOROPHYLL A CRITERIA

Phytoplankton are small, often microscopic plants floating in the water. These organisms form the base of the Chesapeake Bay’s food web, linking nutrients and sunlight

energy with forage fish such as menhaden and bay anchovy, and with bottomdwelling invertebrates such as oysters, clams and worms. The majority of the Bay’s

animals feed directly on phytoplankton or on organisms that consume the phytoplankton. Therefore, the Bay’s “carrying capacity,” or its ability to produce and

maintain a diversity of species, depends in large part on how well phytoplankton

meet the nutritional needs of their consumers. 

A primary characteristic of algae is the presence of photosynthetic pigments. Chlorophyll a is the primary photosynthetic pigment in algae and cyanobacteria (blue-green

algae). Since chlorophyll a is a measure of photosynthesis, it is thus also a measure

of the primary food source of aquatic food webs. 

Chlorophyll a also plays a direct role in reducing light penetration in shallow-water

habitats, which has a direct impact on underwater bay grasses. Uneaten by

zooplankton and filter-feeding fish or shellfish, excess dead algae are consumed by

bacteria, and in the process, remove oxygen from the water column. Phytoplankton

assemblages can become dominated by single species which represent poor food

quality or even produce toxins that impair the animals that feed directly on them.

From a water quality perspective, chlorophyll a is the best available, most direct

measure of the amount and quality of phytoplankton and the potential to lead to

reduced water clarity and low dissolved oxygen impairments.

The EPA is providing the states with a recommended narrative chlorophyll a criteria

applicable to all Chesapeake Bay and tidal tributary waters (Table 3). The EPA

encourages states to adopt numerical chlorophyll a criteria for application to tidal

waters in which algal-related designated use impairments are likely to persist even

after attainment of the applicable dissolved oxygen and water clarity criteria. The

technical information supporting states’ quantitative interpretation of the narrative

chlorophyll a criteria is published in the body of the Chesapeake Bay water quality

criteria document.

The three Chesapeake Bay criteria–dissolved oxygen, water clarity and chlorophyll

a–should be viewed as an integrated set of criteria applied to their respective sets of

designated use habitats and addressing similar and varied ecological conditions and

water quality impairments. They provide the basis for defining the water quality

conditions necessary to protect the five essential Chesapeake Bay tidal-water designated uses.

▼xvi

Executive Summary
CRITERIA IMPLEMENTATION

EPA Region III also is presenting Chesapeake Bay criteria implementation procedures as additional regional guidance in accordance with Section 117(b)(2) of the

Clean Water Act to the Chesapeake Bay watershed states and other agencies, institutions, groups or individuals considering how to apply the criteria to determine the

degree of attainment. The EPA expects that these procedures will promote consistent, baywide application of the criteria across jurisdictional boundaries.

The criteria were derived specifically to protect species and communities in the five

tidal-water designated uses during specific time periods. For example, dissolved

oxygen criteria have been derived for application to each of the five designated uses,

whereas the chlorophyll a criteria apply only to open-water fish and shellfish designated use habitats and the water clarity criteria only to the shallow-water bay grass

designated use habitats. 

In defining what it means for the criteria to be attained, stressor magnitude, duration,

return frequency, spatial extent and temporal assessment period must be accounted

for. Stressor magnitude refers to how much of the pollutant or condition can be

allowed (e.g., 5 mg liter-1) while still achieving the designated uses. Duration refers

to the period of time over which measurements of the pollutant or water quality

parameter is to be averaged (e.g., the 30-day mean). The allowable return frequency

at which the criterion can be violated without a loss of the designated use also must

be considered. Attainment of all three Chesapeake Bay criteria within the respective

designated use habitats should be assessed at the spatial scale of the 78 Chesapeake

Bay segments (spatial extent) using the most recent three consecutive years of applicable tidal water quality monitoring data (temporal assessment period).

As the estuarine habitats gradually attain the three Chesapeake Bay criteria, not only

will the concentrations and values increase (i.e., dissolved oxygen and water clarity)

or decrease (chlorophyll a), but also occurrences of extreme changes in concentrations over a short period of time (e.g., dissolved oxygen concentration changes from

6 mg liter-1 to 2 mg liter-1 in a matter of hours) will be greatly reduced. Even if the

Chesapeake Bay ecosystem is fully restored, it is unlikely that a circumstance of

‘zero violation’ of these criteria will ever be observed, given natural Bay processes

and extreme weather events. As these criteria were developed with conservative

(protective) assumptions, allowing a small percentage of circumstances in which the

criteria may be exceeded will still fully protect the tidal-water designated uses. 

▼xvii

Executive Summary

Table 3. Chesapeake Bay narrative chlorophyll a criteria.

Concentrations of chlorophyll a in free-floating microscopic aquatic plants (algae) shall not exceed levels that 

result in ecologically undesirable consequences—such as reduced water clarity, low dissolved oxygen, food 

supply imbalances, proliferation of species deemed potentially harmful to aquatic life or humans or aesthetically

objectionable conditions—or otherwise render tidal waters unsuitable for designated uses. 
The cumulative frequency distribution methodology for defining criteria attainment

addresses the circumstances under which the criteria may be exceeded in a small percentage of instances, by integrating the five elements of criteria definition and

attainment: magnitude, duration, return frequency, space and time. The methodology

summarizes the frequency of instances in which the water quality threshold 

(e.g., dissolved oxygen concentration) is exceeded, as a function of the area or

volume affected at a given place and over a defined period of time. Acceptable and

protective combinations of the frequency and spatial extent of such instances are

defined using a biologically based reference curve.

Using this approach to define criteria attainment, the EPA recommends a procedure

to quantify the spatial extent (area or volume) to which the water quality criterion

has been achieved or exceeded for each monitoring event. For example, under a

monthly monitoring program, the spatial extent to which the criterion has been

achieved or exceeded would be estimated for each month. This could be accomplished through interpolation of the available point, transect and remote-sensing

data. The criteria measure could thus be estimated at all locations in a given spatial

unit. The spatial extent to which a water quality criterion had been exceeded for a

given monitoring event would be defined as the fraction of the total area or volume

(expressed as a percent) that exceeds the criterion.

Through the integrated application of coupled airshed, watershed and tidal-water

quality Chesapeake Bay models and long-term tidal water quality monitoring data

records, the reductions in air, land and water-based loadings of nitrogen, phosphorus

and sediments required to attain the criteria-defined ambient tidal-water concentrations of dissolved oxygen, water clarity and chlorophyll a can be directly

determined. In effect, the conditions necessary for attaining the three sets of Chesapeake Bay water quality criteria can be translated into watershed-based caps on

nutrient and sediment loadings and further allocated to specific sources and locations

within those watersheds.

▼xviii

Executive Summary
▼xix

Notices

Notices

This document has been developed by the U.S. Environmental Protection Agency

(EPA) Region III Chesapeake Bay Program Office, with the assistance and

support of the EPA Region III Water Protection Division, EPA Region II, EPA Headquarters Office of Water and Office of Research and Development, the states of

Maryland, Virginia, Delaware, Pennsylvania, New York and West Virginia and the

District of Columbia.

The goal of the Clean Water Act is to restore and maintain the chemical, physical and

biological integrity of the nation’s waters and, where attainable, to achieve water

quality that provides for the protection and propagation of fish, shellfish and wildlife

and recreation in and on the water. As a means of meeting this goal, the Clean Water

Act requires states and authorized tribes to establish water quality criteria to protect

designated uses. This document provides regional technical guidance and recommendations to states, authorized tribes and other authorized jurisdictions to develop

water quality criteria and water quality standards under the Clean Water Act to

protect against the adverse effects of nutrient and sediment over-enrichment in the

Chesapeake Bay and its tidal tributary waters. 

States and tribal decision-makers retain the discretion to adopt approaches that differ

from this regional guidance on a case-by-case basis when appropriate and scientifically defensible, consistent with the Clean Water Act. While this document contains

the EPA’s scientific findings and policy recommendations regarding ambient concentrations of dissolved oxygen, water clarity and chlorophyll a that protect Chesapeake

Bay estuarine aquatic resources, it is not a substitute for the Clean Water Act or EPA

regulations; nor is it a regulation. Thus, it cannot impose legally binding requirements on the EPA, states, authorized tribes or the regulated community, and it may

not apply to particular situations or circumstances. The EPA may change this

regional guidance in the future.

This document is available to the public through the Internet at http://www.epa.gov/

waterscience/standards/nutrient.html or www.chesapeakebay.net/baycriteria.htm.

Requests for the document should be sent to the U.S. Environmental Protection

Agency, National Service Center for Environmental Publications, 11029 Kenwood

Road, Building 5, Cincinnati, Ohio 45242 (513-489-8190) or by email (waterpubs@

epamail.epa.gov). Please refer to EPA document number EPA 903-R-03-002.
▼xxi

Acknowledgments

Acknowledgments

These Chesapeake Bay-specific water quality criteria were derived through the

collaborative efforts, collective knowledge and applied expertise of the following

four Chesapeake Bay criteria and standards coordinator teams.

Water Clarity Criteria Team

Richard Batiuk, U.S. EPA Chesapeake Bay Program Office; Peter Bergstrom,

U.S. Fish and Wildlife Service; Arthur Butt, Virginia Department of Environmental

Quality; Ifeyinwa Davis, U.S. EPA Office of Water; Frederick Hoffman, Virginia

Department of Environmental Quality; Charles Gallegos, Smithsonian

Environmental Research Center; Will Hunley, Hampton Roads Sanitation District;

Michael Kemp, University of Maryland Horn Point Laboratory; Ken Moore, Virginia

Institute of Marine Science; Michael Naylor, Maryland Department of Natural

Resources; and Nancy Rybicki, U.S. Geological Survey.

Without the efforts of the authors of the first and second Chesapeake Bay underwater

bay grass technical syntheses, the Bay-specific water clarity criteria could not have

been developed: Steve Ailstock, Anne Arundel Community College; Rick Bartleson,

University of Maryland Horn Point Laboratory; Richard Batiuk, U.S. EPA

Chesapeake Bay Program Office; Peter Bergstrom, U.S. Fish and Wildlife Service;

Steve Bieber, Maryland Department of the Environment; Virginia Carter, U.S.

Geological Survey; William Dennison, University of Maryland Center for

Environmental Studies; Charles Gallegos, Smithsonian Environmental Research

Center; Patsy Heasly, Chesapeake Research Consortium; Edward Hickman,

U.S. Geological Survey; Lee Karrh, Maryland Department of Natural Resources;

Michael Kemp, University of Maryland Horn Point Laboratory; Evamaria Koch,

University of Maryland Horn Point Laboratory; Stan Kollar, Harford Community

College; Jurate Landwehr, U.S. Geological Survey; Ken Moore, Virginia Institute of

Marine Science; Laura Murray, University of Maryland Horn Point Laboratory;

Michael Naylor, Maryland Department of Natural Resources; Robert Orth, Virginia

Institute of Marine Science; Nancy Rybicki, U.S. Geological Survey; Lori Staver,

University of Maryland; Court Stevenson, University of Maryland Horn Point
▼xxii

Acknowledgments

Laboratory; Mirta Teichberg, Woods Hole Oceanographic Institution; and David

Wilcox, Virginia Institute of Marine Science.

Dissolved Oxygen Criteria Team

Richard Batiuk, U.S. EPA Chesapeake Bay Program Office; Denise Breitburg,

Academy of Natural Sciences; Arthur Butt, Virginia Department of Environmental

Quality; Thomas Cronin, U.S. Geological Survey; Ifeyinwa Davis, U.S. EPA Office

of Water; Robert Diaz, Virginia Institute of Marine Science; Frederick Hoffman,

Virginia Department of Environmental Quality; Steve Jordan, Maryland Department

of Natural Resources; James Keating, U.S. EPA Office of Water; Marcia Olson,

NOAA Chesapeake Bay Office; James Pletl, Hampton Roads Sanitation District;

David Secor, University of Maryland Chesapeake Biological Laboratory; Glen

Thursby, U.S. EPA Office of Research and Development; and Erik Winchester, U.S.

EPA Office of Research and Development.

Scientists from across the country, well-recognized for their work in the area of low

dissolved oxygen effects on individual species up to ecosystem trophic dynamics,

contributed their time, expertise, publications and preliminary data and findings to

support the derivation of Chesapeake Bay-specific criteria: Steve Brandt, NOAA

Great Lakes Environmental Research Laboratory; Walter Boynton, University of

Maryland Chesapeake Biological Laboratory; Ed Chesney, Louisiana Universities

Marine Consortium; Larry Crowder, Duke University Marine Laboratory; Peter

deFur, Virginia Commonwealth University; Ed Houde, University of Maryland

Chesapeake Biological Laboratory; Julie Keister, Oregon State University; Nancy

Marcus, Florida State University; John Miller, North Carolina State University; Ken

Paynter, University of Maryland; Sherry Poucher, SAIC; Nancy Rabalais, Louisiana

Universities Marine Consortium; Jim Rice, North Carolina State University; Mike

Roman, University of Maryland Horn Point Laboratory; Linda Schaffner, Virginia

Institute of Marine Science; Dave Simpson, Connecticut Department of

Environmental Protection; and Tim Target, University of Delaware.

Chlorophyll a Criteria Team

Richard Batiuk, U.S. EPA Chesapeake Bay Program Office; Claire Buchanan,

Interstate Commission on the Potomac River Basin; Arthur Butt, Virginia

Department of Environmental Quality; Ifeyinwa Davis, U.S. EPA Office of Water;

Tom Fisher, University of Maryland Horn Point Laboratory; David Flemer, U.S.

EPA Office of Water; Larry Haas, Virginia Institute of Marine Science; Larry

Harding, University of Maryland Horn Point Laboratory/Maryland Sea Grant;

Frederick Hoffman Virginia Department of Environmental Quality; Will Hunley,

Hampton Roads Sanitation District; Richard Lacouture, Academy of Natural

Sciences; Robert Magnien, Maryland Department of Natural Resources; Harold

Marshall, Old Dominion University; Robert Steidel, Hopewell Regional Wastewater

Facility; and Peter Tango, Maryland Department of Natural Resources.
▼xxiii

Acknowledgments

Without the efforts of the Chesapeake Bay Phytoplankton Restoration Goals Team

forging connections between reference phytoplankton communities and resulting

chlorophyll a concentrations would not have been possible: Claire Buchanan,

Interstate Commission on the Potomac River Basin; Richard Lacouture, Academy of

Natural Sciences; Harold Marshall, Old Dominion University; Stella Sellner,

Academy of Natural Sciences; Jacqueline Johnson, Interstate Commission on the

Potomac River Basin/Chesapeake Bay Program Office; Jonathan Champion,

Chesapeake Research Consortium/Chesapeake Bay Program Office; Marcia Olson,

NOAA Chesapeake Bay Office; Fred Jacobs, AKRF, Inc.; John Seibel, PBS & J,

Inc.; and Elgin Perry.

Water Quality Standards Coordinators Team

Richard Batiuk, U.S. EPA Chesapeake Bay Program Office; Jerusalem Bekele,

District of Columbia Department of Health; Libby Chatfield, West Virginia

Environmental Quality Board; Joe Beaman, Maryland Department of the

Environment; Thomas Gardner, U.S. EPA Office of Water (Criteria); Jean Gregory,

Virginia Department of Environmental Quality; Denise Hakowski, U.S. EPA Region

III; Elaine Harbold, U.S. EPA Region III; Wayne Jackson, U.S. EPA Region II;

James Keating, U.S. EPA Office of Water (Standards); Larry Merrill, U.S. EPA

Region III; Garrison Miller, U.S. EPA Region III; Joel Salter, U.S. EPA Office of

Water (Permits); John Schneider, Delaware Department of Natural Resources and

Environmental Control; Mark Smith, U.S. EPA Region III; Scott Stoner, New York

State Department of Environmental Conservation; and Carol Young, Pennsylvania

Department of Environmental Protection.

Without the efforts of the Chesapeake Bay Tidal Monitoring Network Design Team,

the development of the criteria attainment procedures contained in this document

would not have been developed: Claire Buchanan, Interstate Commission on the

Potomac River Basin; Paul Jacobson; Marcia Olson, NOAA Chesapeake Bay Office;

Elgin Perry; Steve Preston, U.S. Geological Survey/Chesapeake Bay Program

Office; Walter Boynton, University of Maryland Chesapeake Biological Laboratory;

Larry Haas, Virginia Institute of Marine Science; Frederick Hoffman, Virginia

Department of Environmental Quality; Bruce Michael, Maryland Department of

Natural Resources; Jacqueline Johnson, Interstate Commission for the Potomac

River Basin; Kevin Summers, U.S. EPA Office of Research and Development; Dave

Jasinski, University of Maryland; Mary Ellen Ley, U.S. Geological Survey/

Chesapeake Bay Program Office; and Lewis Linker, U.S. EPA Chesapeake Bay

Program Office.
▼xxiv

Acknowledgments

The contributions of the 12 independent scientific peer reviewers, selected based on

their recognized national expertise and drawn from institutions and agencies from

across the country, are hereby acknowledged.

Without the contributions of the more than 100 individuals listed as authors or

technical contributors to various syntheses of Chesapeake Bay living resource

habitat requirements over the past two decades, the scientific basis for a set of

designated uses tailored to Chesapeake Bay tidal habitats and species would not have

been forged. Without the efforts of the many individuals involved in all aspects of

collection, management and analysis of Chesapeake Bay Monitoring Program data

over the past two decades, these criteria could not have been derived. Their

collective contributions are hereby fully acknowledged.

The technical editing, document preparation and desk-top publication contributions

of Robin Bisland, Donna An and Susan Vianna are hereby acknowledged.
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

Notices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

National Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Regional Nutrient Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Chesapeake Bay Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

II. Chesapeake Bay Nutrient and Sediment Enrichment Criteria . . . 5

III. Dissolved Oxygen Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Chesapeake Bay science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Natural dissolved oxygen processes . . . . . . . . . . . . . . . . . . . . . . . . . 8

Chesapeake Bay oxygen dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Low dissolved oxygen: historical and recent past . . . . . . . . . . . . . . . 10

Approach to Deriving Dissolved Oxygen Criteria . . . . . . . . . . . . . . . . 12

Chesapeake Bay dissolved oxygen restoration goal framework . . . . 14

Regionalizing the EPA Virginian Province 

saltwater dissolved oxygen criteria . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Applying the EPA freshwater dissolved oxygen criteria . . . . . . . . . . 25

Species listed as threatened or endangered . . . . . . . . . . . . . . . . . . . . 27

Scientific literature findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Instantaneous minimum versus daily mean . . . . . . . . . . . . . . . . . . . . 33

Strengths and limitations of the criteria derivation procedures . . . . . 34

Chesapeake Bay Dissolved Oxygen Criteria Derivation . . . . . . . . . . 40

Migratory fish spawning and nursery designated use criteria . . . . . . 42

Open-water fish and shellfish designated use criteria . . . . . . . . . . . . 46

Deep-water seasonal fish and shellfish designated use criteria . . . . . 52

Deep-channel seasonal refuge designated use criteria . . . . . . . . . . . . 60

▼iii

Contents

Contents
Chesapeake Bay Dissolved Oxygen Criteria . . . . . . . . . . . . . . . . . . . . 65

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

IV. Water Clarity Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

The relationships between water quality, light and 

underwater bay grasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Determining light requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

Strengths and limitations of the criteria derivation procedures . . . . . 85

Water Clarity Criteria Derivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Minimum light requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Light-through-water requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

Chesapeake Bay Water Clarity Criteria . . . . . . . . . . . . . . . . . . . . . . . . 96

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

V. Chlorophyll a Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Scope and magnitude of nutrient enrichment in Chesapeake Bay . . 101

Chlorophyll a: key indicator of phytoplankton biomass . . . . . . . . . . 102

Chesapeake Bay Chlorophyll a Criteria . . . . . . . . . . . . . . . . . . . . . . . . 104

Supporting Technical Information and Methodologies . . . . . . . . . . . . 105

Context for the narrative Chesapeake Bay chlorophyll a criteria . . . 105

Chlorophyll a concentrations characteristic of 

various ecological conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Chlorophyll a concentrations characteristic of 

trophic-based conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

Chlorophyll a concentrations protective against 

water quality impairments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

Methodologies for deriving waterbody-specific 

chlorophyll a criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

VI. Recommended Implementation Procedures . . . . . . . . . . . . . . . . . . . 143

Defining Criteria Attainment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

Dissolved oxygen criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

Water clarity criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

Chlorophyll a criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

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Contents
Addressing Magnitude, Duration, Frequency, Space and Time . . . . . 148

Developing the Cumulative Frequency Distribution . . . . . . . . . . . . . . 152

Step 1. Interpolation of water quality monitoring data . . . . . . . . . . . 152

Step 2. Comparison of interpolated water quality 

monitoring data to the appropriate criterion value . . . . . . . . . . . . . . . 155

Step 3. Identification of interpolator cells that 

exceed the criterion value` . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

Step 4. Calculation of the cumulative probability 

of each spatial extent of exceedance . . . . . . . . . . . . . . . . . . . . . . . . . 156

Step 5. Plot of spatial exceedance vs. the cumulative frequency . . . 159

Diagnosing the Magnitude of Criteria Exceedance . . . . . . . . . . . . . . . 164

Defining the Reference Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

Strengths and limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

Approaches to defining reference curves . . . . . . . . . . . . . . . . . . . . . 167

Reference curves for dissolved oxygen criteria . . . . . . . . . . . . . . . . . 168

Reference curves for water clarity criteria . . . . . . . . . . . . . . . . . . . . . 171

Reference curves for chlorophyll a criteria . . . . . . . . . . . . . . . . . . . . 174

Reference curve implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

Monitoring to Support the Assessment of Criteria Attainment . . . . . 176

Shallow-water monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

Dissolved oxygen criteria assessment . . . . . . . . . . . . . . . . . . . . . . . . 177

Water clarity criteria assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

Chlorophyll a criteria assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

Evaluation of Chesapeake Bay Water Quality Model Output . . . . . . 194

Chesapeake Bay Watershed Model . . . . . . . . . . . . . . . . . . . . . . . . . . 195

Chesapeake Bay Water Quality Model . . . . . . . . . . . . . . . . . . . . . . . 196

Integration of Monitoring and Modeling for Criteria Assessment . . 196

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

VII. Diagnostic Procedures for Natural Processes and 

Criteria Nonattainment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

Addressing Natural Exceedance of the Chesapeake Bay Criteria . . . 201

Natural excursions of low dissolved oxygen conditions . . . . . . . . . . 202

Natural reductions in water clarity levels . . . . . . . . . . . . . . . . . . . . . 206

Natural elevated chlorophyll a concentrations . . . . . . . . . . . . . . . . . . 209

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Contents
Diagnosing Causes of Criteria Nonattainment . . . . . . . . . . . . . . . . . . . 210

Dissolved oxygen criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210

Water clarity criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

Chlorophyll a criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

Appendices 

A. Refined Designated Uses for the Chesapeake Bay and 

Tidal Tributaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1

B. Sensitivity to Low Dissolved Oxygen Concentrations for 

Northern and Southern Atlantic Coast Populations of 

Selected Test Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1

C. Summary of Literature on the Tolerance of Chesapeake Bay

Macrobenthic Species to Low Dissolved Oxygen Conditions . . . . . . C-1

D. Narrative, Numerical and Method-based Chlorophyll a Criteria 

Adopted as Water Quality Standards by States Across the U.S. . . . .D-1

E. 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a

Concentrations by Chesapeake Bay Program Segment . . . . . . . . . . . . E-1

F. Phytoplankton Reference Community Data Analyses . . . . . . . . . . . . . F-1

G. Data Supporting Determination of Adverse Affect Thresholds for

Potentially Harmful Algal Bloom Species . . . . . . . . . . . . . . . . . . . . . .G-1

H. Derivation of Cumulative Frequency Distribution Criteria 

Attainment Reference Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .H-1

I. Analytical Approaches for Assessing Short-Duration 

Dissolved Oxygen Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1

J. Development of Chesapeake Bay Percent Light-at-the-Leaf 

Diagnostic Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J-1

▼vi

Contents
Nutrients are essential to the health and diversity of our surface waters. However,

excessive nutrients lead to low dissolved oxygen, fish kills, algal blooms and imbalances in the aquatic food web. They also pose potential risks to human health, such

as those recently manifested in the harmful algal blooms of the Gulf and East coasts,

including the tidal tributaries of the Chesapeake Bay.

National water quality inventories have repeatedly shown that nutrients are a major

cause of ambient water quality impairments. The EPA’s 1996 Section 305(b) report

identified excessive nutrients as the leading cause of impairments to lakes and the

second leading cause of impairments to rivers, after siltation. In addition, nutrients

were the second leading cause of impairments reported by the states in their 1998

Section 303(d) lists. Nutrients, along with sediment, were the primary causes of

impairments to the Chesapeake Bay and its tidal tributaries on the respective Maryland and Virginia Section 303(d) lists. To meet the objectives of the Clean Water Act,

the EPA’s implementing regulations specify that states must adopt criteria that

contain sufficient parameters to protect existing and designated uses. Until 2000, the

EPA had not published recommended quantitative water quality criteria for nutrients

that states could adopt to protect uses.

In order to achieve and maintain water quality conditions necessary to protect the

aquatic living resources of the Chesapeake Bay and its tidal tributaries from the

effects of nutrient and sediment pollution, EPA Region III has developed Ambient

Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for

the Chesapeake Bay and Its Tidal Tributaries (Regional Criteria Guidance). EPA

Region III has also identified and described five habitats (or designated uses) that

when adequately protected will ensure the protection of the living resources of the

Bay and its tidal tributaries. Those five uses (described in Appendix A) provide the

context in which EPA Region III derived protective Chesapeake Bay water quality

criteria for dissolved oxygen, water clarity and chlorophyll a (see Figure 1 in the

Executive Summary), which are the subject of the Regional Criteria Guidance. EPA

Region III has also published the Technical Support Document for the Identification

of Chesapeake Bay Designated Uses and Attainability. This document provides

▼1

chapter i • Introduction

chapteri

Introduction
further information on the development and geographical extent of the designated

uses to which the criteria may apply.

NATIONAL CRITERIA

Under the Clean Water Act Section 304(a), the EPA issues national criteria recommendations to states and tribes to assist them in developing their water quality

standards. When the EPA reviews a state or tribal water quality standard for approval

under 303(c) of the Clean Water Act, the agency must determine whether the adopted

designated uses are consistent with the Clean Water Act requirements and whether

the adopted criteria protect the designated use. The EPA’s regulations encourage

states and tribes, when adopting water quality criteria as part of their water quality

standards, to employ the EPA’s Section 304(a) guidance, to modify the EPA’s 304(a)

guidance to reflect site-specific conditions or to use other scientifically defensible

methods to derive criteria to protect the designated uses.

REGIONAL NUTRIENT CRITERIA

In 1995 the EPA gathered a group of nationally recognized scientists and managers

to address the national nutrient problem. They recommended that the agency avoid

setting criteria for phosphorus or nitrogen that would apply to all water bodies and

regions of the country. Instead they suggested that the EPA develop guidance

(assessment tools and control measures) for specific bodies of water and ecological

regions across the country and use reference conditions, which reflect pristine or

minimally affected waters, as a basis for developing nutrient criteria.

Using these suggestions as starting points, the EPA published the National Strategy

for the Development of Regional Nutrient Criteria in June 1998. The strategy articulated the EPA’s intention to develop technical guidance manuals for four types of

waters (lakes and reservoirs, rivers and streams, estuaries and coastal waters, and

wetlands) and produce nutrient criteria for specific eco-regions. In addition, the EPA

is committed to working with states and tribes to develop more refined and localized

nutrient criteria based on approaches described in the water body guidance manuals.

The Regional Criteria Guidance provides EPA’s recommendations to the Chesapeake Bay states for use in establishing their water quality standards consistent with

Section 303(c) of the Clean Water Act.

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chapter i • Introduction
CHESAPEAKE BAY CRITERIA

The EPA’s current guidance for dissolved oxygen can be found in the 1986 freshwater dissolved oxygen criteria and 2000 Virginian Province saltwater criteria

documents. EPA Region III developed the criteria presented in this document by

integrating and supplementing the scientific findings and data to fully protect

specific Chesapeake Bay tidal-water habitats. The revised criteria are based on and

consistent with the existing EPA dissolved oxygen criteria.

There are no national 304(a) criteria specific to chlorophyll a or water clarity. In

accordance with sections 117(b) and 303 of the Clean Water Act, EPA Region III

derived the water quality criteria addressing these critical nutrient and sediment

enrichment parameters specifically to protect Chesapeake Bay living resources and

their tidal-water habitats.

The water quality criteria presented in this document are designed to apply to the

Chesapeake Bay and its tidal tributaries and embayments within the tidally influenced waters of the states of Maryland, Virginia and Delaware and the District of

Columbia (Figure I-1). These regional criteria may also apply to other estuarine and

coastal systems, with appropriate modifications.

The regional criteria and designated uses presented in this document and the Technical Support Document are the product of a collaborative effort among the

Chesapeake Bay Program partners. They represent a scientific consensus based on

the best available scientific findings and technical defining water quality conditions

necessary to protect Chesapeake Bay aquatic living resources from effects due to

nutrient and sediment over-enrichment. Various stakeholder groups have been

involved in their development, with contributions from the staffs of federal and state

governments, local agencies, scientific institutions, citizen conservation groups,

business and industry. In the Regional Criteria Guidance the EPA recommends and

expects that the numerical criteria and refined designated uses will be considered by

and appropriately incorporated into the water quality standards of the Chesapeake

Bay jurisdictions with tidal waters—Maryland, Virginia, Delaware and the District

of Columbia.

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chapter i • Introduction
▼4

chapter i • Introduction

West Virginia

Virginia

Pennsylvania

New York

Maryland

DC Delaware

Figure I-1. The Chesapeake Bay watershed crosses the boundaries of six states—Maryland, Virginia,

Delaware, Pennsylvania, New York and West Virginia—and the District of Columbia.
The Chesapeake 2000 agreement committed its signatories (the states of Pennsylvania, Maryland and Virginia; the District of Columbia; the Chesapeake Bay

Commission and the EPA) to “define the water quality conditions necessary to

protect aquatic living resources” in the Chesapeake Bay and its tidal tributaries.

New York, Delaware and West Virginia agreed to the same commitment through a

separate six-state memorandum of understanding with the EPA.

EPA Region III has identified the water quality conditions that are necessary to

protect living resources through the Chesapeake Bay-specific water quality criteria

for dissolved oxygen, water clarity and chlorophyll a published in this document.

The Chesapeake Bay criteria have been derived to protect a series of five refined

tidal-water designated uses which, in turn, reflect important and unique habitats

throughout the Chesapeake Bay and its tidal tributaries (Appendix A). More detailed

descriptions of these refined subcategories of tidal-water designated uses and their

recommended boundaries can be found in the EPA Region III publication, Technical

Support Document for the Identification of Chesapeake Bay Designated Uses and

Attainability. Collectively, these three water quality conditions provide the best and

most direct measures of the effects of too much nutrient and sediment pollution on

the Chesapeake Bay’s aquatic living resources–fish, crabs, oysters, their prey species

and underwater bay grasses.

Fish and other aquatic life require specific levels of dissolved oxygen to survive.

Seasonal algae blooms, when uneaten by fish and shellfish, deplete dissolved

oxygen, potentially rendering the deep waters of the Bay uninhabitable to certain

species during certain times of the year. The Chesapeake Bay dissolved oxygen

criteria were based on the oxygen levels required by different aquatic communities

inhabiting distinct habitats in the Bay’s tidal waters during different times of the year

(Chapter III).

Underwater bay grasses are an essential component of the Chesapeake Bay’s habitat

and an important food source for waterfowl. Decreased water clarity inhibits the

growth of underwater bay grasses. Building on decades of scientific research, the

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chapter ii • Chesapeake Bay Nutrient and Sediment Enrichment Criteria

chapterii

Chesapeake Bay Nutrient and

Sediment Enrichment Criteria
Chesapeake Bay water clarity criteria were derived to protect the minimum light

required by both low and higher salinity underwater plant communities (Chapter IV).

Measurements of chlorophyll a indicate levels of phytoplankton or algal biomass in

the water column. Levels that are too high indicate algal blooms, which lead to a

proliferation of less desirable species, shade the light in shallow-water habitats and

cause low dissolved oxygen conditions, as uneaten algae die off and sink to the

bottom. Narrative Chesapeake Bay chlorophyll a criteria were derived to support

desired ecological conditions and protect against an array of water quality

impairments (Chapter V).

The EPA provides Chesapeake Bay criteria implementation procedures as additional

regional guidance to the Chesapeake Bay watershed states and other agencies, institutions, groups or individuals for consideration of how to apply the criteria in order

to determine the degree of attainment of those criteria (Chapter VI). These implementation procedures are published in this document to promote consistent, baywide

application of the criteria across jurisdictional boundaries.

A series of diagnostic procedures and tools designed to explain the reasons for nonattainment of the water quality criteria are documented (Chapter VII). Approaches

for addressing natural exceedances of the criteria not already accounted for in the

implementation procedures are provided for consistent application across all tidal

water habitats.

The EPA is publishing this Regional Criteria Guidance to further to goals of the

Clean Water Act and, specifically, pursuant to Sections 117(b) and 303(c) of the the

Act.

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chapter ii • Chesapeake Bay Nutrient and Sediment Enrichment Criteria
BACKGROUND

Of all life-supporting environmental constituents, oxygen is one of the most essential. In cells, oxygen stores and liberates the energy that drives vital processes of fish,

crabs and shellfish such as feeding, growth, swimming and reproduction. Low

dissolved oxygen concentrations can increase mortality, reduce growth rates and

alter the distribution and behavior of aquatic organisms, all of which can produce

significant changes in the overall estuarine food web (Breitburg 2002).

The Chesapeake Bay and its tidal tributaries harbor diverse and productive communities of aquatic organisms that are supported by a complex array of food webs. To

establish dissolved oxygen criteria for these living resources and the food webs they

depend upon, we must characterize the dissolved oxygen conditions that lead to

stressful conditions for the living resources of the Chesapeake Bay, ranging from

copepods to sturgeon.

CHESAPEAKE BAY SCIENCE

The development of the scientific underpinnings for Chesapeake Bay-specific

criteria has been under way for decades. The first documentation of seasonal occurrence, low dissolved oxygen conditions in the Chesapeake Bay took place in the

1930s (Newcombe and Horne 1938; Newcombe et al. 1939), with low oxygen conditions documented in the lower Potomac River in the early 1900s (Sale and Skinner

1917). Chesapeake Bay dissolved oxygen dynamics, which are critical to deriving

criteria that reflect the ecosystem process, first became understood during the

research cruises of the Johns Hopkins Chesapeake Bay Institute during the 1950s

through the late 1970s. A five-year, multidisciplinary research program established

in the late 1980s, funded and coordinated by the Maryland and Virginia Sea Grant

programs, yielded significant advances in the understanding of Chesapeake Bay

oxygen dynamics, effects and ecosystem implications (Smith et al. 1992). The coordinated state-federal Chesapeake Bay Water Quality Monitoring Program, initiated

▼7

chapter iii • Dissolved Oxygen Criteria

chapteriii

Dissolved Oxygen Criteria
in 1984, provided decadal scale records of seasonal to interannual variability in

dissolved oxygen conditions throughout the tidal waters. Building on the long-term

baywide monitoring data record, a series of multi-investigator, multi-year National

Science Foundation, NOAA and EPA-funded research programs provided new

insights into Bay ecosystem processes and responses. These investigations laid the

groundwork for management application of the resulting science.

NATURAL DISSOLVED OXYGEN PROCESSES

Dissolved oxygen in any natural body of water is primarily a function of atmospheric

oxygen (which diffuses into the water at the surface), oxygen produced by plants

(microscopic free-floating plants or phytoplankton) during photosynthesis and

aquatic animals, plants and bacteria that consume dissolved oxygen through respiration. Oxygen also is consumed by chemical processes such as sulfide oxidation and

nitrification. The reduction of dissolved oxygen stimulates sulfate reduction and

results in hydrogen sulfide, a more toxic form of sulfur. Oxygen depletion also can

inhibit nitrogen removal via coupled nitrification and denitrification and enhance the

recycling of ammonia and phosphates as well as the release of heavy metals from

bottom sediments into the overlying water column.

The amount of oxygen dissolved in the water changes as a function of temperature,

salinity, atmospheric pressure and biological and chemical processes. Gill and

integumentary respiration, which most Chesapeake Bay aquatic species use, is

accomplished by extracting dissolved oxygen across a pressure gradient (rather than

a concentration gradient). As the partial pressure of dissolved oxygen increases in

the water (e.g., increasing temperature and salinity), it can more readily be extracted

by an organism. Cold-blooded organisms, however, have much higher metabolic

rates and oxygen requirements at higher temperatures, which more than offsets the

oxygen gained at the higher temperature. The interactions among metabolism,

temperature and salinity clearly are complex, but they must be considered in deriving

Chesapeake Bay dissolved oxygen criteria. 

Biological processes such as respiration and photosynthesis can affect the concentration of dissolved oxygen before a new equilibrium can be reached with the

atmosphere. As a result, for relatively short periods of time, or under sustained

conditions of reduced physical mixing (i.e., the stratification of the water column),

dissolved oxygen concentrations can be driven well below the point of saturation.

They can decrease to zero (a condition known as anoxia), especially in deep or stratified bodies of water, or increase to a concentration of 20 mg liter-1 (a condition

known as supersaturation) during dense algal blooms.

CHESAPEAKE BAY OXYGEN DYNAMICS

It is critical to take into account the natural processes that control oxygen dynamics

in order to establish criteria that reflect natural conditions and protect different habitats. The Chesapeake Bay tends to have naturally reduced dissolved oxygen

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chapter iii • Dissolved Oxygen Criteria
conditions in its deeper waters because of its physical morphology and estuarine

circulation. As in other estuarine systems (e.g., Boynton et al. 1982; Nixon 1988;

Caddy 1993; Cloern 2001), the Chesapeake’s highly productive waters, combined

with sustained stratification, long residence times, low tidal energy and its tendency

to retain and recycle nutrients, set the stage for lower dissolved oxygen conditions.

The mesohaline mainstem Chesapeake Bay and lower reaches of the major tidal

rivers have a stratified water column, which essentially prevents waters near the

bottom from mixing with oxygenated surface waters. The recycling of nutrients and

water-column stratification lead to severe reductions in dissolved oxygen concentrations during the warmer months of the year in deeper waters within and below the

pycnocline layer.

This reduction in dissolved oxygen generally results from a host of additional

biological and physical factors (e.g., Kemp and Boynton 1980; Kemp et al. 1992;

Sanford et al. 1990; Boynton and Kemp 2000). The annual spring freshet delivers

large volumes of fresh water to the Bay. The contribution of significant quantities of

nutrients in the spring river flows, combined with increasing temperatures and light,

produces a large increase in phytoplankton biomass. Phytoplankton not consumed

by suspension feeders (such as zooplankton, oysters and menhaden) sink to the

subpycnocline waters, where they are broken down by bacteria over a period of days

or weeks (e.g., Malone et al. 1986; Tuttle et al. 1987; Malone et al. 1988). This loss

of oxygen due to bacterial metabolism is exacerbated by restricted mixing with

surface waters because of the onset of increased water-column stratification.

The Chesapeake Bay’s nearshore shallow waters periodically experience episodes of

low to no dissolved oxygen, in part because bottom water has been forced into the

shallows by a combination of internal lateral tides and sustained winds (Carter et al.

1978; Tyler 1984; Seliger et al. 1985; Malone et al. 1986; Breitburg 1990; Sanford

et al. 1990). Low dissolved oxygen conditions in the shallow waters of tidal tributaries are more often the result of local production and respiration than the incursion

of bottom waters. Climatic conditions such as calm winds and several continuous

cloudy days in a row can contribute to oxygen depletion in these shallow-water habitats. They can be exposed to episodes of extreme and rapid fluctuations in dissolved

oxygen concentrations (Sanford et al. 1990). In depths as shallow as 4 meters,

dissolved oxygen concentrations may decline to 0.5 mg liter-1 for up to 10 hours

(Breitburg 1990).

Diel cycles of low dissolved oxygen conditions often occur in nonstratified shallow

waters where water-column respiration at night temporarily reduces dissolved

oxygen levels (D’Avanzo and Kremer 1994). In nearshore waters of the mesohaline

mainstem Chesapeake Bay, near-bottom dissolved oxygen concentrations are characterized by large diel fluctuations and daily minima during the late night and early

morning hours of July and August (Breitburg 1990). 

The timing and extent of reduced dissolved oxygen conditions in the Chesapeake

Bay vary from year to year, driven largely by local weather patterns, the timing and

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chapter iii • Dissolved Oxygen Criteria
magnitude of freshwater river flows, the concurrent delivery of nutrients and sediments into tidal waters and the corresponding springtime phytoplankton bloom

(Officer et al. 1984; Seliger et al. 1985; Boynton and Kemp 2000; Hagy 2002). In

the Chesapeake Bay’s mesohaline mainstem, these conditions generally occur from

June through September but have been observed to occur as early as May. They may

persist through early October, until the water column is fully mixed in the fall. The

deeper waters of several Chesapeake Bay major tidal tributaries also can exhibit

hypoxic and anoxic conditions (Hagy 2002). 

LOW DISSOLVED OXYGEN: HISTORICAL AND RECENT PAST

Dissolved oxygen levels vary naturally in lakes, estuaries and oceans over varying

temporal and spatial scales due to many biological, chemical and physical processes.

In estuaries such as the Chesapeake Bay, freshwater inflow that influences watercolumn stratification; nutrient input and cycling; physical processes such as

density-driven circulation; and tides, winds, water temperature and bacterial activity

are among the most important factors. These processes can lead to large natural

seasonal and interannual variability in oxygen levels in many parts of the Chesapeake Bay and its tidal tributaries.

Superimposed on this natural dissolved oxygen variability is a progressive increase

in the intensity and frequency of hypoxia and anoxia over the past 100 to 150 years,

most notably since the 1960s. This human-induced eutrophication is evident both

from instrumental data and geochemical and faunal/floral ‘proxies’ of dissolved

oxygen conditions obtained from the sedimentary record.

The instrumental record, while incomplete prior to the inception of the multi-agency

Chesapeake Bay Monitoring Program in 1984, suggests that as early as the 1930s

(Newcombe and Horne 1938) and especially since the 1960s (Taft et al. 1980),

summer oxygen depletion has been recorded in the Chesapeake Bay. Officer et al.

(1984), Malone (1992), Harding and Perry (1997) and Hagy (2002) provide useful

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chapter iii • Dissolved Oxygen Criteria

Anoxia is the absence of oxygen. Because most field dissolved oxygen meters are only

precise to ± 0.1 or 0.2 mg liter-1, areas with measured oxygen concentrations of 0.2 mg liter-1

or less are sometimes classified as anoxic. There is no accurate consensus on the scientific

definition of hypoxia, but it is often defined as oxygen concentrations below 2 mg liter-1 (U.S.

scientific literature) or 2 mg liter-1 (European scientific literature). These specific

concentration-based definitions are problematic when applied in an effects context, because

many species show reduced growth and altered behavior at oxygen levels above 2 mg liter-1,

and sensitive species experience mortality during prolonged exposure at these low

concentrations. As an operational definition, hypoxia should be considered to be oxygen

concentrations reduced from full saturation that impair living resources. 

Anoxia is the absence of oxygen. Because most field dissolved oxygen meters are only precise to 

± 0.1 or 0.2 mg liter -1, areas with measured oxygen concentrations of 0.2 mg liter -1 or less are

sometimes classified as anoxic. There is no accurate consensus on the scientific definition of

hypoxia, but it is often defined as oxygen concentrations below 2 mg liter -1 (U.S. scientific

literature) or 2 ml liter -1 (European scientific literature). These specific concentration-based

definitions are problematic when applied in an effects context, because many species show reduced

growth and altered behavior at oxygen levels above 2 mg liter -1, and sensitive species experience

mortality during prolonged exposure at these low concentrations. As an operational definition,

hypoxia should be considered to be oxygen concentrations reduced from full saturation that impair

living resources.
discussions of the instrumental record of dissolved oxygen and related parameters

such as chlorophyll a across this multi-decade data record.

At issue is whether, and to what degree, dissolved oxygen reductions are a naturally

occurring phenomenon in the Chesapeake Bay. Long sediment core records (17

meters to greater than 21 meters in length) indicate that the Chesapeake Bay formed

about 7,500 years ago (Cronin et al. 2000; Colman et al. 2002) when the rising sea

level after the final stage of Pleistocene deglaciation flooded the Susquehanna

channel. The modern estuarine circulation and salinity regime probably began in the

mid- to late Holocene epoch, about 4,000-5,000 years ago (in the regional climate of

the early Holocene, Chesapeake Bay’s salinity differed from that of the late

Holocene). This is based on the appearance of ‘pre-colonial’ benthic foraminiferal,

ostracode and dinoflagellate assemblages. It is against this mid- to late Holocene

baseline that we can view the post-European settlement and modern dissolved

oxygen regime of the Chesapeake Bay. 

During the past decade, studies of the Chesapeake Bay’s late Holocene dissolved

oxygen record have been carried out using several proxies of past dissolved oxygen

conditions, which are preserved in sediment cores that have been dated using the most

advanced geochronological methods. These studies, using various indicators of past

dissolved oxygen conditions, are reviewed in Cronin and Vann (2003) and provide

information that puts the monitoring record of the modern Chesapeake Bay into a

long-term perspective and permits an evaluation of natural variability in the context

of restoration targets. The following types of measurements of oxygen-sensitive

chemical and biological indicators have been used: nitrogen isotopes (Bratton et al.

2003); biogenic silica and diatom communities (Cooper and Brush 1991; Cooper

1995; Colman and Bratton 2003); molybdenum and other metals (Adelson et al.

2000; Zheng et al., in press); lipid biomarkers; acid volatile sulfur (AVS)/chromium

reducible sulfur (CRS) ratios; total nitrogen and total organic carbon (Zimmerman

and Canuel 2000); elemental analyses (Cornwell et al. 1996) and paleo-ecological

reconstructions based on dinoflagellate cysts (Willard et al. 2003); and benthic

foraminiferal assemblages (Karlsen et al. 2000). Although space precludes a comprehensive review of these studies, and the time period studied and level of

quantification vary, several major themes emerge, which are summarized here. 

First, the 20th century sedimentary record confirms the limited monitoring record of

dissolved oxygen, documenting that there has been a progressive decrease in

dissolved oxygen levels, including the periods of extensive anoxia in the deepchannel region of the Chesapeake Bay that have been prominent during the last 40

years. Most studies provide strong evidence that there was a greater frequency or

duration of seasonal anoxia beginning in the late 1930s and 1940s and again around

1970, reaching unprecedented frequencies or duration in the past few decades in the

mesohaline Chesapeake Bay and the lower reaches of several tidal tributaries. Clear

evidence of these low dissolved oxygen conditions has been found in all geochemical and paleo-ecological indicators studied principally through their great impact on

benthic and phytoplankton (both diatom and dinoflagellate) communities. 

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chapter iii • Dissolved Oxygen Criteria
Second, extensive late 18th and 19th century land clearance also led to oxygen reduction and hypoxia, which exceeded levels characteristic of the previous 2,000 years.

Best estimates for deep-channel mid-bay seasonal oxygen minima from 1750 to

around 1950 are 0.3 to 1.4-2.8 mg liter-1 and are based on a shift to dinoflagellate

cyst assemblages of species tolerant of low dissolved oxygen conditions. This shift

is characterized by a four- to fivefold increase in the flux of biogenic silica, a greater

than twofold (5-10 millileter-1) increase in nitrogen isotope ratios (15N) and periods

of common (though not dominant) Ammonia parkinsoniana, a facultative anaerobic

foraminifer. These patterns are likely the result of increased sediment influx and

nitrogen and phosphorous runoff due to extensive land clearance and agriculture.

Third, before the 17th century, dissolved oxygen proxy data suggest that dissolved

oxygen levels in the deep channel of the Chesapeake Bay varied over decadal and

interannual time scales. Although it is difficult to quantify the extremes, dissolved

oxygen probably fell to 3 to 6 mg liter-1, but rarely if ever fell below 1.4 to 2.8 mg

liter-1. These paleo-dissolved oxygen reconstructions are consistent with the Chesapeake Bay’s natural tendency to experience seasonal oxygen reductions due to its

bathymetry, freshwater-driven salinity stratification, high primary productivity and

organic matter and nutrient regeneration (Boicourt 1992; Malone 1992; Boynton et

al. 1995).

In summary, the main channel of the Chesapeake Bay most likely experienced reductions in dissolved oxygen before large-scale post-colonial land clearance took place,

due to natural factors such as climate-driven variability in freshwater inflow.

However, this progressive decline in summer oxygen minima, beginning in the 18th

century and accelerating during the second half of the 20th century, is superimposed

on interannual and decadal patterns of dissolved oxygen variability. Human activity

during the post-colonial period has caused the trend towards hypoxia and most

recently (especially after the1960s) anoxia in the main channel of the Chesapeake

Bay and some of its larger tidal tributaries. The impact of these patterns has been

observed in large-scale changes in benthos and phytoplankton communities, which

are manifestations of habitat loss and degradation.

APPROACH FOR DERIVING 

DISSOLVED OXYGEN CRITERIA

Against this backdrop, a set of dissolved oxygen criteria have been derived to protect

Chesapeake Bay estuarine species living in different habitats that are influenced by

the Bay’s natural processes. The Chesapeake Bay dissolved oxygen criteria directly

reflect natural oxygen dynamics. For example, instantaneous minimum to daily

mean criterion values reflect short-term variations in oxygen concentrations, and

seasonal application of deep-water and deep-channel criteria account for the natural

effects of water-column stratification on oxygen concentrations. Oxygen dynamics

and natural low- to no-oxygen conditions also were taken into account in developing

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chapter iii • Dissolved Oxygen Criteria
the refined tidal-water designated uses (see Appendix A; U.S. EPA 2003a), which

factor in natural conditions leading to low dissolved oxygen concentrations.

The derivation of these regional criteria followed the methodologies outlined in the

EPA’s Guidelines for Deriving Numerical National Water Quality for the Protection

of Aquatic Organisms and their Uses (U.S. EPA 1985), the risk-based approach used

in developing the Ambient Aquatic Life Water Quality Criteria for Dissolved Oxygen

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chapter iii • Dissolved Oxygen Criteria

Chesapeake Bay Dissolved Oxygen Criteria Team

member Dr. Thomas Cronin, of the U.S. Geological Survey (USGS), (surveyed five scientists1 who

have studied the history of anoxia and hypoxia in

the Chesapeake Bay over decadal and centennial

time scales, using geochemical and biological

proxies from sediment cores and instrumental and

historical records. The consensus of the five scientists is that the Chesapeake Bay was seasonally

anoxic between 1900 and 1960. The seasonal

anoxia was extensive in the deep channel and probably lasted several months. Similarly, between

1600 and 1900, the near-unanimous consensus is

that the Bay was seasonally anoxic for probably

weeks to months in the deep channel. One

researcher had reservations about his group’s

earlier conclusion on definitive evidence of anoxia

prior to 1900, but cannot exclude the possibility of

anoxia during this period. Anoxia during the

1900–1960 period was probably geographically

less extensive in the Bay and perhaps occurred less

frequently (i.e., not every year) than after the

1960s. In addition to the geochemical and faunal

proxies of past trends in oxygen depletion, experts

cite the Sale and Skinner (1917) instrumental documentation of hypoxia and probable anoxia in the

lower Potomac in 1912.

For the period prior to European colonization

(~1600 AD), the consensus is that the deep

channel of the Bay may have been briefly hypoxic

(< 2 mg liter-1), especially during relatively wet

periods (which did occur, based on the paleoclimate record). Anoxia probably occurred only

during exceptional conditions. It should be noted

that the late 16th and much of the 17th century was

an extremely dry period which was not conducive

to oxygen depletion.

In sum, hypoxia, and probably periodic spatiallylimited anoxia, occurred in the Bay prior to the

large-scale application of fertilizer, but since the

1960s oxygen depletion has become much more

severe.

These experts also unanimously believe that

restoring the Bay to mid-20th century, pre-1960

conditions might be possible but very difficult

(one expert suggested an 80 percent nitrogen

reduction was necessary), in light of remnant

nutrients in sediment in the Bay and behind dams,

likely increased precipitation as the climate

changes, population growth and other factors.

Most researchers believe that restoring the Bay to

conditions prior to 1900 is either impossible, or

not realistic, simply due to the fact that the

temporal variability (year-to-year and decadal) in

‘naturally occurring’ hypoxia renders a single

target dissolved oxygen level impossible to define. 

1T. M. Cronin (USGS, Reston, Virginia), S. Cooper (Bryn Athyn College), J. F. Bratton (USGS, Woods Hole, Massachusetts),

A. Zimmerman (Pennsylvania State University), G. Helz (University of Maryland, College Park).
(Saltwater): Cape Cod to Cape Hatteras (U.S. EPA 2000) and the Biological

Evaluation on the CWA 304(a) Aquatic Life Criteria as part of the National Consultations, Methods Manual (U.S. EPA, U.S. Fish and Wildlife Service and NOAA National

Marine Fisheries Service, in draft). The resulting criteria factored in the physiological

needs and habitats of the Chesapeake Bay’s living resources and are designed to

protect five distinct tidal-water designated uses (Appendix A; U.S. EPA 2003a).

Criteria for protecting the migratory fish spawning and nursery, shallow-water bay

grass and open-water fish and shellfish designated uses were set at levels to protect

the growth, recruitment and survival ecologically, recreationally and commercially

important fish and shellfish species. Criteria applicable to deep-water seasonal fish

and shellfish designated uses were set at levels to protect shellfish and juvenile and

adult fish, and to foster the recruitment success of the bay anchovy. Criteria for deepchannel seasonal refuge designated uses were set to protect the survival of bottom

sediment-dwelling worms and clams. These summer deep-water and deep-channel

designated uses take into account the natural historic presence of low oxygen in

these habitats and the likelihood that such conditions may persist (U.S. EPA 2003a).

CHESAPEAKE BAY DISSOLVED OXYGEN 

RESTORATION GOAL FRAMEWORK

The Chesapeake Bay dissolved oxygen restoration goal was published in 1992 in

response to the Chesapeake Executive Council’s commitment to “develop and adopt

guidelines for the protection of water quality and habitat conditions necessary to

support the living resources found in the Chesapeake Bay system and to use these

guidelines” (Chesapeake Executive Council 1987). The 1992 goal contained specific

target dissolved oxygen concentrations for application over specified averaging

periods and locations (Table III-1; Jordan et al. 1992).

Information on the effects of low dissolved oxygen concentrations was compiled for

14 target species of fish, mollusks and crustaceans, as well as for other benthic and

planktonic communities in the Bay food web. These species were selected from a

larger list of important species reported in Habitat Requirements for Chesapeake

Bay Living Resources, Second Edition (Funderburk et al. 1991). The selection of

target dissolved oxygen concentrations and their temporal and spatial applications

followed an analysis of dissolved oxygen concentrations that would provide the

levels of protection needed to achieve the restoration goal. Where data gaps existed,

best professional judgment was used.

The original Chesapeake Bay dissolved oxygen restoration goal and its supporting

framework made three significant breakthroughs for the derivation and management

application of the Bay-specific dissolved oxygen criteria. First, the 1992 dissolved

oxygen target concentrations varied with the vertical depth of the water column and

horizontally across the Chesapeake Bay and its tidal tributaries, reflecting variations

in the levels of water quality required for the protection of different habitats (see

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chapter iii • Dissolved Oxygen Criteria
Appendix A; U.S. EPA 2003a). Second, the averaging period for each target concentration was tailored to each habitat, understanding that short-term exposures to

concentrations below the target concentrations were tolerable and could still protect

living resources (see “Chesapeake Bay Dissolved Oxygen Criteria Derivation,” page

40). Finally, the 1992 dissolved oxygen restoration goal contained a methodology

through which water quality monitoring data and model scenario outputs, collected

over varying time periods, could be assessed to calculate the percentage of time that

areas of bottom habitat or volumes of water-column habitat would meet or exceed

the applicable target dissolved oxygen concentrations (see Chapter VI).

REGIONALIZING THE EPA VIRGINIAN PROVINCE SALTWATER

DISSOLVED OXYGEN CRITERIA

The EPA’s Ambient Water Quality Criteria for Dissolved Oxygen (Saltwater): Cape

Cod to Cape Hatteras (U.S. EPA 2000), here referred to as the Virginian Province

criteria document, involved the development of an extensive database on dissolved

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chapter iii • Dissolved Oxygen Criteria

Table III-1. 1992 Chesapeake Bay dissolved oxygen goal for restoration of living resource habitats.

Target Dissolved Oxygen Concentrations Time and Location

Dissolved oxygen 1 mg liter-1 All times, everywhere.

1.0 mg liter-1  dissolved oxygen 3 mg liter-1 For no more than 12 hours, interval between

excursions at least 48 hours, everywhere.

Monthly mean dissolved oxygen  5 mg liter-1 All times, throughout above-pycnocline1

 waters.

Dissolved oxygen  5 mg liter-1

All times, throughout above-pycnocline waters

in spawning reaches, spawning rivers, and

nursery areas.

1The pycnocline is the portion of water column where density changes rapidly becaue of salinity and temperature.

Source: Jordan et al. 1992

The Chesapeake Bay dissolved oxygen goal for the restoration of living resource habitats is to

provide for sufficient dissolved oxygen to support the survival, growth and reproduction of

anadromous, estuarine and marine fish and invertebrates in the Chesapeake Bay and its tidal

tributaries by achieving, to the greatest spatial and temporal extent possible, the following 

target concentrations of dissolved oxygen, and by maintaining the existing minimum

concentration of dissolved oxygen in areas of the Chesapeake Bay and its tidal tributaries 

where dissolved oxygen concentrations fall above the recommended targets.

Dissolved oxygen ≥ 1 mg liter -1
oxygen effects (Miller et al. 2002) and a close evaluation and synthesis of earlier

data, published in peer-reviewed literature. Ultimately the criteria were derived using

both traditional methodologies and a new biological risk-assessment framework. A

mathematical model was used to integrate effects over time, replacing the concept of

an averaging period, and protection limits were established for different life stages

(i.e., larvae versus juveniles and adults). Where practical, data were selected and

analyzed to conform to Guidelines for Deriving Numerical National Water Quality

Criteria for the Protection of Aquatic Organisms and their Uses (or EPA Guidelines,

U.S. EPA 1985).

The Virginian Province criteria document addressed three areas of protection:

1) juvenile and adult survival, 2) growth effects and 3) larval recruitment effects. In

doing so, it segregated effects on juveniles and adults from those on larvae. To

address cumulative effects of low dissolved oxygen on larval recruitment to the juvenile life stage (i.e., larval survival time), a new biological approach using a

mathematical model was taken. The model evaluated the effects of dissolved oxygen

conditions on larvae by tracking the intensity and duration of low dissolved oxygen

effects across the larval recruitment season (U.S. EPA 2000). Criteria to protect

larvae were derived using data based on varying dissolved oxygen exposures for

larval stages of nine sensitive estuarine and coastal organisms.

The juvenile and adult survival and growth criteria presented in the Virginian

Province document set boundaries for judging the dissolved oxygen status of a given

site. If dissolved oxygen concentrations are above the Virginian Province chronic

growth criterion (4.8 mg liter-1), then the site meets the objectives for protection. If

the dissolved oxygen conditions remain above the Virginian Province juvenile/adult

survival criterion (2.3 mg liter-1) over a 24-hour period, the site meets the objectives.

When the dissolved oxygen conditions fall between these two values, then the site

requires further evaluation.

The Virginian Province criteria document supported the derivation of region-specific

dissolved oxygen criteria tailored to the species, habitats and dissolved oxygen exposure regimes of varying estuarine, coastal and marine waters. The segregation by

life stage allows the criteria to be tailored to protect the individual refined Chesapeake Bay tidal-water designated uses, which reflect the use of different habitats by

different life stages (Appendix A). This segregation by life stage differs significantly

in approach from traditional aquatic life water quality criteria. However, the

Virginian Province criteria were not designed to address natural variations in

dissolved oxygen concentrations from surface waters to greater water-column

depths. If Chesapeake Bay-specific dissolved oxygen criteria had been derived using

only a strict application of this criteria methodology, they would not be flexible

enough to tailor each set of criteria to the refined tidal-water designated uses

presented in Appendix A. The resulting criteria would be driven solely by larval

effects data, irrespective of depth and season.

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chapter iii • Dissolved Oxygen Criteria
Therefore, the dissolved oxygen criteria specific to the Chesapeake Bay were

derived through the regional application of the Virginian Province criteria and the

application of both EPA published traditional toxicological and new EPA biologicalbased methodologies. Chesapeake Bay-specific science was factored into each step

of the process. The extensive Virginian Province data base was supplemented with

additional Chesapeake Bay-specific data from the scientific literature. The Virginian

Province larval recruitment model parameters were adjusted to better reflect Chesapeake Bay conditions, data and species. Finally, steps were taken to ensure

protection of species listed as threatened or endangered in Chesapeake Bay tidal

waters following both national EPA guidelines and joint U.S. EPA, U.S. Fish and

Wildlife and National Marine Fisheries Service national Endangered Species Act

consultation methodologies. The Chesapeake Bay-specific dissolved oxygen criteria

were derived with the full support of and technical assistance from the U.S. EPA

Office of Research and Development’s Atlantic Ecology Division and the U.S. EPA

Office of Water’s Office of Science and Technology.

Chesapeake Bay Species

A total of 36 species of fish, crustaceans and mollusks were included in the Virginian

Province criteria data base (U.S. EPA 2000). Only four are not resident Chesapeake

Bay species (Table III-2, U.S. EPA 1998), including the green crab and the mysid

Americamysis bahia. Both the American lobster and Atlantic surf clam have been

observed in the Chesapeake Bay, but only near the Bay mouth, in high salinities.

American lobster larvae require relatively low temperatures (20oC) and high salinities (30 ppt) for successful development, and these conditions do not normally occur

in the Chesapeake Bay.

The EPA guidelines on criteria recalculation, which allow regional and site-specific

criteria derivation, state that species should be deleted from the effects data base only

if the class is absent (U.S. EPA 1994). Emphasis is placed on deriving criteria using

an effects data base that represents the range of sensitivity of tested and untested

species from, in this case, the Chesapeake Bay and its tidal tributaries. As described

below, including these four non-Chesapeake Bay species in the effects data base

does not change the Bay-specific dissolved oxygen criteria. To ensure consistency

with national EPA guidelines, no species were dropped from the original Virginian

Province effects data base when deriving these Chesapeake Bay-specific criteria.

Juvenile and Adult Survival Criteria

The criterion minimum concentration, or CMC, provides a lower limit for a 24-hour

averaged concentration to protect juvenile and adult survival. The CMC for juvenile

and adult survival was recalculated using a Chesapeake Bay-specific effects data

base of 32 species of fish, crustaceans and mollusks (Table III-2). Dropping the four

non-Chesapeake Bay species from the original Virginian Province data base resulted

in a recalculated Chesapeake Bay-specific juvenile/adult survival CMC value of 

▼17

chapter iii • Dissolved Oxygen Criteria
▼18

chapter iii • Dissolved Oxygen Criteria

Table III-2. U.S. EPA Virginian Province criteria data base species found in the Chesapeake Bay.

Common Name Scientific Name Found in Chesapeake Bay Notes

Species Genus Only

American lobster Homarus americanus (Yes) - 1

Amphipod Ampelisca abdita Yes -

Atlantic menhaden Brevoortia tyrannus Yes -

Atlantic rock crab Cancer irroratus Yes -

Atlantic silverside Menidia menidia Yes -

Atlantic surfclam Spisula solidissima (Yes) - 2

Blue crab Callinectes sapidus Yes -

Burry’s octopus Octopus burryi No Yes 4

Daggerblade grass shrimp Palaemonetes pugio Yes -

Eastern oyster Crassostrea virginica Yes -

Flatback mud crab Eurypanopeus depressus Yes -

Fourspine stickleback Apeltes quadracus Yes -

Green crab Carcinus maenas No No 6

Hard clam Mercenaria mercenaria Yes -

Harris mud crab Rhithropanopeus harrisii Yes -

Inland silverside Menidia beryllina Yes -

Longfin squid Loligo pealeii (Yes) - 3

Longnose spider crab Libinia dubia Yes -

Marsh grass shrimp Palaemonetes vulgaris Yes -

Mysid shrimp Americamysis bahia No No 7

Naked goby Gobiosoma bosc Yes -

Northern sea robin Prionotus carolinus Yes -

Pipe fish Syngnathus fuscus Yes -

Rock crab Cancer irroratus Yes -

Sand shrimp Crangon septemspinosa Yes -

continued

Found in the Chesapeake Bay
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chapter iii • Dissolved Oxygen Criteria

Table III-2. U.S. EPA Virginian Province criteria data base species found in the Chesapeake Bay (continued).

Species Genus Only

Scup Stenotomus chrysops Yes -

Sheepshead minnow Cyprinodon variegatus Yes -

Skillet fish Gobiesox strumosus Yes -

Striped bass Morone saxatilis Yes -

Striped blenny Chasmodes bosquianus Yes -

Spot Leiostomus xanthurus Yes -

Summer flounder Paralichthys dentatus Yes -

Tautog Tautoga onitis Yes -

Windowpane flounder Scophthalmus aquosus Yes -

Winter flounder Pleuronectes americanus Yes -

Notes:

1. Occasionally found in the Chesapeake Bay mouth region outside of the Bay Bridge/tunnel during blue

crab winter dredge surveys.

2. Found near the Chesapeake Bay mouth at high salinities.

3. Found in the region around the Chesapeake Bay mouth.

4. Octopus americanus is found in the higher salinity reaches of the Chesapeake Bay.

5. Genus Dyspanopeus supercedes genus Neopanope (See Weiss, H. 1995. Marine Animals of Southern

New England and New York, State Geological and Natural History Survey of Connecticut).

6. If found in the Chesapeake Bay, Carcinus maenas would be at the extreme southern edge of its range

(See Gosner, K. 1979. Field Guide to the Atlantic Seashore : Invertebrates and Seaweeds of the Atlantic

Coast from the Bay of Fundy to Cape Hatteras, Houghton Mifflin. Boston.). This species has not been

documented in the Comprehensive List of Chesapeake Bay Basin Species (U.S. EPA 1998).

7. Americamysis bahia supercedes Mysidopsis bahia. (See Price W. W., R. W. Heard, L. Stuck 1994.

Observations on the genus Mysidopsis Sars, 1864 with the designation of a new genus, Americamysis,

and the descriptions of Americamysis alleni and A. stucki (Peracarida: Mysidacea: Mysidae), from the

Gulf of Mexico. Proceedings of the Biological Society of Washington 107:680-698).

Sources: U.S. EPA 1998, 2000. 

Dyspanopeus sayi 5 Say mud crab Yes –

–

–

–

–

–

–

–

–

–

–

Notes:

1. Occasionally found in the Chesapeake Bay mouth region outside of the Bay Bridge/tunnel during blue

crab winter dredge surveys.

2. Found near the Chesapeake Bay mouth at high salinities.

3. Found in the region around the Chesapeake Bay mouth.

4. Octopus americanus is found in the higher salinity reaches of the Chesapeake Bay.

5. Genus Dyspanopeus supercedes genus Neopanope (See Weiss, H. 1995. Marine Animals of Southern

New England and New York, State Geological and Natural History Survey of Connecticut).

6. If found in the Chesapeake Bay, Carcinus maenas would be at the extreme southern edge of its range

(See Gosner, K. 1979. Field Guide to the Atlantic Seashore : Invertebrates and Seaweeds of the Atlantic

Coast from the Bay of Fundy to Cape Hatteras, Houghton Mifflin. Boston.). This species has not been

documented in the Comprehensive List of Chesapeake Bay Basin Species (U.S. EPA 1998).

7. Americamysis bahia supercedes Mysidopsis bahia. (See Price W. W., R. W. Heard, L. Stuck 1994.

Observations on the genus Mysidopsis Sars, 1864 with the designation of a new genus, Americamysis,

and the descriptions of Americamysis alleni and A. stucki (Peracarida: Mysidacea: Mysidae), from the

Gulf of Mexico. Proceedings of the Biological Society of Washington 107:680-698).

Sources: U.S. EPA 1998, 2000.

Common Name Scientific Name Found in the Chesapeake Bay Notes

Species Genus Only
2.24 mg liter-1, very close to the EPA Virginian Province criterion value of 2.27 mg

liter-1 (U.S. EPA 2000). To maintain consistency with EPA Virginian Province

criteria and national EPA guidelines, no changes were made to the Virginian

Province criteria value of 2.27 mg liter-1 (rounded off to 2.3 mg liter-1 for purposes

of this criteria document), applied as a 1-day mean concentration.

Larval and Juvenile Growth Criteria

The criterion value protecting against adverse effects on growth under continuous

exposures, called the criterion continuous concentration (or CCC), when recalculated for only Chesapeake Bay species, increased 0.2 mg liter-1 to a Chesapeake

Bay-specific value of 5.0 mg liter-1. To maintain consistency with EPA Virginian

Province criteria and the national EPA criteria derivation guidelines, no changes

were made to the Virginian Province criteria value of 4.8 mg liter-1.

Larval Recruitment Model Application

The Virginian Province criteria larval recruitment model was used only to confirm

that the criterion values selected for the migratory fish spawning and nursery,

shallow-water and open-water criteria fully protected larval recruitment. Only in the

case of the deep-water criteria was application of the larval recruitment model

central to deriving Chesapeake Bay-specific dissolved oxygen criteria values.

Virginian Province Larval Recruitment Model. The recruitment model is a

discrete time, density-independent model consisting of several equations that allow

the cumulative impact of low dissolved oxygen to be expressed as a proportion of the

potential annual recruitment of a species. The model is run by inputting the necessary bioassay and biological information, selecting dissolved oxygen durations to

model, and then, through an iterative process, assessing various dissolved oxygen

concentrations until the desired percent recruitment impairment is obtained. The

resulting pairs of duration and dissolved oxygen concentration become the recruitment curve. The process has been incorporated in a spreadsheet for simplicity. The

model can be set up to handle unlimited and various life history stages. Its application for dissolved oxygen effects is to model larval recruitment to the juvenile stage.

The model’s equations and the major assumptions used in its application are

explained in Appendix E of the Virginian Province document (U.S. EPA 2000). The

life history parameters in the model include larval development time, larval season,

attrition rate and spatial distribution (e.g., vertical distribution). The magnitude of

effects on recruitment is influenced by each of the four life history parameters. For

instance, larval development time establishes the number of cohorts that entirely or

partially co-occur within the interval of low dissolved oxygen stress. The second

parameter, the length of the larval season, is a function of the spawning period, and

also influences the relative number of cohorts that fall within the window of hypoxic

▼20

chapter iii • Dissolved Oxygen Criteria
stress. The third life history variable, natural attrition rate, gauges the impact, if any,

of slower growth and development of the larvae in response to low dissolved oxygen

by tracking the associated increase in natural mortality (e.g., predation). The model

assumes a constant rate of attrition, so increased residence time in the water column

due to delayed development translates directly to decreased recruitment. Finally, the

distribution of larvae in the water column determines the percentage of larvae from

each cohort that would be exposed to reduced dissolved oxygen under stratified

conditions. 

The recruitment model assumes that the period of low dissolved oxygen occurs

within the larval season (hypoxic events always begin at the end of the development

time of the first larval cohort), and that hypoxic days are contiguous. Use of the

current model also assumes that a new cohort occurs every day of the spawning

season, and that each cohort is equal in size. Use of the model, however, does not

require that a fresh cohort be available every day. Successful calculation of recruitment impairment only requires knowing the total number of cohorts available during

a recruitment season (i.e., it does not matter whether they were created daily, weekly,

monthly, etc.) and whether a cohort is exposed to hypoxia. The application of the

model is further simplified by assuming that none of the life history parameters

change in response to hypoxia.

Chesapeake Bay Larval Recruitment Model Refinements. A series of

refinements were made to the Virginian Province criteria parameters for length of

recruitment season and duration of larval development. These values were revised to

reflect Chesapeake Bay-specific conditions (Table III-3).

Crustaceans. The Virginian Province criteria document states that the larval model

for crustaceans includes all larval stages and the transition from larval to megalopal

(post-larval) stage, but not the megalopal stage in its entirety (U.S. EPA 2000).

Therefore, the duration used in the model was based on the duration of larval development, plus one day for molting to the megalopal stage. The following Chesapeake

Bay-specific estimates of the duration of larval development are rounded to the

nearest whole day: rock crab—22 days; say mud crab—17 days; lobster—15 days;

spider crab—6 days; and grass shrimp—15 days. These estimates also are supported

by a wide array of literature (Anger et al. 1981a; Anger et al. 1981b; Broad 1957;

Chamberlain 1957; Costlow and Bookhout 1961; Johns 1981; Logan and Epifanio

1978; Maris 1986; Ryan 1956; Sandifer 1973; Sandifer and Van Engel 1971; Sasaki

et al. 1986; Sastry 1970; Sastry 1977; Sastry and McCarthy 1973; Sulkin and

Norman 1976; Wass 1972; Williams 1984).

The literature supports a larval release season (here termed the reproductive season)

of 120 days or more for rock crab, say mud crab and spider crab, based on the presence of gravid females and larvae in field collections (Anger et al. 1981a; Anger et

al. 1981b; Broad 1957; Chamberlain 1957; Costlow and Bookhout 1961; Johns

1981; Logan and Epifanio 1978; Maris 1986; Ryan 1956; Sandifer 1973; Sandifer

▼21

chapter iii • Dissolved Oxygen Criteria
and Van Engel 1971; Sasaki et al. 1986; Sastry 1970; Sastry 1977; Sastry and

McCarthy 1973; Sulkin and Norman 1976; Wass 1972; Williams 1984). Lobster

larvae and adults are rarely found in the Chesapeake Bay, therefore, collection data

were not available. 

Grass shrimp have an extremely long reproductive season that extends even longer

than the brachyurans. The Virginian Province criteria document implies that the

actual period over which most of these crustaceans release larvae is only 30 to 40

days (except for grass shrimp). This was not supported in the literature for the Chesapeake Bay. However, given the interest in capturing “the period of predominant

recruitment, rather than observance of the first and last dates for zoeal presence in

the water column” (U.S. EPA 2000), one could reasonably state that brachyuran

larvae are released over a 75-day period in the Chesapeake Bay. Grass shrimp larvae

are released over a period of at least 100 days due to their greater reproductive flexibility. These reproductive season values, added to the duration of the larval

development, provided the following values for the length of the recruitment season

in the Chesapeake Bay: rock crab—100 days; mud crab—90 days; spider crab—

80 days; and grass shrimp—120 days (Table III-3).

▼22

chapter iii • Dissolved Oxygen Criteria

Table III-3. Original U.S. EPA Virginian Province saltwater dissolved oxygen criteria larval

recruitment values and the revised recruitment season and larval development

values reflecting Chesapeake Bay-specific conditions.

Species

Length of

Recruitment

Season (days) 1 

Duration of Larval

Development 

(days) 1 

Attrition

Rate (percent

per day)

Percentage

Population

Exposed to

Hypoxic Event

Rock crab 65/100 35/22 5% 20%

Say mud crab 66/90 21/17 5% 75%

Flatback mud crab 66/90 21/17 5% 75%

Lobster 95 35/15 5% 20%

Spider crab 66/80 21/6 5% 50%

Silverside 42/150 14 5% 50%

Striped bass 49/70 28 5% 50%

Grass shrimp 100/120 12/15 5% 50%

Red drum 49/140 21 5% 50%

1

 First value is the original Virginian Province-wide value; the second value following the slash is the

Chesapeake Bay-specific value.
Fishes. In the Chesapeake Bay, striped bass spawn over a 30- to 40-day period. By

adding in the duration of larval development of 28 to 50 days, a reasonable estimate

for the recruitment season is 70 days (Grant and Olney 1991; McGovern and Olney

1996; Olney et al. 1991; Rutherford and Houde 1995; Secor and Houde 1995;

Ulanowicz and Polgar 1980). It should be noted that most spawning in a given tributary may occur over a much shorter period of 7 to 21 days (Rutherford 1992; Olney

et al. 1991). However, given the inability to predict which portion of the reproductive season will result in recruitment, it is important to provide water quality

conditions that support recruitment for the duration of spawning season (Secor 2000;

Secor and Houde 1995).

Silversides, along with other East Coast estuarine-dependent species, tend to show

differences in the date of initiation of spawning and spawning duration from north to

south (e.g., southern sites have longer durations). Silversides are serial batch

spawners that spawn over a less than two-month period in the northern regions of the

east coast, from two to three months around New York, and from three to four

months in the Maryland portion of the Chesapeake Bay (Conover and Present 1990;

Conover 1992; Gleason and Bengston 1996). A 140-day recruitment season factors

in a 90-day reproductive season and a 50-day duration of larval development.

Red drum also are serial batch spawners. Documentation of the red drum spawning

season is mostly for southern systems and varies between two months (Wilson and

Neiland 1994; Rooker and Holt 1997) and three months (McMichael and Peters

1987). The 140-day recruitment season applied here factors in a 90-day reproductive

season and a 50-day duration of larval development.

Impairment Percentage. Population growth of estuarine and coastal organisms

may be more affected by mortality of the juvenile and adult stages than the larval

stage. In nature only a small fraction of a season’s larvae will make it to the juvenile/adult stage. Thus, removal of a single larva from exposure to low dissolved

oxygen (which has a high probability of being removed naturally) is not nearly as

important as the loss of a single juvenile (at each successive life stage—from egg to

larva to juvenile to adult—the probability of survival to the next stage increases).

Juveniles are much closer to the reproductive stage and represent the loss not only

of the individual, but also of the potential larvae from that individual for the next

season. In this regard, an individual larvae is not as important to the population as an

individual juvenile or adult. Therefore, populations can tolerate different levels of

impact at different stages of individual development (U.S. EPA 2000). At the same

time, the criteria need to protect members of a species at all life stages so they can

develop from an egg to an adult. 

Protection against a greater than 5 percent cumulative reduction in larval seasonal

recruitment due to exposure to low oxygen conditions was applied in the Chesapeake

Bay-specific larval recruitment effects models, consistent with the level of protection

selected for the Virginian Province criteria (U.S. EPA 2000). The selection of a 

5 percent impairment of early life stages accords the same level of protection as that

▼23

chapter iii • Dissolved Oxygen Criteria
set for adult and juvenile life stages through the CMC criteria. The 5 percent impairment also is consistent with EPA guidelines for deriving ambient aquatic life water

quality criteria (U.S. EPA 1985). The 5 percent impairment sets the potential reduction

in seasonal recruitment of affected species due to low dissolved oxygen exposure at a

low level, relative to the cumulative effects of other natural and anthropogenic factors.

The EPA’s criteria derivation guidelines and technical support documents do not

state that the purpose of criteria is to prevent any losses; the purpose of the criteria

is to prevent “unacceptable” losses. The EPA has acknowledged throughout the

history of the criteria development process that criteria may allow some adverse

effects to occur, e.g., the use of 95th percentile means that there is the possibility that

5 percent of the communities’ genera will experience some impact (U.S. EPA 1985).

The EPA recognizes that large losses of larval life stages occur naturally. Some

species may be able to withstand a greater than 5 percent loss of larvae from exposure to low dissolved oxygen or other causes without an appreciable effect on

juvenile recruitment. However, this may not be the case for certain highly sensitive

species or populations that already are highly stressed, such as threatened/

endangered species where the 5 percent impairment is not applied.

In the absence of data showing how much impairment may be caused by low

dissolved oxygen conditions alone and still have a minimal effect on natural larval

recruitment to the juvenile stage for all species protected, a conservative level of

acceptable impairment has been applied. The goal is to provide a level of protection

from exposure to low dissolved oxygen that will not cause unacceptable loss to the

juvenile recruitment class above what is expected to occur naturally.

Regional Species Effects

The same species from different regions may react differently to low dissolved

oxygen conditions. For example, populations from traditionally warmer waters may

be less sensitive because they have adapted to lower concentrations of oxygen associated with native warmer temperatures. Alternatively, higher temperatures may

cause warmer-water populations to need more dissolved oxygen and thereby make

them more sensitive to lower concentrations.

Most of the effects data in the EPA Virginian Province saltwater dissolved oxygen

criteria document were from EPA-sponsored laboratory tests conducted with species

collected in the northern portion of the province. To determine whether such

geographic differences exist, northern (Rhode Island) and southern populations

(Georgia or Florida) of two invertebrates, the mud crab and the grass shrimp, and one

fish, the inland silverside, were tested in the laboratory at non-stressful temperatures.

Exposure-response relationships were similar for northern and southern populations of

each species, supporting the use of data from one region to help develop safe dissolved

oxygen limits for other regions (Coiro et al., unpublished data; see Appendix B).

▼24

chapter iii • Dissolved Oxygen Criteria
Temperature/Dissolved Oxygen Interactions

This document includes effects data collected at temperatures that are greater than

20oC and many greater than 25oC. Where there are data for the same species at

multiple temperatures, for example, grass shrimp larvae tested at temperatures

ranging from 20oC to 30oC (see Appendix B), there is no evidence for a temperature

effect on sensitivity to hypoxia over the range of temperatures tested.

The findings reported in detail in Appendix B indicate that the low dissolved oxygen

effects data were gathered over a range of different temperatures that did not influence the resulting effects findings. These findings further confirmed that test

organisms from the northern portion of the Virginian Province were no more or less

sensitive than organisms collected well south of the province boundaries. (See

“Strengths and Limitations of the Criteria Derivation Procedures,” p. 34, for a description of the potential interaction between dissolved oxygen effects and stressful

temperatures.)

APPLYING THE EPA FRESHWATER DISSOLVED OXYGEN CRITERIA

The Virginian Province saltwater criteria were derived largely from laboratory-based

effects data using test conditions with salinities ranging from oligohaline to oceanic.

Although a majority of the tests were run at salinities of greater than 15 ppt, data

from the literature included tests whose estuarine species were exposed to salinities

as low as 5 ppt. Many of the estuarine species tested tolerate a wide range of salinities, but the location of the U.S. EPA Office of Research and Development Atlantic

Ecology Division laboratory at Narragansett Bay, Rhode Island, dictated that the

tests be run at higher salinities. With extensive tidal-fresh (0-0.5 ppt) and oligohaline

(> 0.5-5ppt) habitats in the upper Chesapeake Bay and upper reaches of most tidal

tributaries, criteria established for these less saline habitats must protect resident

species. To bridge this gap, the applicable EPA freshwater dissolved oxygen criteria

were applied to ensure that the Chesapeake Bay-specific criteria protected freshwater species inhabiting tidal waters.

Freshwater Dissolved Oxygen Criteria

The EPA freshwater criteria document, published in 1986, stipulated five limits for

dissolved oxygen effects on warm-water species (Table III-4, U.S. EPA 1986). To

protect early life stages, the criteria include a 7-day mean of 6 mg liter-1 and an

instantaneous minimum of 5 mg liter-1. To protect other life stages, additional criteria

were derived. These are a 30-day mean of 5.5 mg liter-1, a 7-day mean of 4 mg 

liter-1 and an instantaneous minimum of 3 mg liter-1. Some of the most sensitive

survival and growth responses reported for warm-water species in the freshwater

criteria document were for early life stages of channel catfish and largemouth bass,

both of which are present in tidal-fresh habitats throughout the Chesapeake Bay and

its tidal tributaries (Murdy et al. 1997).

▼25

chapter iii • Dissolved Oxygen Criteria
The freshwater dissolved oxygen criteria documentation contains data for effects on an

extensive array of fish species. In addition, the

freshwater document focuses on growth effects

to early life stages, which are the more sensitive stages. Recognizing that the 1986

freshwater dissolved oxygen criteria were not

derived following the EPA’s 1985 criteria derivation guidelines, the EPA conducted a

preliminary survey of the literature since the

1986 freshwater document was published and

did find additional data that were consistent

with the 1985 EPA guidelines. However, the

effects data that were found (additional field

observations and short-term [several hours]

laboratory exposures), most of which focused

on respiratory effects, indicated that the 1986 freshwater criteria were protective.

Therefore, the EPA believes that its existing freshwater criteria accurately portray the

expected effects of low dissolved oxygen on freshwater aquatic species.

Early Life Stages

The EPA freshwater early life stage criteria were based on embryonic and larval data

for the following eight species: largemouth bass, black crappie, white sucker, white

bass, northern pike, channel catfish, walleye and smallmouth bass (U.S. EPA 1986).

Fishes of Chesapeake Bay (Murdy et al. 1997) documents smallmouth bass as “occasional to common in Chesapeake Bay tributaries from Rappahannock northward,

rare to occasional south of the Rappahannock, and absent from Eastern Shore

streams and rivers.” Regarding white suckers: “Found in all tributaries to Chesapeake Bay throughout the year, the white sucker occurs in nearly every kind of

habitat...” The largemouth bass is “common to abundant in all tributaries of Chesapeake Bay.” Black crappie were reported to be “occasional to abundant inhabitants

in major tributaries of Chesapeake Bay.” Finally, channel catfish were “common in

all tributaries of Chesapeake Bay.” (All references Murdy et al. 1997.) 

Given that five of these species—largemouth bass, black crappie, white sucker,

channel catfish and smallmouth bass—are resident in Bay tidal-fresh waters, the

freshwater early life stage criteria are fully applicable to Chesapeake Bay tidal-fresh

habitats. (See Figure 1 on page 14 and the text on pages 17-18 in the EPA’s Ambient

Water Quality Criteria for Dissolved Oxygen [Freshwater] for more details; U.S.

EPA 1986.) No efforts were made to recalculate the national freshwater criteria using

only Chesapeake Bay species, given the limited number of species used in deriving

the 1986 criteria. Dropping any of the eight species would not provide an effects data

set meeting the EPA’s guidelines for criteria recalculation to address site-specific

conditions (U.S. EPA 1994).

▼26

chapter iii • Dissolved Oxygen Criteria

Table III-4. U.S. EPA freshwater dissolved oxygen

water quality criteria (mg liter-1) 

for warm-water species.

Early Other 

Duration Life Stages1 Life Stages

30-day mean NA2 5.5

7-day mean 6 NA

7-day mean minimum NA 4

1-day minimum3 5 3

1Includes all embryonic and larval stages and all juvenile 

forms to 30 days following hatching.

2Not applicable.

3All minima should be considered as instantaneous 

concentrations to be achieved at all times.

Source: U.S. EPA 1986.
Other Life Stages

The warm-water freshwater criteria that protect other life stages were derived from

a much wider array of fish and invertebrate species, many of which occur in Chesapeake Bay tidal-fresh habitats (U.S. EPA 1998). These criteria apply to Chesapeake

Bay habitats with salinities of less than 0.5 ppt. The national EPA freshwater criteria

protecting other warm-water species life stages were not recalculated using only

Chesapeake Bay species, for the same reasons described above.

Given the differences in the available effects data, the methodologies followed in

deriving the freshwater dissolved oxygen criteria differed from those used in developing

the Virginian Province dissolved oxygen criteria. In-depth descriptions of both methodologies can be found in each respective criteria document (U.S. EPA 1986, 2000).

SPECIES LISTED AS THREATENED OR ENDANGERED

When a threatened or endangered species occurs at a site and sufficient data indicate

that it is sensitive at concentrations above the recommended criteria, site-specific

dissolved oxygen criteria may be derived (U.S. EPA 2000). Based on a review of all

federal and Chesapeake Bay tidal water state lists of threatened or endangered

species (U.S. Fish and Wildlife Service; National Oceanic and Atmospheric

Administration; the states of Maryland, Virginia and Delaware and the District of

Columbia), the only federally listed endangered species found to need protection

from the effects of low dissolved oxygen conditions was shortnose sturgeon 

(U.S. EPA 2003b). 

Shortnose sturgeon occur in the Chesapeake Bay and several tidal tributaries

(Skjeveland et al. 2000; Mangold 2003; Spells 2003). Genetic evidence suggests that

the shortnose captured in the Chesapeake Bay share the same gene pool with

Delaware Bay shortnose sturgeon, and movement has been documented between the

two bays through the C & D Canal (Welsh et al. 2002; Wirgin et al., in review).

Shortnose sturgeon have been federally protected since 1967 (National Marine Fisheries Service 1998). Chesapeake Bay shortnose sturgeon are listed as a Distinct

Population Segment in the National Oceanic and Atmospheric Administration’s

National Marine Fisheries Service Shortnose Sturgeon Recovery Plan. Since 1996,

50 sub-adult and adult shortnose sturgeon have been captured in the upper Chesapeake Bay, Potomac River and Rappahannock River (Skjeveland et al. 2000).

Mitochondrial DNA analysis indicated that these were a subset of the Delaware

population’s gene pool. 

Currently two views are held on the status of shortnose sturgeon in the Chesapeake

Bay. One view holds that shortnose sturgeon may continue to reproduce in the Bay,

arguing that the genetic evidence is inconclusive or that the Delaware Bay and

Chesapeake Bay populations may share the same gene pool. The other opinion is

that the C & D Canal serves as an important migration corridor, and shortnose occurrences in the Chesapeake Bay result from immigration from the Delaware Bay.

▼27

chapter iii • Dissolved Oxygen Criteria
Further, due to salinity preferences it is conceivable that their immigration (and

recent occurrences) has been favored by the recent series of wet years. Several sturgeon population geneticists, ecologists and icthyologists favor this latter view (Secor

2003; Wirgin et al. in review; I. Wirgin, personal communication; J. Waldman,

personal communication; J. Musick, personal communication). Regardless of

whether shortnose sturgeon populations remain in the Chesapeake Bay, the

Chesapeake Bay dissolved oxygen criteria have been derived to be protective of all

life stages of both shortnose and Atlantic sturgeon.

Sturgeon Dissolved Oxygen Sensitivity

Sturgeon in Chesapeake Bay and elsewhere are more sensitive to low dissolved

oxygen conditions than most other fish. In comparison with other fishes, sturgeon

have a limited behavioral and physiological capacity to respond to hypoxia (multiple

references reviewed and cited by Secor and Niklitschek 2003). Sturgeon basal

metabolism, growth, consumption and survival are all very sensitive to changes in

oxygen levels, which may indicate their relatively poor ability to oxyregulate. In

summer, temperatures greater than 20°C amplify the effect of hypoxia on sturgeon

and other fishes due to a temperature-oxygen ‘habitat squeeze’ (Coutant 1987). Deep

waters with temperatures that sturgeon prefer tend to have dissolved oxygen concentrations below the minimum that they require. Sturgeon are therefore either forced to

occupy unsuitable habitats or have a reduction in habitat.

Several studies have directly addressed the lethal effects of hypoxia on sturgeon

species important to the Chesapeake Bay. Jenkins et al. (1993) examined the effects

of different salinities and dissolved oxygen levels on juveniles of the shortnose sturgeon Acipenser brevirostrum. The dissolved oxygen tests were all conducted at a

mean temperature of 22.5°C. The authors state:

Due to various constraints including limitations of facilities and test

animals, strictly controlled and standardized methods could not be

followed in all tests. The findings reported should be considered as

preliminary until such time as more rigorous testing can be accomplished.

In addition, the authors report nominal2 oxygen levels rather than those specific

dissolved oxygen concentrations experienced during each replicate experiment. All

experiments were conducted in freshwater. Still, strong evidence was presented that

younger fish were differentially susceptible to low oxygen levels in comparison to

older juveniles. Fish older than 77 days experienced minimal mortality at nominal

levels 2.5 mg liter-1, but at 2 mg liter-1 experienced 24 to 38 percent mortality.

▼28

chapter iii • Dissolved Oxygen Criteria

2The authors report that dissolved oxygen levels were monitored every 30 minutes throughout the 

6-hour tests, and state that each parameter remained at “satisfactory levels.” The dissolved oxygen

values reported are 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 and 7.5 mg liter-1. Since up to five replicates were

used with as many as 12 measurements, it seems unlikely that these exact dissolved oxygen concentration values were maintained consistently throughout all the tests.
Younger fish experienced 18 to 38 percent mortality in the 3 mg liter-1 treatment and

>80 percent mortality in the 2.5 mg liter-1 treatment. Mortality of juveniles 77 days

or older at treatment levels ≥3.5 mg liter-1 was not significantly different than control

levels. Because only nominal dissolved oxygen concentrations were reported, the

EPA could not derive LC50 values for criteria derivation purposes based upon

responses reported by Jenkins et al. (1993).

Criterion Protective of Sturgeon

More rigorous tests with shortnose sturgeon were recently performed using youngof-the-year fish 77 to 134 days old (Campbell and Goodman 2003). Campbell and

Goodman (2003) present four 24-hr LC50 values for shortnose sturgeon (Acipenser

brevirostrum). Three of these are from tests with non-stressful temperatures

(22–26°C) for this species. The fourth test was conducted at 29°C and was considered to be a stressful temperature by the authors (Larry Goodman, personal

communication). Fish from this fourth test also were exposed to temperatures as

high as 31°C during the acclimation period immediately preceding their exposure to

hypoxia. Since the data from the fourth test also include an effect due to temperature

stress, they should be considered separately from data from the other three tests.

The current draft (December 2002) of the “National Consultation” on threatened and

endangered species (being negotiated between the U.S. EPA, the U.S. Fish and

Wildlife Service and the NOAA National Marine Fisheries Service) states:

Where acute toxicity data are available for the species of interest, only

these data will be used for designating the LC50 for this species. If these

data include more than one test, the geometric mean of the LC50s of these

tests will be used in risk calculations. If only one toxicity test has been

conducted, the lower 95% confidence interval of the LC50 from this test

will be used.

Following this guidance the final LC50 for shortnose sturgeon under ambient conditions of non-stressful temperatures would be the geometric mean of 2.2, 2.2 and

2.6 mg liter-1, or 2.33 mg liter-1. Under stressful temperatures, the LC50 value that

should be used would be 3.1 mg liter-1 (this is the LC50 of the 29°C test, since the

3.1 mg liter-1 treatment resulted in exactly 50 percent mortality there was no 

95 percent confidence interval) (Campbell and Goodman 2003). 

Long-term exposures (10 days) of Atlantic sturgeon, Acipenser oxyrinchus, youngof-the-year (150 to 200 days old) to 2.8 to 3.3 mg liter-1 at 26°C resulted in complete

mortality over a 10-day period in three of four replicates (Secor and Gunderson

1998). The fourth replicate experienced 50 percent mortality. At 19°C and 2.3 to 

3.2 mg liter-1, only 12 to 25 percent mortality was recorded. There was insufficient

data to calculate an LC50 for 19°C (it was less than 2.70 mg liter-1 3, but could not

determine how much less). However, based on survival data present in Secor and

▼29

chapter iii • Dissolved Oxygen Criteria

3Based on daily dissolved oxygen data provided by the lead author, Dr. David Secor, University of

Maryland Center for Environmental Science, Chesapeake Biological Laboratory, Solomons, Maryland.
Gunderson (1998), a 96-hour LC50 of 2.89 mg liter-1 3 was estimated for Atlantic

sturgeon at 26°C. This value is very similar to the “high temperature” value of 

3.1 mg liter-1 calculated for shortnose sturgeon by Campbell and Goodman (2003).

Data from Secor and Niklitschek (2001) show that shortnose sturgeon are more

tolerant of higher temperatures than Atlantic sturgeon, which could explain why

26°C is not a stressful temperature for shortnose sturgeon (Campbell and Goodman

2003), but is for Atlantic sturgeon (Secor and Gunderson 1998). Alternatively, the

temperature difference between the two species could be because the shortnose sturgeon were from Savannah River progeny and were held at higher temperatures than

the Atlantic sturgeon, which came from Hudson River progeny.

Using the above data, the EPA calculated acute criteria for the protection of sturgeon

survival in the Chesapeake Bay under both non-stressful and stressful temperatures.

The only LC50 value available for non-stressful temperatures that meets the requirements for criteria derivation based on the EPA’s 1985 guidelines (U.S. EPA 1985) is

the 24-hour 2.33 mg liter-1 calculated above from Campbell and Goodman (2003).

To be consistent with EPA guidelines, this value was used with the original Virginian

Province criteria acute data set to recalculate the Final Acute Value (FAV). The new

FAV, 2.12 mg liter-1, is more protective than the 1.64 mg liter-1 from the Virginia

Province document, but still substantially lower than the 2.33-3.5 mg liter-1 derived

directly from the empirical study on shortnose sturgeon. Therefore, the EPA

defaulted to the 2.33 mg liter-1 value, multiplying it by 1.384 to arrive at a new CMC,

3.2 mg liter-1 (rounded to two significant figures). This value is expected to be

protective of sturgeon survival at non-stressful temperatures. Campbell and

Goodman (2003) indicate that most of the mortality for shortnose sturgeon occurs

within the first 2 to 4 hours of a test. Therefore, using this value as an instantaneous

value should protect sturgeon under most conditions.

A higher dissolved oxygen criterion would be needed in areas and times of the year

where sturgeon are to be protected and temperatures are likely to be considered

stressful (e.g., 29°C and above for shortnose sturgeon). The simplest approach is to

use the LC50 value of 3.1 mg liter-1 from the fourth test of Campbell and Goodman

(2003). Multiplying this by 1.38 results in a high temperature CMC for shortnose

sturgeon of 4.3 mg liter-1.

To determine a criterion value that would also protect sturgeon from nonlethal

effects, bioenergetic and behavioral responses were considered which had been

derived from laboratory studies conducted on juvenile Atlantic and shortnose sturgeon (Niklitschek 2001; Secor and Niklitschek 2001). Growth was substantially

reduced at 40 percent oxygen saturation compared to normal oxygen saturation

▼30

chapter iii • Dissolved Oxygen Criteria

4 This value is the geometric mean of the LC5/LC50 ratios from the Virginian Province document 

(U.S. EPA 2000). The ratio for the shortnose sturgeon tests from Campbell and Goodman (2003) was

1.30 based on an analysis of raw data provided by the co-author, Larry Goodman, U.S. Environmental

Protection Agency, office of Research and Development, Gulf Ecology Division, Gulf Breeze,

Florida. To be consistent with the Virginian Province document, EPA applied the 1.38 ratio.
conditions (greater than or equal to 70 percent saturation) for both species at temperatures of 20oC and 27oC. Metabolic and feeding rates declined at oxygen levels

below 60 percent oxygen saturation at 20oC and 27oC. In behavior studies, juveniles

of both sturgeon species actively selected 70 percent or 100 percent oxygen saturation levels over 40 percent oxygen saturation levels. Based on these findings, a 

60 percent saturation level was deemed protective for sturgeon. This corresponds to

5 mg liter-1 at 25oC. Therefore, a 5 mg liter-1 Chesapeake Bay criterion protecting

against adverse growth effects would protect sturgeon growth as well.

In accordance with Section 7 of the Endangered Species Act, the EPA is continuing

consultation with the NOAA National Marine Fisheries Service to promote the

recovery and protection of the endangered shortnose sturgeon in the Chesapeake Bay

and its tidal tributaries.

Historical and Potential Sturgeon Tidal Habitats 

in Chesapeake Bay

Atlantic and shortnose sturgeon probably most recently colonized the Chesapeake

Bay 5,000–8,000 years ago after the last glaciation, when climate and the watershed’s hydraulic regime became more stable (Custer 1986; Miller 2001; also see

page 11). The Chesapeake Bay during this period already exhibited the two-layer

circulation pattern. Thus, we should expect that deep-channel habitats during periods

of strong stratification were hypoxic during the past 5,000 years, albeit not at the

same spatial extent or severity that has occurred over the past 50 years (Officer et al.

1984; Cooper and Brush 1991). Atlantic sturgeon in other estuarine and coastal

systems will use habitats greater than 15 meters in depth (see below), but these other

systems do not exhibit the same characteristics of estuarine circulation, watershed

areal extent and bathymetry that contribute to natural deep-water and deep-channel

hypoxia in the mesohaline Chesapeake Bay. 

The geochemical, paleo-ecological and instrumental record of the 20th century indicates that deep-channel regions have not served as potential habitats for sturgeon

because seasonal (summer) anoxia and hyopxia have occurred most years, reaching

levels below those required by sturgeon. Hypoxia, and probably periodic, spatiallylimited anoxia occurred in the Chesapeake Bay prior to the large-scale application of

fertilizer, but since the 1960s oxygen depletion has become much more severe (Hagy

2002), prohibiting sturgeon use of this habitat during summer months. Analysis of

recent U.S. Fish and Wildlife Service sturgeon capture location data showed absence

of sturgeon occurrences in deep-channel habitats during summer months (June 1

through September 30), but substantial numbers of occurrences in these same habitats during other seasons (U.S. EPA 2003b). In summary, based upon the recent

relevant history of the Chesapeake Bay ecosystem, deep-channel regions in summer

are not considered sturgeon habitats. 

Deeper water-column regions may continue to provide temperature refuges, migration corridors and foraging for sturgeon in the absence of strong water-column

▼31

chapter iii • Dissolved Oxygen Criteria
stratification (which results in dissolved oxygen concentrations well below

saturation levels, due to restricted mixing with the well-oxygenated surface waters.)

Recent fisheries-dependent data did not show overlap during summer months 

(June 1–September 30) between deep-water regions and sturgeon occurrences, but

most gear deployed were for shallow waters (i.e., pound nets). During other months

(October–May), deeper fishing gill nets captured sturgeon in both deep-channel and

deep-water regions (U.S. EPA 2003b). Fishery-independent gill netting in the upper

Chesapeake Bay above the Bay Bridge resulted in several Atlantic sturgeon captured

in June and July at one station in pycnocline waters. 

In other systems where strong water-column stratification does not occur to the

degree observed in the Chesapeake Bay and its tidal tributaries, both sturgeon

species are known to use deep-water habitats in summer months as thermal refuges.

During the period of 1990–1999, very little summer deep-water habitat was

predicted to support sturgeon production based on a bioenergetics model, due principally to pervasive hypoxia (Secor and Niklitschek, in press). Further, sturgeons are

able to respond behaviorally to favorable gradients in dissolved oxygen (Secor and

Niklitschek 2001). 

Based on this evidence, pycnocline deep-water habitat does not comprise ‘potential’

habitats for sturgeon during periods of strong water-column stratification limiting

exchange with overlying, more oxygenated waters. In the absence of strong watercolumn stratification, these deeper water-column habitats are considered open-water

habitat and comprise ‘potential’ habitats for sturgeon.

Atlantic sturgeon occur at depths between 1 meter to more than 25 meters; shortnose

sturgeon occur at depths between 1 and 12 meters (Kieffer and Kynard 1993; Savoy

and Shake 2000: Welsh et al. 2000). In winter, Atlantic sturgeon select deeper

habitats occurring in the Chesapeake Bay’s deep channel (Secor et al. 2000; Welsh

et al. 2000).

Distribution studies and laboratory experiments support the view that shortnose sturgeon prefer riverine and estuarine habitats over marine ones (e.g., Secor 2003).

Shortnose adults have been reported occasionally in coastal waters up to 31 ppt, but

typically occur within several kilometers of their natal estuaries (Dadswell et al. 1984;

Kynard 1997). This contrasts with the sympatric Atlantic sturgeon, which are considered true anadromous fish that must migrate into coastal waters to complete their life

cycles (Kynard 1997). In general, shortnose sturgeon do not invade salinities greater

than 15 ppt, with centers of concentrations at less than 5 ppt for all life history stages

during summer months (Dadswell et al. 1984; Brundage and Meadows 1982; Dovel

et al. 1992; Geoghegan et al. 1992; Collins and Smith 1996; Bain 1997; Haley 1999).

Atlantic sturgeon older than one year fully tolerate marine salinities and are expected

to be distributed across all salinities, depending on season, reproduction and foraging

conditions after their first year of life (Dovel and Berggen 1983; Dovel et al. 1992;

Kieffer and Kynard 1993; Colligan et al. 1998; Secor et al. 2000). 

▼32

chapter iii • Dissolved Oxygen Criteria
Thus, Atlantic sturgeon are not limited by bathymetry and salinity in the Chesapeake

Bay and would be expected to inhabit all tidal waters, including pycnocline and subpycnocline waters, if water quality conditions permitted. Shortnose sturgeon habitats

would overlap those of Atlantic sturgeon for salinities less than 15 ppt. But there is

strong evidence that both species historically have not used deep-water and deepchannel designated use habitats during the summer months (U.S. EPA 2003b) due to

naturally pervasive low dissolved oxygen conditions (see above and the prior section

titled “Low Dissolved Oxygen: Historical and Recent Past”). 

SCIENTIFIC LITERATURE FINDINGS

For each tidal-water designated use-based set of Chesapeake Bay dissolved oxygen

criteria, a review was conducted of the relevant dissolved oxygen effects literature

beyond those data contained in the Virginian Province criteria document, to include

recent published findings and Chesapeake Bay-specific data. These findings were

used to confirm the derived criteria values and support the adoption of criteria with

instantaneous minimum durations. In the case of the deep-channel designated use,

the scientific literature formed the basis for the seasonal-based Chesapeake Bay

deep-channel criterion value.

INSTANTANEOUS MINIMUM VERSUS DAILY MEAN

The scientific literature provides clear evidence that mortality occurs rapidly from

short-term exposure (less than 6 to 12 hours) to low oxygen concentrations

(Magnusson et al. 1998; Breitburg 1992; Jenkins et al. 1993; Chesney and Houde

1989; Campbell and Goodman 2003). In a recent comprehensive review of the

effects of hypoxia on coastal fishes and fisheries, Breitburg (2002) stated:

Oxygen concentrations below those that result in the standardly calculated

50% mortality in 24 to 96 h exposure test can lead to mortality in minutes

to a few hours. For example, in the case of naked gobies, exposure to

dissolved oxygen concentrations of 0.25 mg liter-1 leads to death in a

matter of a few minutes (Breitburg 1992). As exposure time increases, the

oxygen saturation that causes death approaches the saturation level that

results in reduced respiration—typically a saturation level 2 to 3 times

higher than found to be lethal in 24 h tests (Magnusson et al. 1998). 

Temperature is often an important cofactor determining when lethal

conditions are reached because it can affect both the amount of oxygen

that can dissolve in water, and the metabolic requirements of fish. Studies

to date indicate that fish require higher oxygen saturations and higher

dissolved oxygen concentrations for survival at higher temperatures....

The effects of exposure duration and temperature are thus very important

to consider in setting water quality standards for dissolved oxygen

concentration, highlighting the need to set absolute minima, instead of

time-averaged minima, and the need to consider geographic variation in

maximum water temperatures.

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chapter iii • Dissolved Oxygen Criteria
Data on laboratory tests of asphyxia and field data on fish kills associated with intrusions of hypoxic bottom water indicate that mortality rapidly occurs from short-term

exposure to very low dissolved oxygen concentrations. Asphyxia occurs at about half

the dissolved oxygen concentration resulting in reductions in respiration

(Magnusson et al. 1998). For the species illustrated, respiration declines at dissolved

oxygen concentrations of about 85 percent of the LC50 concentration (see Figure 2

in Magnusson et al. 1998). 

Asphyxia, as stated above, has been reported at dissolved oxygen concentrations

well below the reported LC50 concentrations. To ensure full protection of each of the

five designated uses, an instantaneous minimum criterion has been recommended. In

addition, a daily mean criterion value has been recommended for the deep-water use

to ensure full protection of the open-water juvenile and adult fish that use deep-water

habitats for short periods in summer to forage for food.

STRENGTHS AND LIMITATIONS OF 

THE CRITERIA DERIVATION PROCEDURES

As with any science-based set of criteria, the approach used in deriving these criteria

has its strengths and limitations. The dissolved oxygen criteria are designed to

protect the five proposed designated uses under the conditions in which the underlying effects data were generated. Elevated temperatures, for example, will stress

organisms regardless of the dissolved oxygen concentrations. The proposed conditions will protect the designated uses along with the application of other appropriate

water quality criteria that protect against temperature, chemical contaminant and

other related stresses. 

The EPA recognizes that interactions among other stressors and dissolved oxygen

exist. Conservative assumptions, documented in this chapter and associated appendices, were made to reflect these remaining uncertainties with regard to interactions

with other stressors. Incorporation of arbitrary ‘margins of safety’ were not part of

the Chesapeake Bay criteria derivation process, consistent with national EPA guidelines (U.S. EPA 1985). The EPA believes that the criteria provided in this document

are protective under water quality conditions in which aquatic organism are not

otherwise unduly stressed by other factors.

Salinity Effects

The Virginian Province criteria document is geared toward >15 ppt salinities, with a

subset of tests run at much lower salinities (e.g., striped bass larvae). However, low

dissolved oxygen effects synthesized from the science literature used in deriving the

EPA criteria included tests run at salinities lower than 15 ppt salinity (e.g., Burton et

al. 1980, research on menhaden and spot). All tests were run at salinities found to be

nonstressful to the respective organisms. These results and a review of the literature

indicated that nonstressful salinity levels do not influence an organism’s sensitivity

to low dissolved oxygen.

▼34

chapter iii • Dissolved Oxygen Criteria
Temperature Effects

With the exception of the criterion derived to protect shortnose sturgeon, Chesapeake

Bay criteria do not explicitly address potential interactions between varying stressful

temperature levels and the effects of low dissolved oxygen. The amount of available

dissolved oxygen changes as temperature changes, and the metabolic rates of organisms increase as temperature increases. In both cases, temperature directly affects

organisms and their responses to dissolved oxygen conditions.

High temperatures and low dissolved oxygen concentrations often appear together.

Generally, low dissolved oxygen concentrations would be more lethal at water

temperatures approaching the upper thermal limit for a species. Surface or shoal

regions of high temperature will cause fish to seek cooler habitats, yet these deeper

habitats are more likely to contain hypoxic waters. The resulting ‘habitat squeeze’

(Coutant 1985) curtails summertime habitats and production (Brandt and Kirsch

1993; Secor and Niklitschek 2001). A number of species have shown heightened

sensitivity to low dissolved oxygen concentrations at higher, yet nonlethal, temperatures (Breitburg et al. 2001). At this time sufficient data exist only for specific life

history stages of some species (i.e., juvenile shortnose and Atlantic sturgeon) to fully

quantify and build temperature and dissolved oxygen interactions into a set of

Chesapeake Bay-specific dissolved oxygen criteria. Clearly, given the welldocumented role of temperature and dissolved oxygen interactions in constraining

the potential habitats of striped bass, sturgeon and other Chesapeake Bay fishes,

more research and model development are needed.

The EPA does not think that a margin of safety for temperature effects is needed.

Although having more data specific to an issue is always desirable, the available data

are sufficient to derive dissolved oxygen criteria for the Chesapeake Bay that are

protective of most species most of the time (which was the original intent of the

EPA’s 1985 national aquatic life criteria derivation guidelines). The data in Appendix

B show that high, but nonstressful temperatures will not alter the dissolved oxygen

criteria (some of these temperatures were as high as 30°C). The only rigorous data

that are available for a single Chesapeake Bay species using nonstressful and

stressful temperatures are for shortnose sturgeon (Campbell and Goodman 2003).

These data have been used in the revised the Chesapeake Bay open-water dissolved

oxygen criteria to derive protection limits specifically aimed at shortnose sturgeon in

higher, stressful temperature waters.

pH Effects

The interaction between pH levels and dissolved oxygen concentrations is more of an

issue in laboratory experimentation and the analysis of laboratory-based effects data

than in deriving and applying the dissolved oxygen criteria themselves. Given the great

buffering capacity of seawater, pH, although a potentially important factor, is unlikely

to change much in seawater. Existing pH water quality criteria, along with the application of the appropriate dissolved oxygen criteria, should be protective of the use.

▼35

chapter iii • Dissolved Oxygen Criteria
Behavioral Effects

As Breitburg (2002) concluded from a recent extensive review of the scientific literature, clear evidence exists of behavioral responses to low dissolved oxygen

conditions.

Field studies have repeatedly shown that as oxygen concentrations

decline, the abundance and diversity of demersal fishes decrease (e.g.,

Howell and Simpson 1994; Baden and Pihl 1996; Eby 2001; Breitburg et

al. 2001). Bottom waters below approximately 2 mg liter-1 have extremely depauperate fish populations. Some individual species appear to

have threshold concentrations below which their densities decline precipitously (Howell and Simpson 1994; Baden and Pihl 1996; Eby 2001).

However, because fish species vary in both physiological tolerance and

behavior, total fish abundance and fish species richness tend to decline

gradually with declining oxygen concentrations.

Longer duration exposures to low oxygen and more severe hypoxia lead

to avoidance of and emigration from affected habitat. All larval, juvenile

and adult fishes that have been tested to date respond to oxygen gradients

by moving upwards or laterally away from waters with physiologically

stressful or potentially lethal dissolved oxygen towards higher oxygen

concentrations (e.g., Deubler and Posner 1963; Stott and Buckley 1979;

Breitburg 1994; Wannamaker and Rice 2000). Mortality from direct

exposure to hypoxic and anoxic conditions is less than might otherwise

occur because of this potential capacity for behavioral avoidance.

Habitat loss due to hypoxia in coastal waters is, however, far greater than

would be calculated based on the spatial extent of lethal conditions,

because most fish avoid not only lethal oxygen concentrations but also

those that would reduce growth and require greatly increased energy

expenditures for ventilation. Field and sampling and laboratory experiments indicate that oxygen concentrations that are avoided tend to be 2 to

3 times higher than those that lead to 50 percent mortality in 24-to 96-

hour exposures, and approximately equal to concentrations that have been

shown to reduce growth rates in laboratory experiments.

The net result of emigration and mortality is reduced diversity, abundance

and production of fishes within the portion of the water column affected

by low dissolved oxygen. Emigration leading to reduced densities of

fishes even at oxygen concentrations approaching 40 to 50 percent saturation (3 to 4 mg liter-1) is supported by the pattern of increasing number

of species in trawl samples with increasing dissolved oxygen concentration in Long Island Sound (Howell and Simpson 1994) and Chesapeake

Bay (Breitburg et al. 2001) and the increasing number of finfish individuals caught per trawl hour within increasing bottom dissolved oxygen off

the Louisiana Coast in the Gulf of Mexico (Diaz and Solow 1999).

Concentrations associated with avoidance are very similar to those observed to result

in adverse effects on growth (Breitburg 2002). Figure III-1 illustrates the relationship

▼36

chapter iii • Dissolved Oxygen Criteria
▼37

chapter iii • Dissolved Oxygen Criteria

Figure III-1. Relationship between lethal dissolved oxygen concentrations and those resulting in reduced

growth and behavioral avoidance of affected habitat. (a) LC50 vs. avoidance behavior, (b) LC50 vs. growth

reduction, and (c) growth vs. avoidance behavior. Two identical points in (c) are indicated by the number 2

next to the data point. Data sources are as follows. Avoidance vs. mortality: Burton et al. 1980; Coutant

1985; Petersen and Petersen 1990; Pihl et al. 1991; Scholz and Waller 1992; Schurmann and Steffensen

1992; Howell and Simpson 1994; Petersen and Pihl 1995; Poucher and Coiro 1997; Wannamaker and Rice

2000; U.S. EPA 2000; Eby 2001. Growth vs. mortality: Burton et al. 1980; Petersen and Petersen 1990; Pihl et

al. 1991; Scholz and Waller 1992; Schurmann and Steffensen 1992; Petersen and Pihl 1995; Chabot and Dutil

1999; U.S. EPA 2000; McNatt 2002. Avoidance vs. growth: Couton 1985; Pihl et al. 1991; Scholz and Waller

1992; Howell and Simpson 1994; Petersen and Pihl 1995; Poucher and Coiro 1997; U.S. EPA 2000; Eby 2001;

and McNatt 2002. Only studies utilizing a range of dissolved oxygen concentrations are included in figures.

Data from multiple studies on the same species were averaged. If responses were tested at several

temperatures, the temperature with the most dissolved oxygen effects tested was selected. 

Source: Breitburg 2002.

LC50 (mg liter-1)

0.0 0.5 1.0 1.5 2.0 2.5

Significant avoidance (mg liter-1)

1

2

3

4

5

LC50 (mg liter-1)

0.0 0.5 1.0 1.5 2.0 2.5

Growth reduction (mg liter-1)

1

2

3

4

5

Growth reduction (mg liter-1)

012345

Significant avoidance (mg liter-1)

1

2

3

4

5

2

r

2

=0.65, p=0.052

y=-1.65 + 3.29X

r

2

=0.82, p=0.035

y=0.31 + 0.89X

r

2

=0.71, p=0.002

y=0.36 + 1.98X

Dissolved oxygen concentrations 

causing avoidance = 2.25*LC50

Dissolved oxygen concentrations 

causing growth reduction = 2.28*LC50

Dissolved oxygen concentrations 

causing avoidance = 0.98 * dissolved oxygen

causing growth reduction

(A)

(B)

(C)
between dissolved oxygen concentrations that are lethal and those resulting in

reduced growth and behavioral avoidance of the affected habitat. Regressions, calculated from data from a variety of sources, included LC50 versus avoidance behavior,

LC50 versus growth reduction and growth versus avoidance behavior. Dissolved

oxygen concentrations associated with avoidance were found to be 2.25 times the

LC50 concentration (Figure III-1a). Dissolved oxygen concentrations causing growth

reduction were 2.28 times the LC50 concentration (Figure III-1b). Dissolved oxygen

concentrations causing avoidance were essentially the same as those concentrations

causing growth reduction (Figure III-1c). Reduced growth and avoidance by fish

occur at similar oxygen concentrations relative to lethal levels. Thus, protecting for

one factor should protect for the other, if appropriate time durations are used.

The relationship between the average number of species per trawl across a range of

dissolved oxygen concentrations provides additional evidence for a strong dissolved

oxygen/behavioral connection that transcends individual estuarine and coastal

systems (Figure III-2; Breitburg 2002). Using data from the Chesapeake Bay, Long

Island Sound and Kattegat Sea, the number of species collected per trawl was shown

to increase with increasing dissolved oxygen concentration in all three estuarine and

coastal systems.

Individual species habitat requirements and the characteristics of habitats both determine the extent to which an ecosystem’s habitats are used and contribute to the

health and production of Chesapeake Bay living resources. Each species’ behavioral

responses, their predators and their prey can also be considered in deriving dissolved

oxygen criteria. Based on the limited data on behavioral responses, we are not sure

of the actual adverse effects that behavioral responses such as avoidance have on

individuals, much less on whole populations. Although considerable data on behavioral avoidance of low oxygen habitats exist, we are unable to predict individual or

population-level consequences of such avoidance. 

Although it is true that we cannot directly evaluate the effects of avoidance in the

same way that we can with effects on growth and survival, the EPA does not believe

that a margin of safety for avoidance behavior is needed. The data reviewed by Breitburg (2002) clearly show that concentrations that have an effect on avoidance are

nearly identical to those that affect growth. Therefore, criteria that protect growth

should also be protective of habitat squeeze due to avoidance.

Larval Recruitment Model

The larval recruitment model was used only in the actual derivation of the deepwater criteria when it was applied specifically to bay anchovy egg and larval life

stages. In deriving the migratory spawning and nursery and open-water criteria, the

larval recruitment model results for nine different species were used to ensure that

the criteria based on other effects data would be fully protective of larval life stages.

▼38

chapter iii • Dissolved Oxygen Criteria
▼39

chapter iii • Dissolved Oxygen Criteria

Figure III-2. Average number of species per trawl at a range of dissolved oxygen concentrations along the

a) western shore of the Chesapeake Bay near the Calvert Cliffs Nuclear Power Plant (Breitberg and Kolesar,

unpublished data), b) Long Island Sound (redrawn from Howell and Simpson 1994, figures 3 and 4), and 

c) Kattegat (Baden et al. 1990; Baden and Phil 1996). Data are averaged in approximately 0.5 mg liter-1

intervals and for all data >0.5 mg liter-1. Note variation in scale of vertical axes.

Source: Breitburg 2002.

A. Chesapeake Bay

B. Long Island Sound

Mean number of species per trawl

C. Kattegat

4

3

2

1

0

12

10

8

6

4

2

0

14

12

10

8

6

4

2

0

Bottom layer dissolved oxygen (mg liter-1)

0 1 2 3 4 ≥5
Uncertainties remain with respect to the percent of the population exposed to low

dissolved oxygen, the length of the actual spawning period and the protection of

spawning events concentrated over short periods of time. In addition, the assumption

implicit in the larval recruitment model is that all spawning days are equal. 

Due to meteorological, food web and other influences, eggs hatched at different

times during the spawning season are not expected to contribute equally to

successful survival to juvenile and adult stages, nor are eggs produced continuously

throughout the spawning season. In particular, species show spawning behaviors

and early survival rates that depend on lunar tidal patterns, weather-driven changes

to water quality (e.g., winds and temperature changes) and available forage for

young. For example, it is well-documented that most striped bass survival can come

from a relatively narrow period of time during the entire spawning period

(Ulanowicz and Polgar 1980; Secor and Houde 1995; Secor 2000). Since we cannot

predict when this smaller window may occur relative to specific hypoxic events,

conservative assumptions must be made. These include always assuming in the

recruitment model that hypoxia will occur during times of maximum offspring

production. 

A number of reports exist on the consequences of slow growth in terms of increased

predation mortality. The model does not contain a variable for growth (it only deals

with larval survival), however, it does increase the mortality (i.e., changes the sensitivity to hypoxia) with increasing exposure duration.

The EPA acknowledges uncertainties with the parameters in the larval recruitment

model. This is why specific parameters within the model were chosen to be conservative. Specifically, spawning periods reflect when the bulk of spawning occurs, not

just the first and last possible occurrence of a given species larvae in the water

column. In addition, the model always assumes that a hypoxic event occurs during

the spawning season of each species modeled. The percentages of each cohort that

is exposed during a hypoxic event were also intended to be conservative.

CHESAPEAKE BAY DISSOLVED OXYGEN 

CRITERIA DERIVATION

Chesapeake Bay dissolved oxygen criteria were established to protect estuarine

living resources inhabiting five principal habitats: migratory spawning and nursery,

shallow-water, open-water, deep-water and deep-channel. These five categories are

drawn from the refined designated uses for the Chesapeake Bay and its tidal tributary waters (Figure III-3). See Appendix A and U.S. EPA 2003a for more detailed

descriptions of the refined designated uses.

The EPA’s Guidelines for Deriving Numerical National Water Quality Criteria for

the Protection of Aquatic Organisms and their Uses (U.S. EPA 1985) is the primary

source on how to establish numerical criteria. Consistent with the national guidelines

provided, scientific judgment took precedence over the specifics of the guidelines,

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chapter iii • Dissolved Oxygen Criteria
when warranted. A similar judgment was applied in the development of the 2000

EPA Virginian Province saltwater and the 1986 EPA freshwater dissolved oxygen

criteria documents (U.S. EPA 1986, 2000).

The Chesapeake Bay dissolved oxygen criteria were derived using methodologies

documented in the EPA Virginian Province saltwater criteria document and using

criteria originally published in the EPA freshwater criteria document. The scientific

rationale for modifications to the 1985 EPA guidelines for deriving the saltwater

dissolved oxygen criteria for the Virginian Province and the national freshwater

dissolved oxygen criteria are detailed in those peer-reviewed, EPA documents. 

▼41

chapter iii • Dissolved Oxygen Criteria

Figure III-3. Conceptual illustration of the five Chesapeake Bay designated use zones.

Deep-Channel

Seasonal Refuge Use

Deep-Channel

Seasonal Refuge Use

A. Cross-Section of Chesapeake Bay or Tidal Tributary

B. Oblique View of the Chesapeake Bay and its Tidal Tributaries

Shallow-Water

Bay Grass Use Open-Water

Deep-Water Fish and Shellfish Use

Seasonal Fish and

Shellfish Use

Deep-Water

Seasonal Fish and

Shellfish Use

Migratory Fish

Spawning and

Nursery Use

Shallow-Water

Bay Grass Use Open-Water

Fish and Shellfish Use
Criteria for migratory fish spawning and nursery, shallow-water bay grass and openwater fish and shellfish designated use habitats were set at levels to protect the

survival, growth and reproduction of all species. Criteria that apply to deep-water

seasonal fish and shellfish habitats in summer were set at levels to protect shellfish,

the survival of juvenile and adult fish, and the recruitment success of the bay

anchovy. Criteria for deep-channel seasonal refuge designated use habitats in

summer were set to protect the survival of sediment-dwelling worms and clams. 

MIGRATORY FISH SPAWNING AND 

NURSERY DESIGNATED USE CRITERIA

Criteria that support the migratory fish spawning and nursery designated use must fully

protect the “survival, growth and propagation of balanced indigenous populations of

ecologically, recreationally and commercially important anadromous, semi-anadromous and tidal-fresh resident fish species inhabiting spawning and nursery grounds

from February 1 through May 31” (Appendix A; U.S. EPA 2003a). This covers the

survival and growth of all life stages—eggs, larvae, juveniles and adults—for a given

number of species and their underlying food sources. As described below, the criteria

are based on establishing dissolved oxygen concentrations to protect against losses in

larval recruitment, growth effects on larvae and juveniles and the survival and growth

effects on the early life stages of resident tidal-fresh species.

Criteria Components

Protection against Larval Recruitment Effects. Applying the Virginian

Province criteria larval recruitment effects model generates a relationship illustrated

as a curve, projecting the cumulative loss of recruitment caused by exposure to low

dissolved oxygen (Figure III-4). The number of acceptable days of exposure to low

dissolved oxygen decreases as the severity of the low oxygen conditions increases.

The migratory fish spawning and nursery designated use criteria must ensure protection of larvae as they are recruited into the juvenile/adult population. The Virginian

Province criteria larval recruitment curve levels out at approximately 4.6 mg liter-1

beyond 30 days of exposure (Figure III-4). By dropping non-Chesapeake Bay

species and applying Chesapeake Bay-specific modifications to the larval recruitment model parameters, as described previously, a curve is generated that closely

follows the original Virginian Province criteria curve but levels off just above 4.6 mg

liter-1. Dissolved oxygen concentrations and exposure durations falling above the

Chesapeake Bay-specific curve, e.g., above 4.6 mg liter-1 for 30 days, 3.4-3.5 mg

liter-1 for up to seven days and 2.7-2.8 liter-1 at all times, would protect against larval

recruitment effects.

Protection for Early Life Stages for Resident Tidal-Fresh Species. The

EPA freshwater dissolved oxygen criteria set a 7-day mean of 6 mg liter-1 and an

instantaneous minimum of 5 mg liter-1 to protect early life-stage, warm-water, freshwater species (Table III-4) (U.S. EPA 1986).

▼42

chapter iii • Dissolved Oxygen Criteria
Protection against Growth Effects. To ensure recruitment to the adult population, the Chesapeake Bay criteria must ensure protection against growth effects on

rapidly developing larvae and juveniles. The Virginian Province criteria document

recommends 4.8 mg liter-1 as the threshold above which long-term, continuous

exposures should not cause unacceptable growth effects (U.S. EPA 2000). As

described previously, if the non-Chesapeake Bay species were removed from the

Virginian Province criteria dissolved oxygen growth effects data base, a recalculated

Chesapeake Bay-specific criterion would be 5 mg liter-1.

These values were derived by observing the effects of low dissolved oxygen on

larval and early juvenile life stages. Growth effects on these stages served as the

basis for the chronic criterion because: 1) growth is generally the more sensitive

endpoint measure upon exposure to low dissolved oxygen compared with survival;

2) results for other sublethal endpoints such as reproduction were limited; 3) the

limited data available indicated that thresholds protecting against growth effects are

likely to protect against reproductive effects; and 4) larval and juvenile life stages

were more sensitive to effects from low dissolved oxygen than were adults (U.S.

EPA 2000). In addition to higher dissolved oxygen requirements, fish eggs and

larvae also are more vulnerable to low dissolved oxygen because of limitations in

behavioral avoidance (Breitburg 2002).

Protection against Effects on Threatened/Endangered Listed Species.

As documented previously, short-term exposures to dissolved oxygen concentrations

▼43

chapter iii • Dissolved Oxygen Criteria

Figure III-4. Comparison of the Virginian Province-wide (—) and Chesapeake Bay-wide 

(---) larval recruitment effects.

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 10 20 30 40 50 60 70

Time (days)

Chesapeake Bay curve

Virginia Province curve

 2.7-2.8 mg liter-1 at 0 days

3.4-3.5 mg liter-1 at 7 days

>4.6 mg liter-1 past 30 days

-1) Dissolved Oxygen Concentration (mg liter
of > 3.2 mg liter-1 on the order of several hours at nonstressful temperatures and longerterm exposures of 30 days or longer at > 5 mg liter-1 will not impair the survival or

growth of Atlantic and shortnose sturgeon (Secor and Niklitschek 2001, 2003;

Niklitschek 2001; Secor and Gunderson 1998; Campbell and Goodman 2003). At

stressful temperatures above 29°C, short-term exposures to dissolved oxygen concentrations >4.3 mg liter-1 will not impair the survival of shortnose sturgeon.

Additional Scientific Literature Findings. Results from Brandt et al. (1998)

indicate that striped bass food consumption and growth decline as oxygen levels

decline. Continuous exposure to dissolved oxygen concentrations of 4 mg liter-1 or

less caused striped bass to lose weight, even though food was always unlimited.

Previous experiments on the effects of oxygen levels on striped bass also have shown

that dissolved oxygen concentrations of less than 3 to 4 mg liter-1 adversely affect

feeding (Chittenden 1971).

Jordan et al. (1992) summarized the literature supporting the adoption of the Chesapeake Bay restoration goal target concentration protecting anadromous spawning

and nursery areas as follows.

This target DO concentration (>5 mg liter-1 at all times) was selected to

protect the early life stages of striped bass, white perch, alewife, blueback

herring, American shad, hickory shad and yellow perch. This concentration of DO will allow eggs to hatch normally (Bradford et al. 1968;

O’Malley and Boone 1972; Marcy and Jacobson 1976; Harrell and

Bayless 1981; Jones et al. 1988), as well as allow survival and growth of

larval and juvenile stages of all anadromous target species (Tagatz 1961;

Bogdanov et al. 1967; Krouse 1968; Bowker et al. 1969; Chittenden 1969,

1972, 1973; Meldrim et al. 1974; Rogers et al. 1980; Miller et al. 1982;

Coutant 1985; ASMFC 1987; Jones et al. 1988). For example, concentrations of DO below 5 mg liter-1 for any duration will not support normal

hatching of striped bass eggs (O’Malley and Boone 1972). Although one

hatchery operation was able to maintain striped bass fingerlings at DO

concentrations of 3 to 4 mg liter-1 (Churchill 1985). Bowker et al. (1969)

found DO > 3.6 mg liter-1 was required for survival of juveniles.

Across an array of temperatures (13–25oC) and salinities (5–25 ppt), Krouse (1968)

observed complete mortality of striped bass at 1 mg liter-1, ‘minimal mortality’ at 

5 mg liter-1 and ‘intermediate survival’ at 3 mg liter-1 upon exposure of 72 hours.

Some field observations have indicated that juveniles and adults of anadromous

species prefer dissolved oxygen concentrations > 6 mg liter-1 (Hawkins 1979;

Christie et al. 1981; Rothschild 1990). However, no lethal or sublethal effects other

than possible avoidance have been documented for dissolved oxygen concentrations

between 5 and 6 mg liter-1.

Rationale

The migratory spawning and nursery designated use criteria must ensure full protection for warm-water freshwater species’ egg, larval and juvenile life stages, which

▼44

chapter iii • Dissolved Oxygen Criteria
co-occur with the tidal-fresh and low-salinity migratory spawning and nursery habitats (Table III-5). To ensure full protection for resident tidal-fresh warm-water

species’ early life stages, a 7-day mean criterion of 6 mg liter-1 and an instantaneous

minimum criterion of 5 mg liter-1 were selected, consistent with the EPA freshwater

criteria (U.S. EPA 1986).

To ensure protection not only of survival and recruitment of larvae into the juvenile

population but also to eliminate any potential for adverse effects on growth during

the critical larvae and early juvenile life stages, an instantaneous minimum criterion

of 5 mg liter-1 was selected. The Virginian Province saltwater criteria document

states that exposures to dissolved oxygen concentrations above this concentration

should not result in any adverse effects on growth (U.S. EPA 2000). Given the lack

of information on the population level consequences of short- versus long-term

reductions in growth on the survival of larvae and juveniles, a specific averaging

▼45

chapter iii • Dissolved Oxygen Criteria

Table III-5. Migratory fish spawning and nursery designated use dissolved oxygen criteria components.

Criteria Components Concentration Duration Source

Protection against growth effects > 4.8 mg liter-1 - U.S. EPA 2000

Protection against larval

recruitment effects

> 4.6 mg liter-1

> 3.4-3.5 mg liter-1

> 2.7-2.8 mg liter-1

30 to 40 days

7 days

instantaneous

minimum

U.S. EPA 2000

Protection of early life stages for

resident tidal freshwater species

> 6 mg liter-1

> 5 mg liter-1

U.S. EPA 1986

Protection against effects on

threatened/endangered species

(shortnose sturgeon)

> 5 mg liter-1

> 3.5 mg liter-1

> 3.2 mg liter-1 1

> 4.3 mg liter-1 2

30 days

6 hours

2 hours

2 hours

Secor and Niklitschek

2003; Niklitschek 2001;

Secor and Gunderson

1998; Jenkins et al. 1993;

Campbell and Goodman

2003

Additional published findings

- Growth effects on striped bass

- Protect early life stages

- Intermediate striped bass

survival

- Full survival

- Preferred concentrations

< 3 to 4 mg liter-1

> 5 mg liter-1

> 3mg liter-1

>5 mg liter-1

 6 mg liter-1

-

-

72 hours

72 hours

-

Brandt et al. 1998;

references in text

Krouse 1968

Krouse 1968

Hawkins 1979; Christie

et al. 1981; Rothschild

1990

1

 Protective of survival at nonstressful temperatures.

2

 Protective of shortnose sturgeon at stressful temperatures (>29C).

7-day mean

instantaneous

minimum
period was not recommended in the Virginian Province saltwater criteria. In the case

of anadromous species, a narrow set of natural conditions (e.g., salinity, temperature)

is required and a narrow time window exists for a successful spawn. Natural mortalities for larvae already are extremely high. As even short-term reductions in growth

could influence advancement to the next stage through the impairment of survival

and the ability to avoid predators, the criterion value that protects against growth

effects is applied as an instantaneous minimum.

Setting the criterion duration of exposure as an instantaneous minimum is consistent

with the instantaneous minimum duration for the 5 mg liter-1 concentration criterion

value from the EPA freshwater dissolved oxygen criteria for ensuring full protection

of warm-water freshwater species’ early life stages against short-term exposures

(Table III-4; U.S. EPA 1986). The instantaneous minimum of the 5 mg liter-1 criterion value also protects the survival and growth of shortnose sturgeon (Table III-5).

Migratory Spawning and Nursery Criteria

The following dissolved oxygen criteria fully support the Chesapeake Bay migratory

fish spawning and nursery designated use when applied from February 1 through

May 31: a 7-day mean > 6 mg liter-1 applied to tidal-fresh waters with long-term

averaged salinities up to 0.5 ppt; and an instantaneous minimum > 5 mg liter-1

applied across all the migratory fish spawning and nursery designated use habitats,

regardless of salinity. See U.S. EPA 2003a for details on the selection of February 1

through May 31 as the time period for applying the migratory spawning and nursery

designated use.

OPEN-WATER FISH AND SHELLFISH DESIGNATED USE CRITERIA

Criteria that support the open-water designated use must fully protect the “survival,

growth and propagation and growth of balanced, indigenous populations of ecologically, recreationally and commercially important fish and shellfish inhabiting

open-water habitats” (Appendix A; U.S. EPA 2003a). The dissolved oxygen requirements for the species and communities inhabiting open- and shallow-water habitats

are similar enough to ensure protection of both the open-water and shallow-water

designated uses with a single set of criteria. The open-water criteria were based on

establishing dissolved oxygen concentrations to protect against losses in larval

recruitment, growth effects on larvae and juveniles and the survival of juveniles and

adults in tidal-fresh to high-salinity habitats.

Criteria Components

Protection against Larval Recruitment Effects. Applying the Virginian

Province criteria model generates a relationship illustrated as a curve that projects

the cumulative loss of recruitment caused by exposure to low dissolved oxygen. The

number of acceptable days of exposure to low dissolved oxygen decreases as the

▼46

chapter iii • Dissolved Oxygen Criteria
severity of the low oxygen conditions increases. The open-water designated use

criteria must ensure protection of larvae as they recruit into the juvenile/adult

population.

The Virginian Province larval recruitment effects curve levels out at approximately

4.6 mg liter-1 beyond 30 days’ exposure (Figure III-5). Dropping the non-Chesapeake Bay resident species and then applying a series of Chesapeake Bay-specific

modifications to the larval recruitment model parameters, as described previously,

yields a curve that follows the original Virginian Province criteria curve and also

levels out around 4.6 mg liter-1 beyond 30 days exposure (Figure III-5). Setting the

larval exposure level to 100 percent5 results in a curve that levels out at 4.8 mg liter1. The effects curves illustrated in Figure III-5 reflect the combined dissolved oxygen

concentration and duration of exposure protective against a five percent or greater

impact, thereby protecting 95 percent or greater of the seasonally produced

offspring. Dissolved oxygen concentrations/exposure durations falling above the

curve, e.g., above 2.7-2.9 mg liter-1 at all times, above 3.4-3.6 mg liter-1 for up to

seven days, and above 4.6-4.8 mg liter-1 for 30 days, would protect against larval

recruitment effects greater than five percent in open-water designated use habitats.

▼47

chapter iii • Dissolved Oxygen Criteria

Figure III-5. Comparison of the Virginian Province-wide (—) and Chesapeake Bay specific

larval recruitment effects at variable (---) and 100 percent ( . . . ) exposures.

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 10 20 30 40 50 60 70

Time (days)

Dissolved Oxygen Concentration (mg liter-1)

Chesapeake Bay curve

at 100% Exposure

Chesapeake Bay curve

Virginia Province curve

 >4.6-4.8 mg liter-1 past 30 days

 3.4-3.6 mg liter-1 at 7 days

2.7-2.9 mg liter-1 at 0 days

5 The larval recruitment model has a parameter for what percentage of a given cohort is exposed to low

dissolved oxygen conditions.
Protection of Juvenile/Adult Survival. The Virginian Province criteria document recommends 2.3 mg liter-1 as the threshold above which long-term, continuous

exposures should not cause unacceptable lethal conditions for juvenile and adult fish

and shellfish. As described below, this value does not protect the survival of shortnose sturgeon.

Protection against Growth Effects. To ensure recruitment to the adult population, the open-water designated use dissolved oxygen criteria must protect against

growth effects on rapidly developing larvae and juveniles. The Virginian Province

document recommends 4.8 mg liter-1 as the threshold above which long-term,

continuous exposures should not cause unacceptable growth effects. If the nonChesapeake Bay species were to be removed from the Virginian Province growth

effects data base, the recalculated Bay-specific criterion protective against growth

effects would be 5 mg liter-1.

This chronic criterion value was derived from laboratory evaluations of the effects of

low dissolved oxygen on growth, principally with larval and early juvenile life

stages. Growth effects on these early life stages were used as the basis of the chronic

criterion because: 1) growth is generally the more sensitive endpoint measure upon

exposure to low dissolved oxygen compared with survival; 2) results for other nonmortality related endpoints such as reproduction were very limited; 3) the limited

data indicated that thresholds protecting against growth effects are likely to protect

against negative reproductive effects; and 4) larval life stages were more sensitive to

effects from low dissolved oxygen than were juveniles/adults (U.S. EPA 2000). The

derivation of a dissolved oxygen criterion value of 5 mg liter-1 to protect against

growth effects is consistent with findings reported by Breitburg (2002) that dissolved

oxygen concentrations causing growth reductions were 2.28 times the LC50 concentration (2.28 x 1.64 = 3.7 mg liter-1, where 1.64 is the Final Acute Value from the

EPA Virginian Province document).

Protection of Resident Tidal-Fresh Species. The open-water fish and shellfish

designated use criteria must also fully protect warm-water freshwater species that

co-occur in tidal-fresh and low-salinity open- and shallow-water habitats. The EPA

freshwater dissolved oxygen criteria set a 30-day mean of 5.5 mg liter-1; a 7-day

mean minimum of 4.0 mg liter-1, and an instantaneous minimum of 3.0 mg liter-1 to

protect life stages for warm-water species beyond early life stages (Table III-4) (U.S.

EPA 1986).

Protection against Effects on Threatened/Endangered Listed Species.

As documented previously, short-term exposures of several hours to dissolved

oxygen concentrations of > 3.2 mg liter-1 at nonstressful temperatures and longerterm exposures of 30 days or more at > 5 mg liter-1 would protect the survival and

growth of Atlantic and shortnose sturgeon (Secor and Niklitschek 2001, 2003;

Niklitschek 2001; Secor and Gunderson 1998; Campbell and Goodman 2003). At

▼48

chapter iii • Dissolved Oxygen Criteria
stressful temperatures above 29°C, short-term exposures to dissolved oxygen

concentration > 4.3 mg liter-1 will not impair the survival of shortnose sturgeon.

Additional Scientific Literature Findings. As striped bass larvae begin their

metamorphoses to the juvenile stage, they move into shallow-water habitats near

shore and in shoal areas less than 2 meters deep (Boreman and Klauda 1988;

Boynton et al. 1981; Setzler-Hamilton et al. 1981). Nursery areas for juvenile striped

bass with dissolved oxygen concentrations greater than 5 mg liter-1 are preferable,

given findings that concentrations below 4 mg liter-1 can adversely affect juvenile

growth rates, feeding rates, habitat use and susceptibility to predation (e.g., Kramer

1987; Breitburg et al. 1994). Mortality of juvenile striped bass has been observed at

dissolved oxygen concentrations < 3 mg liter-1 (Chittenden 1972; Coutant 1985;

Krouse 1968).

Results from trawls in Long Island Sound showed significant reductions in both

species diversity and abundance at sites with dissolved oxygen < 2 mg liter-1 (Howell

and Simpson 1994). At open water-column sites with dissolved oxygen concentrations > 3 mg liter-1, 15 of the 18 target species caught occurred with greater frequency

compared with sites with concentrations < 2 mg liter-1. Further research indicated that

the total abundance of fish was relatively insensitive to low dissolved oxygen conditions, reaching normal levels at 1.5 mg liter-1. However, total fish biomass and species

richness were particularly sensitive, declining at dissolved oxygen concentrations of

3.7 mg liter-1 and 3.5 mg liter-1, respectively (Simpson 1995).

Rationale

To ensure the full protection of survival and recruitment of larvae into the juvenile

population, reduce the potential for adverse effects on growth and protect threatened

or endangered species across tidal-fresh to high-salinity habitats, dissolved oxygen

criteria values of a 30-day mean of 5.5 mg liter-1 applied to tidal-fresh habitats with

long- term averaged salinities up to 0.5 ppt; a 30-day mean of 5 mg liter-1 applied to

all other open-water habitats (> 0.5 ppt salinity); a 7-day mean of 4 mg liter-1; and

an instantaneous minimum of 3.2 mg liter-1 were selected (Table III-6). At temperatures stressful to shortnose sturgeon (>29°C), a 4.3 mg liter-1 instantaneous

minimum criteria should apply.

The 5 mg liter-1 value is based on the Virginian Province criterion protecting against

growth effects (U.S. EPA 2000). The Virginian Province criteria document states

that exposures to dissolved oxygen concentrations above this concentration will not

result in any adverse effects on growth. However, the document recommended no

specific duration. The extensive open-water habitats provide better opportunities for

avoiding predators and seeking food than the more confined, geographically limited

migratory spawning and nursery habitats. The 30-day mean averaging period for the

5 mg liter-1 criterion value was selected to reflect current uncertainties over how

much impact growth reduction has on juvenile and adult survival and reproduction

in the shallow- and open-water Chesapeake Bay habitats. The 30-day mean averaging

▼49

chapter iii • Dissolved Oxygen Criteria
▼50

chapter iii • Dissolved Oxygen Criteria

Table III-6. Open-water fish and shellfish designated use dissolved oxygen criteria components.

Criteria Components Concentration Duration Source

Protection against larval

recruitment effects

> 4.6-4.8 mg liter-1

> 3.4-3.6 mg liter-1

> 2.7-2.9 mg liter-1

30 to 40 days

7 days

< 24 hours

U.S. EPA 2000

Protection against growth

effects

> 4.8 mg liter-1 - U.S. EPA 2000

Protection of juvenile/adult

survival

> 2.3 mg liter-1 24 hours U.S. EPA 2000

Protection for resident tidal

freshwater species

> 5.5 mg liter-1

> 4 mg liter-1

> 3 mg liter-1

30 days

7 days

instantaneous 

minimum

U.S. EPA 1986

Protection against effects on

threatened/endangered

species (shortnose sturgeon)

> 5 mg liter-1

> 3.5 mg liter-1

> 3.2 mg liter-1 1

> 4.3 mg liter-1 2

30 days

6 hours

2 hours

2 hours

Secor and Niklitschek

2003; Niklitschek 2001;

Secor and Gunderson

1998; Jenkins et al. 1994;

Campbell and Goodman

2003

Additional published

findings

S Preferred striped

bass juvenile habitat

S Juvenile striped

bass growth,

feeding effects

S Juvenile striped

bass mortality

S Total fish biomass

declining

S Total fish species

richness

> 5 mg liter-1

< 4 mg liter-1

< 3 mg liter-1

< 3.7 mg liter-1

< 3.5 mg liter-1

-

-

-

-

-

Kramer 1987; Breitburg et

al. 1994

Kramer 1987; Breitburg et

al. 1994

1 Protective of survival at nonstressful temperatures.

2 Protective of shortnose sturgeon at stress temperatures (> 29°C).

< 3 mg liter-1

< 3.7 mg liter-1

< 3.5 mg liter-1

-

-

-

Chittenden 1972; Coutant

1985; Krouse 1968

Simpson 1995

Simpson 1995
period is consistent with and fully protects against effects on larval recruitment (see

Figure III-5 and text below) and is consistent with the duration protection of freshwater species.

The criterion values of a 30-day mean of 5 mg liter-1, a 7-day mean of 4 mg liter-1

and an instantaneous minimum of 3.2 mg liter-1 fully protect larval recruitment.

Depending on an assumption of partial or 100 percent exposure to low dissolved

oxygen concentrations, larval recruitment would be protected at concentrations

ranging between 4.6 and 4.8 mg liter-1 beyond 30 days of exposure (Figure III-5). At

seven days of exposure, concentrations between 3.4 and 3.6 mg liter-1, extracted

from the range of larval recruitment curves protects against effects. The 7-day mean,

4 mg liter-1 concentration criterion value, therefore, protects recruitment. The instantaneous minimum 3.2 mg liter-1 criterion would protect larval recruitment, given that

the instantaneous minimum exposure level concentrations are between 2.7 to 2.9 mg

liter -1.

The instantaneous minimum 3.2 mg liter-1 criterion will also protect the survival of

juvenile and adult fish and shellfish species inhabiting shallow- and open-water habitats, given it has a higher value than the Virginian Province value of 2.3 mg liter-1

(U.S. EPA 2000).

The 30-day mean 5.5 mg liter-1 criterion value is consistent with the EPA freshwater

dissolved oxygen criteria to protect warm-water freshwater species (U.S. EPA 1986).

The other two components of the proposed open-water criteria—7-day mean of 4 mg

liter-1 and instantaneous minimum of 3.2 mg liter-1—are also consistent with the

EPA warm-water freshwater criteria (Table III-4).

The instantaneous minimum 3.2 mg liter-1 criterion protects against lethal effects

from short-term exposures to low dissolved oxygen for both Bay species of sturgeon.

A 30-day mean 5 mg liter-1 criterion protects against growth effects for longer-term

exposures (Secor and Niklitschek 2001, 2003; Niklitschek 2001; Secor and

Gunderson 1998). Application of the 3.2 mg liter-1 criterion as an instantaneous

minimum concentration is justified on the basis that effects on shortnose sturgeon

were observed after just two hours’ exposure (Campbell and Goodman 2003).

From October 1 through May 31, when the open-water fish and shellfish designated

use extends through the water column into the seasonally defined deep-water seasonal

fish and shellfish and deep-channel seasonal refuge designated use habitats, these

habitats are important both to blue crabs and larger finfish species seeking refuge in

deeper, warmer waters (e.g., striped bass, white perch, Atlantic croaker, shortnose

sturgeon and Atlantic sturgeon) during the cooler months of the year (see Appendix

A; U.S. EPA 2003a). The criterion values described above will provide the necessary

levels of protection for all of these species, for both juvenile and adult life stages.

Open-Water Criteria

The following criteria fully support both the Chesapeake Bay open-water fish and

shellfish and shallow-water bay grass designated uses when applied year-round: a

▼51

chapter iii • Dissolved Oxygen Criteria
30-day mean > 5.5 mg liter- 1 applied to tidal-fresh habitats only with long-term

averaged salinities of up to 0.5 ppt; a 30-day mean > 5 mg liter-1; a 7-day mean 

> 4 mg liter-1; and an instantaneous minimum > 3.2 mg liter-1. At temperatures

stressful to shortnose sturgeon (>29°C), a 4.3 mg liter-1 instantaneous minimum

criteria should apply.

DEEP-WATER SEASONAL FISH AND SHELLFISH 

DESIGNATED USE CRITERIA

In deep-water habitats, where the physical exchange of higher oxygenated waters in

the upper water-column habitats is much reduced by density stratification and pycnocline waters are not reoxygenated by riverine or oceanic bottom waters, dissolved

oxygen concentrations will naturally be lower during the warmer months of the year.

Criteria to support the deep-water seasonal fish and shellfish designated use must

fully “protect the survival, growth and propagation of balanced, indigenous populations of ecologically, recreationally and commercially important fish and shellfish

species inhabiting deep-water habitats” (Appendix A; U.S. EPA 2003a).

In the Chesapeake Bay, the bay anchovy is an abundant, ecologically significant fish

likely to be affected by low dissolved oxygen conditions, given its life history.

Although it is not a commercial species, the bay anchovy is prey for bluefish, weakfish and striped bass (Hartman and Brandt 1995), forms a link between zooplankton

and predatory fish (Baird and Ulanowicz 1989) and represents from 60 to 90 percent

of piscivorus fish diets on a seasonal basis (Hartman 1993). Bay anchovy spawn

from May to September in the Chesapeake Bay, with a peak in June and July (Olney

1983; Dalton 1987) across a broad range of temperatures and salinities throughout

the Chesapeake Bay (Dovel 1971; Houde and Zastrow 1991). Their spawning and

nursery periods coincide with the presence of low dissolved oxygen conditions in the

Chesapeake Bay and its tidal tributaries.

The hatchability of fish eggs is known to be influenced by the oxygen concentrations

to which they are exposed during incubation (Rombough 1988). Chesney and Houde

(1989) conducted laboratory experiments to test the effects of low dissolved oxygen

conditions on the hatchability and survival of bay anchovy eggs and yolk-sac larvae.

Their findings demonstrated that survival rates of bay anchovy eggs and larvae are

likely to be affected when exposed to dissolved oxygen concentrations less than 3

mg liter-1 and 2.5 mg liter-1, respectively. Breitburg (1994) found very similar effects

for 3- to 13-day post-hatch bay anchovy larvae, where 50 percent survival was

observed at 2.1 mg liter-1.

Bay anchovy routinely inhabit waters within the pycnocline region. Bay anchovy

eggs have been found throughout the water column regardless of bottom layer

oxygen concentrations in mesohaline areas of tributaries (Keister et al. 2000), but

were retained in surface and pycnocline waters in the mesohaline mainstem Bay

(North 2001; Breitburg et al., unpublished data; Figure III-6). MacGregor and Houde

(1996) found that most bay anchovy eggs were distributed in water above the

▼52

chapter iii • Dissolved Oxygen Criteria
▼53

chapter iii • Dissolved Oxygen Criteria

Figure III-6. Illustration of bay anchovy egg densities observed in the surface mixed

layer, within pycnocline and below pycnocline waters in the mesohaline portion of the

Chesapeake Bay (a) and the mesohaline portion of the Patuxent River (b).

NOTE: one surface value of 45 eggs meter-3 for the Chesapeake Bay and two bottom values of 

274 and 413 eggs meter-3 for Patuxent River deleted for clarity.

Sources: Breitburg et al. 2003; Breitburg et al., unpublished data; Keister et al. 2000.

0

1

2

3

4

5

0 0.1 0.2 0.3 0.4 0.5

bottom water

pycnocline

surface water

a.

0

25

50

75

100

125

150

02468

bottom dissolved oxygen (mg liter -1)

bottom water

pycnocline

surface water

b.

number eggs meter -3

-1 bottom dissolved oxygen (mg liter )

-3 number eggs meter

(note: 2 bottom values 274 and 413 eggs/m3 deleted for clarity)

(note: 1 surface value of 45/m3 deleted for clarity)
pycnocline when below pycnocline waters had dissolved oxygen concentrations of 

< 2 mg liter-1. Rilling and Houde (1999) observed bay anchovy eggs and larvae

throughout the water column during June and July. Bay anchovy larvae are found

throughout the water column when bottom oxygen concentrations are above 2 mg

liter-1 (Keister et al. 2000).

Environmental conditions present during the egg, larval or juvenile life stages

strongly influence fish population dynamics. Key among these are changes in food

supply for first-feeding larvae and factors that modify predation mortality for the

highly vulnerable larval life stages. The majority of the species for which larval

effects data are available within the Virginian Province criteria document do not

routinely inhabit waters in the pycnocline layer. To derive a criteria to protect deep

waters located within the pycnocline layer that are generally inhabited by bay

anchovy and their eggs and larvae, a Chesapeake Bay-specific larval recruitment

effects model was generated for the bay anchovy.

Criteria Components

Protection against Egg/Larval Recruitment Effects. Two larval recruitment

effect models were derived that are specific to the Chesapeake’s bay anchovy, based

on the original Virginian Province larval recruitment effects model (U.S. EPA 2000).

The bay anchovy eggs effects model was based on a 5 percent impairment of eggs

hatching to yolk-sac larvae, assuming a 100-day recruitment period and 1-day development period (Chesney and Houde 1989). The larvae-based recruitment effects

model, also based on a 5 percent impairment, assumed that yolk-sac larvae and postyolk larvae or feeding larvae had the same sensitivity.

A development period of 32 days was applied, based on Houde’s work (1987), which

documented an egg-to-larval duration of 33 days. One day was subtracted to reflect

the egg stage (Chesney and Houde 1989), yielding the 32-day development period.

A 132-day recruitment period was calculated by adding the 32-day development

period to the 100-day recruitment period mentioned above.

A 50 percent exposure6 to low dissolved oxygen concentrations was built into both

the egg and larvae recruitment effects models. Field-based observations have indicated widespread distributions of bay anchovy eggs and larvae across the Bay’s

mainstem waters and throughout the water column except in subpycnocline waters

with extremely low dissolved oxygen concentrations (MacGregor and Houde 1996;

Rilling and Houde 1999; Keister et al. 2000; Breitburg et al. 2003).

The final separate survival curves for both the egg and larval recruitment effect

models, illustrated in Figure III-7, were based on comparing the effects data from

▼54

chapter iii • Dissolved Oxygen Criteria

6 The larval recruitment model has a parameter for what percentage of a given cohort is exposed to

low dissolved oxygen conditions.
Chesney and Houde (1989) with the final survival curve from Figure 5 in the

Virginian Province saltwater criteria document (U.S. EPA 2000). A single combined

egg/larval recruitment effects curve, based on the midpoint between the two individual effects curves, also is illustrated in Figure III-7. The effects curves illustrated

in Figure III-7 reflect the combined dissolved oxygen concentration and duration of

exposure protective against a 5 percent or greater impact, thereby protecting 95

percent of the seasonally produced offspring. Dissolved oxygen concentrations and

exposure durations falling above the combined bay anchovy egg/larval recruitment

curve—3 mg liter-1 for 30 days and 1.7 mg liter-1 at all times—would protect against

egg and larval recruitment effects greater than 5 percent.

Protection of Juvenile/Adult Survival. The Virginian Province document

recommends 2.3 mg liter-1 as the threshold above which long-term, continuous exposures should not cause lethal conditions for juvenile and adult fish and shellfish (U.S.

EPA 2000). 

Additional Scientific Literature Findings. Breitburg et al. (2001) provide a

synthesis of the acute sensitivities of an array of species that may inhabit the water

column or near-bottom habitats in the deep-water designated use habitats.

Adults and juveniles of most Chesapeake Bay species that have been

tested have 24 hr LC50 values near 1 mg l-1 (i.e., approximately 13% saturation at 25oC and 18 psu). Acute toxicity tests have yielded 50%

mortality rates with 24-hr exposures at 0.5-1.0 mg l-1 for species such as

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chapter iii • Dissolved Oxygen Criteria

Figure III-7. Chesapeake Bay bay anchovy egg and larval recruitment effects curves.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 10 20 30 40 50 60 70

Time (days)

Dissolved Oxygen Concentration (mg liter-1)

Bay anchovy eggs curve

Bay anchovy larvae curve

Combined bay anchovy

egg/larval recruitment curve

 3 mg liter-1 at 30 days

 1.7 mg liter-1 at 0 days
hogchoker (Trinectes maculatus), northern sea robin (Prionotus

carolinus), spot (Leiostomus xanthurus; but LC50 reported as >1 mg l-1 by

Phil et al. 1991) tautog (Tautoga onitis), windowpane flounder

(Scopthalmus aquosus), and fourspine stickleback (Apeltes quadracus),

and 50% mortality rates between 1.1 and 1.6 mg l-1 for Atlantic menhaden

(Brevoortia tyrannus), scup (Stenotomus chrysops), summer flounder

(Paralichthyus dentatus), pipefish (Syngnathus fuscus) and striped bass

(Morone saxatilis) (Pihl et al. 1991; Poucher and Coiro, 1997; Thursby et

al. 2000 [cited here as U.S. EPA]). Thus, for nearly all species tested, the

range of tolerances is quite low; only a 1.0 mg l-1 difference separates the

most and least sensitive species described above.

Although fewer species have been tested during the larval stage, larvae of

species that occur in Chesapeake Bay appear to be somewhat more sensitive to low oxygen exposure than are most adults and juvenile. For

example, 50% mortality with 24-h exposure occurs between 1.0 and 

1.5 mg l-1 for skilletfish (Gobiesox strumosus), naked goby (Gobiosoma

bosc), and inland silverside (Menidia beryllina) larvae, while 50%

mortality occurs at 1.8 to 2.5 mg l-1 for larval red drum (Sciaenops ocellatus), bay anchovy (Anchoa mitchilli), striped blenny (Chasmodes

bosquianus) and striped bass (Saksena and Joseph 1972; Breitburg 1994;

Poucher and Coiro 1997). Field and laboratory observations indicate that

lethal dissolved oxygen concentrations for skilletfish, naked goby and

striped blenny adults are 1.0 mg l-1 (Breitburg, unpublished data).

Embryo tolerances vary inconsistently in relation to tolerances of later

stages; 50% mortality in 12-96 h occurs at a higher dissolved oxygen

concentration than that for larval mortality for bay anchovy (2.8 mg l-1),

at a similar oxygen concentration as for larvae for inland silverside (1.25

mg l-1), and at lower concentrations than that leading to larval mortality

for winter flounder (Pleuronectes americanus; 0.7 mg l-1) and naked 

goby (approximately 0.6 mg l-1) (Chesney and Houde 1989; Poucher and

Coiro 1997).

Roman et al. (1993) examined the distribution of two species of zooplankton copepods—Acartia tonsa and Oithona colcarva—through the water column in the

Chesapeake Bay. Acartia tonsa, which regularly migrate from open water down to

subpycnocline waters, were not found in bottom waters when oxygen concentrations

were < 1 mg liter -1. The highest concentration of zooplankton were found at the

pycnocline level. 

In a recent review of zooplankton responses to and ecological consequences of

zooplankton exposure to low dissolved oxygen, Marcus (2001) synthesized the

following literature findings.

Vargo and Sastry (1977) reported that 2-h LD50 values for Acartia tonsa

and Eurytemora affinis adults collected from the Pettaquamscutt River

Basin, Rhode Island ranged from dissolved oxygen concentrations of

0.36 to 1.40 mg l-1 and 0.57 to 1.40 mg l-1 respectively. Roman et al.

(1993) tested the oxygen tolerance of adults of Acartia tonsa and Oithona

▼56

chapter iii • Dissolved Oxygen Criteria
colcarva from Chesapeake Bay. Survival was considerably less after 24 h

in < 2.0 mg l-1 oxygenated water.

Stalder and Marcus (1997) examined the 24-h survival of three coastal

copepod species in response to low oxygen. Acartia tonsa showed excellent survival at concentrations as low as 1.43 mg l-1. Between 1.29 and

0.86 mg l-1 survival declined markedly and at 0.71 mg l-1 mortality was

100%. Labidocera aestiva and Centropages hamatus were more sensitive

to reduced dissolved oxygen concentrations. The survival of these species

was significantly lower at 1.43 mg l-1. The survival of nauplii of Labidocera aestiva and Acartia tonsa at low dissolved oxygen concentrations

was generally better than adult survival.

Rationale

Protecting the recruitment of bay anchovy eggs and larvae into the juvenile and adult

population is crucial to the integrity of the Chesapeake Bay ecosystem. The bay

anchovy is a primary food source for many fish species. To protect bay anchovy

recruitment, criteria values of a 30-day mean of 3 mg liter-1 and an instantaneous

minimum of 1.7 mg liter-1 were selected to best reflect the shape of the final

combined bay anchovy egg and larval recruitment effects curve illustrated in 

Figure III-7.

This approach to criteria derivation is consistent with the approach to derive criteria

protective against larval effects in open-water habitats. These approaches followed

the guidelines published by the EPA in the Virginian Province dissolved oxygen

criteria document (U.S. EPA 2000). The bay anchovy 12- to 24-hour post larvae

hatch values from Chesney and Houde (1989) place bay anchovy larvae within the

upper range of larval life stage sensitivities for all 17 fish and invertebrate species

documented in the Virginian Province document (see Figure 4 on page 13 and

Appendix D). The criteria derived to protect bay anchovy early life stages should be

protective of other species that routinely inhabit deeper, pycnocline habitats.

The 1.7 mg liter-1 criterion value was derived as the dissolved oxygen concentration

where the combined egg/larval recruitment effects curve intercepted the y-axis

(Figure III-7). Given that the y-axis intercept reflects ‘time zero,’ an instantaneous

minimum duration was applied to the 1.7 mg liter-1 criterion value. The 3 mg liter-1

criterion value was derived as the approximate point where the combined egg/larval

recruitment effects curve levels out. The flattening of the curve beyond this point

indicates that dissolved oxygen concentrations much greater than 3 mg liter-1 should

not cause increased impairment of egg/larval recruitment over longer periods of

exposure. The 3 mg liter-1 concentration corresponded with 30 days on the x-axis

(Figure III-7).

These criteria values and durations are supported by findings published in the scientific literature. Chesney and Houde (1989) evaluated 12- to 14-hour-old yolk-sac bay

anchovy larvae over 12 hours, yielding the effects data used in running the bay

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chapter iii • Dissolved Oxygen Criteria
anchovy egg/larval recruitment models. In deep-water habitats, field observations

support the presence of effects at durations of less than 24 hours, which supports the

selection of the instantaneous minimum versus a daily average criterion concentration (Breitburg 1992). Magnusson et al. (1998) have indicated that asphyxia, as

described previously, has been reported at dissolved oxygen concentrations well

below (> 50 percent) the reported LC50 concentrations. Given that the reported LC50

values for bay anchovy larvae range from 2.1 to 2.8 mg liter-1 (Chesney and Houde

1989; Breitburg 1994), an instantaneous minimum criteria value above 1.4 mg 

liter-1 (50 percent of 2.8 mg liter-1) is required to prevent lethal conditions at exposures of less than 24-hour averaged conditions. Given that the reported laboratory

and field effects were manifested in less than 12 hours, an instantaneous minimum

concentration is further justified as the temporal period for application of the 1.7 mg

liter-1 criterion value.

In addition to early life stages of bay anchovy, the instantaneous minimum of 1.7 mg

liter-1 protects juvenile and adult survival of those fish species commonly inhabiting

water-column and bottom habitats within the pycnocline (e.g., spot, summer

flounder and winter flounder; Table III-7). See also Table 1, page 8 in U.S. EPA

(2000) for additional supporting effects data. This criterion value also will protect

zooplankton, the principal prey of the bay anchovy and many other fish during their

early life stages (Table III-7; Marcus 2001; Roman et al. 1993). Application of the

Virginian Province saltwater criteria for juvenile/adult survival, 2.3 mg liter-1 as a 1-

day mean, will provide the required level of protection to short-term exposures to

low dissolved oxygen in deep-water habitats (U.S. EPA 2000).

The open-water criteria that apply to the summer-only deep-water designated use

habitats from October 1 through May 31 will protect Atlantic and shortnose sturgeon

inhabiting these deep waters in the winter (Secor et al. 2000; Welsh et al. 2000).

From June 1 to September 30, the deep-water designated use criteria will not fully

protect Atlantic and shortnose sturgeon.

Historically, natural low dissolved oxygen conditions (< 3 mg liter-1) in deep-water

and deep-channel regions would have curtailed sturgeon access. Over the past several

hundred years, sturgeon probably have not used deep-water and deep-channel designated use habitats during summer months due to ‘naturally’ pervasive hypoxia (see

the sections above titled, “Low Dissolved Oxygen: Historical and Recent Past” and

“Historical Potential Sturgeon Tidal Habitats”). Behavioral studies indicate that sturgeon are capable of avoiding these hypoxic regions (Niklitschek 2001) and probably

have done so for centuries. On the other hand, deep-water and deep-channel designated use habitats do recover to normoxic conditions during the fall, winter and spring

months. During these periods evidence supports a past and recent role for habitats as

thermal refuge and migration corridors in the Chesapeake Bay.

This criterion also will protect open-water species with higher dissolved oxygen

sensitivities that search for prey within these pycnocline habitats for short periods of

time. Field data from other estuarine and coastal systems, such as Long Island Sound

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chapter iii • Dissolved Oxygen Criteria
▼59

chapter iii • Dissolved Oxygen Criteria

Table III-7. Deep-water seasonal fish and shellfish designated use criteria components.

Criteria Components Concentration Duration Source

Protection against egg/larval 

recruitment effects 

3 mg liter-1

1.7 mg liter-1

30 days 

instantaneous 

minimum 

Chesney and Houde 1989; 

Breitburg 1994; U.S. EPA 2000 

Protection of juvenile/adult 

survival 

> 2.3 mg liter-1 24 hours U.S. EPA 2000 

Additional literature findings 

- 50 percent mortality 

for hogchoker, 

northern sea robin, 

spot 

- 50 percent mortality 

for tautog, 

windowpane 

flounder adults 

- 50 percent mortality 

for menhaden, 

summer flounder, 

pipefish, striped bass 

adults 

- 50 percent mortality 

for skilletfish, naked 

goby, silverside 

larvae 

- 50 percent mortality 

for red drum, bay 

anchovy, striped 

blenny larvae 

- Zooplankton habitat 

avoidance 

- Reduced copepod 

nauplii abundance 

- 50 percent mortality 

for Acartia tonsa and 

Eurytemora affinis

- Mortality for Acartia 

tonsa and Oithona 

colcarva

- 100 percent mortality 

for copepods 

- Reduced survival for 

copepods 

- Acartia tonsa

survival 

0.5-1 mg liter-1

> 1 mg liter-1

1.1-1.6 mg liter-1

1-1.5 mg liter-1

1.8-2.5 mg liter-1

< 1 mg liter-1

< 1 mg liter-1

0.36-1.4 mg liter-1

< 2 mg liter-1

0.71 mg liter-1

<.86-1.3 mg liter-1

> 1.43 mg liter-1

24 hours 

24 hours 

24 hours 

24 hours 

24 hours 

- 

- 

2 hours 

24 hours 

24 hours 

24 hours 

24 hours 

Reviewed in Breitburg et al. 

2001 

Reviewed in Breitburg et al. 

2001; Pihl et al. 1991; 

Reviewed in Breitburg et al. 

2001; Pihl et al. 1991; Poucher 

and Coiro 1997; U.S. EPA 

2000 

Breitburg 1994; Poucher and 

Coiro 1997 

Saksena and Joseph 1972; 

Breitburg 1994; Poucher and 

Coiro 1997 

Roman et al. 1993 

Qureshi and Rabalais 2001 

Vargo and Sastry 1977 

Roman et al. 1993 

Stalder and Marcus 1997 

Stalder and Marcus 1997 

Stalder and Marcus 1997 
and Albemarle-Pamlico Sound, clearly indicate that open-water species will use

pycnocline region habitats if dissolved oxygen concentrations are above levels that

result in avoidance (e.g., Howell and Simpson 1994; Simpson 1995; Eby 2001). 

Recommended Criteria

The following criteria fully support the seasonal-based Chesapeake Bay deep-water

designated use when applied from June 1 through September 30: a 30-day mean 

3 mg liter-1, a 1-day mean 2.3 mg liter-1 and an instantaneous minimum 1.7 mg liter-1.

DEEP-CHANNEL SEASONAL REFUGE DESIGNATED USE CRITERIA

Deep-channel habitats are defined as the very deep water-column and adjacent

bottom surficial sediment habitats located principally in the river channel at the

lower reaches of the major rivers (e.g., the Potomac River) and along the spine of the

middle mainstem Chesapeake Bay at depths below which seasonal anoxic (< 0.2 mg

liter-1 dissolved oxygen) to severe hypoxic conditions (< 1 mg liter-1 dissolved

oxygen) routinely set in and persist for extended periods of time under current conditions (Appendix A; U.S. EPA 2003a). From late spring to early fall, many of these

deep-channel habitats are naturally exposed to very low dissolved oxygen concentration conditions. Under low dissolved oxygen conditions of 1 to 2 mg liter-1, these

habitats are suitable only for survival of benthic infaunal and epifaunal organisms.

Criteria that support the deep-channel designated use must fully protect the “survival

of balanced, indigenous populations of ecologically important benthic infaunal and

epifaunal worms and clams that provide food for bottom-feeding fish and crabs”

(Appendix A; U.S. EPA 2003a). The seasonal-based deep-channel criteria are based

on establishing dissolved oxygen concentrations to protect the survival of bottom

sediment-dwelling worms and clams.

Components

The infauna of the deep-channel habitat are the most tolerant of all infaunal benthic

organisms in the Chesapeake Bay. Even if there were no problems with low

dissolved oxygen conditions, the benthic organisms inhabiting unconsolidated mud

habitats in these deep-channel designated use habitats probably would not change.

Looking at benthos from deep-channel habitats in the Chesapeake Bay that are not

hypoxic or anoxic, one finds the same benthic community species. On an annual

basis, productivity is about the same for hypoxic versus non-hypoxic deep unconsolidated mud bottom sediment habitats in the mesohaline Chesapeake Bay (Diaz and

Schaffner 1990). The factors that control what is present in these mesohaline benthic

habitats are salinity and sediment type. Hypoxic conditions run a distant third

(Holland et al. 1977). Hypoxic conditions change the benthic community structure

periodically, but the pool from which these low oxygen habitats are recolonized after

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chapter iii • Dissolved Oxygen Criteria
a severe low-oxygen to no-oxygen event is still the limiting factor for a benthic

community.

Benthic infauna have high tolerances to low dissolved oxygen conditions (~1 mg

liter-1) and many macrofaunal species demonstrate behavioral reactions before they

eventually die (Diaz and Rosenberg 1995). For the mesohaline zone of estuaries, the

critical dissolved oxygen level appears to be around 0.6–1.0 mg liter-1 (Diaz and

Rosenberg 1995; Table III-8). At the high end of this dissolved oxygen range, the

bottom-dwelling community starts to lose moderately tolerant species, with more

tolerant species dying off at the low end of the range. In estuaries and coastal systems

exposed to seasonally varying low dissolved oxygen, the critical dissolved oxygen

concentration is closer to 1 mg liter-1 (Llanso 1992), with subtle reductions in

dissolved oxygen concentration from 1 to 0.5 mg liter-1 causing a full range of

responses from behavioral to death (Llanso and Diaz 1994). In their synthesis of

dissolved oxygen concentrations causing acute and chronic effects on Chesapeake

Bay benthic infaunal organisms, Holland et al. (1989) found a similar range of oxygen

concentrations that cause mortality or severe behavioral effects (Appendix C). 

In the deep channel of the Chesapeake Bay, communities of mud-burrowing worms

and clams have a broad tolerance to a wide range of sediment types, salinities,

dissolved oxygen concentrations and organic loadings. Several keystone Bay

bottom-dwelling polychaete worm species—Paraprionospio pinnata, Streblospio

benedicti, Loimia medusa and Heteromastus filiformis—are resistant to dissolved

oxygen concentrations as low as 0.6 mg liter-1 (Llanso and Diaz 1994; Diaz et al.

1992; Llanso 1991).

Extensive mortality is likely only under persistent exposure to very low dissolved

oxygen concentrations (< 1 mg liter-1) at higher summer temperatures in the Chesapeake Bay (Holland et al. 1977). Similar findings have been reported for other

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chapter iii • Dissolved Oxygen Criteria

Table III-8. Deep-channel designated use criteria effects data.

Effects Observed Concentration Source

S Mesohaline community

mortality of moderately

tolerant species

S Mesohaline community

mortality of more tolerant

species

S Behavioral to lethal responses

observed

S Behavior, growth and

production effects observed

S Epifaunal community survival

1 mg liter-1

0.6 mg liter-1

0.5-1 mg liter-1

< 2 mg liter-1

0.5-2 mg liter-1

Numerous references cited in Diaz

and Rosenberg 1995

Numerous references cited in Diaz

and Rosenberg 1995

Llanso 1992; Llanso and Diaz 1994;

references cited in Holland et al. 1989

Diaz et al. 1992

Sagasti et al. 2000

–

–

–

–

–
estuarine and coastal systems (Rosenberg 1977; Jorgensen 1980; Stachowitsch 1984;

Gaston 1985).

While the macrobenthic community itself often is found to be insensitive to low

dissolved oxygen concentrations around 2 mg liter-1, exposure of these bottom habitats to brief periods of dissolved oxygen concentrations < 2 mg liter-1 affects

behavior (resulting in decreased burrowing depth and exposure at the sediment

surface), growth and production (Diaz et al. 1992). From a synthesis of 12 years of

diverse observations and 5 years of remotely operated vehicle videotapes, Rabalais

et al. (2001) reported stressed behavior, such as emergence from the sediments by

burrowing invertebrates, at dissolved oxygen concentrations below 1.5 to 1 mg 

liter-1. At dissolved oxygen concentrations of 1 to 1.5 mg liter-1, they observed “even

the most tolerant burrowing organisms, principally polychaetes, emerge partially or

completely from their burrows and lie motionless on the bottom.” Demersal feeding

fish change their feeding habits quickly to take advantage of stressed macrobenthos

that come to the sediment surface (Stachowitsch 1984; Jorgensen 1980), where they

become more vulnerable to predation during or following a low dissolved oxygen

event (Pihl et al. 1991, 1992).

Epifaunal communities living along the surfaces of bottom sediments in the Chesapeake Bay can persist with minimal changes in species composition and abundance

under brief exposures to dissolved oxygen concentrations in the range of 0.5 to 2.0

mg liter-1 (Sagasti et al. 2000).

For the unconsolidated mud benthic infaunal community of the mesohaline Chesapeake Bay where the deep-channel designated use habitats are located, 1 mg liter-1

is protective of survival. The global scientific literature points towards 2 mg liter-1 as

the protective dissolved oxygen value, but this is the oxygen tolerance for higher

salinity, more structured benthic communities and species. Between 2 and 3.5 mg

liter-1 there are definite behavioral changes for many species and mortality for sensitive species in these higher salinity habitats. For Chesapeake Bay species in similar

higher salinity (polyhaline) habitats, 2 mg liter-1 would be the dissolved oxygen

minimum requirement. Benthic communities in these polyhaline habitats in the

Chesapeake Bay will be protected by applying the open-water dissolved oxygen

criteria year-round. However, for the mesohaline Chesapeake Bay where the hypoxic

and anoxic conditions are focused during the summer months, the scientific literature for unconsolidated mud mesohaline benthic communities supports 1 mg liter-1

as the bottom-line requirement. Dissolved oxygen concentrations of less than 1 mg

liter-1 lead to mortality for even tolerant species.

Rationale

To ensure protection of the survival of bottom-dwelling worms and clams, an instantaneous minimum criterion of 1 mg liter-1 was selected (Table III-9). As documented

through the extensive scientific literature reported here, this value will protect

against lethal effects from exposure to low dissolved oxygen. However, behavioral

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chapter iii • Dissolved Oxygen Criteria
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chapter iii • Dissolved Oxygen Criteria

Table III-9. Response patterns of Chesapeake Bay benthic organisms to declining dissolved oxygen 

concentrations (mg liter-1).

Response Dissolved Oxygen Species Reference

Avoidance

Infaunal swimming 1.1 Paraprionospio pinnata Diaz et al. 1992

0.5 Nereis succinea Sagasti et al. 2001

Epifaunal off

bottom

0.5 Neopanope sayi Sagasti et al. 2001

0.5 Callinectes sapidus Sagasti et al. 2001

1 Stylochus ellipticus Sagasti et al. 2001

1 Mitrella lunata Sagasti et al. 2001

0.5 Dirodella obscura Sagasti et al. 2001

1 Cratena kaoruae Sagasti et al. 2001

Fauna, unable to leave or escape, initiate a series of sublethal responses

Cessation of

feeding

0.5 Balanus improvisus Sagasti et al. 2001

0.6 Streblospio benedicti Llanso 1991

1 Loimia medusa Llanso and Diaz

1994

1.1 Capitella sp. Warren 1977; Forbes

and Lopez 1990

Decreased

activities not

related to

respiration

0.5 Balanus improvisus Sagasti et al. 2001

0.5 Conopeum tenuissimum Sagasti et al. 2001

0.5 Membranipora tenuis Sagasti et al. 2001

1 Cratena kaoruae Sagasti et al. 2001

1 Stylochus ellipticus Sagasti et al. 2001

1 Streblospio benedicti Llanso 1991

Cessation of

burrowing

1.1 Capitella sp. Warren 1977

continued
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chapter iii • Dissolved Oxygen Criteria

Table III-9. Response patterns of Chesapeake Bay benthic organisms to declining dissolved oxygen 

concentrations (mg liter-1) (continued).

Emergence from

tubes or burrows

0.1-1.3 Ceriathiopis americanus Diaz, unpublished

data

0.5 Sabellaria vulgaris Sagasti et al. 2001

0.5 Polydora cornuta Sagasti et al. 2001

0.7 Micropholis atra Diaz et al. 1992

1 Hydroides dianthus Sagasti et al. 2001

10% saturation Nereis diversicolor Vismann 1990

Siphon stretching

into water column

0.1-1.0 Mya arenaria, Abra alba Jorgensen 1980

Siphon or body

stretching

0.5 Molgula manhattensis Sagasti et al. 2001

0.5 Diadumene leucolena Sagasti et al. 2001

Floating on surface

of water

0.5 Diadumene leucolena Sagasti et al. 2001

Formation of

resting stage

0.5 Membranipora tenuis Sagasti et al. 2001

0.5 Conopeum tenuissimum Sagasti et al. 2001

Sources: Diaz and Rosenberg 1995; Sagasti et al. 2001.

Response Dissolved Oxygen Species Reference
changes leading to increased opportunities for predation are not protected by this

criterion. These changes may benefit bottom-feeding fish and crabs, giving them

direct access to food, albeit under potentially stressful water quality conditions.

The deep-channel criteria protect survival but not necessarily the growth of benthic

infaunal and epifaunal species from June through September. However, Diaz and

Schaffner (1990) reported that their evaluation of annual secondary productivity of

hypoxic habitats in the Bay’s deep-channel habitats indicated no significant reduction in productivity from low dissolved oxygen conditions. Therefore, the

deep-channel criteria’s failure to provide full protection against growth impairments

is counteracted by growth during the rest of the year, when dissolved oxygen

concentrations are naturally higher than 1 mg liter-1, which leads to a net result of

protection against growth impairment on an annual basis.

The instantaneous minimum value of 1 mg liter-1 is much more protective of benthic

infaunal organisms than a 1- or 7-day average. In the case of bottom-dwelling organisms, it is not the average condition that is most detrimental to the organisms but the

absolute minimum dissolved oxygen. When dissolved oxygen drops significantly

below 1 mg liter-1 for even short periods of time (on the order of hours) mortality

increases, even for tolerant species. Other deep-channel criteria with higher concentrations than 1 mg liter-1 and with 1-, 7- or 30-day averaging periods were not

derived for deep-channel designated use habitats, since dissolved oxygen concentrations are not expected to exceed 2 mg liter-1 from June through September due to

natural constraints.

Deep-Channel Criteria

The instantaneous minimum 1 mg liter-1 criterion fully supports the seasonal-based

Chesapeake Bay deep-water designated use when applied from June 1 through

September 30. 

CHESAPEAKE BAY DISSOLVED OXYGEN CRITERIA

The Chesapeake Bay dissolved oxygen criteria are structured to protect the five tidalwater designated uses and reflect the needs and habitats of Bay estuarine living

resources (Table III-10). Criteria for the migratory fish spawning and nursery,

shallow-water bay grass and open-water fish and shellfish designated uses were set

at levels to protect the reproduction and survival of all organisms and against impairments to their growth. Criteria for deep-water habitats during seasons when the water

column is significantly stratified were set at levels to protect juvenile and adult fish,

shellfish and the recruitment success of the bay anchovy. Criteria for deep-channel

habitats in summer were set to protect the survival of bottom sediment-dwelling

worms and clams. 

▼65

chapter iii • Dissolved Oxygen Criteria
▼66

chapter iii • Dissolved Oxygen Criteria Designated Use Criteria Concentration/Duration Protection Provided Temporal Application 7-day mean > 6 mg liter-1 (tidal habitats with 0-0.5 ppt salinity) Survival/growth of larval/juvenile tidal-fresh resident fish; protective of threatened/endangered species. Instantaneous minimum > 5 mg liter-1 Survival and growth of larval/juvenile migratory fish; protective of threatened/endangered species. February 1 - May 31 Migratory fish spawning and nursery use Open-water fish and shellfish designated use criteria apply June 1 - January 31 Shallow-water bay grass use Open-water fish and shellfish designated use criteria apply Year-round 30-day mean > 5.5 mg liter-1 (tidal habitats with 0-0.5 ppt salinity) Growth of tidal-fresh juvenile and adult fish; protective of threatened/endangered species.

30-day mean > 5 mg liter-1(tidal habitats with >0.5 ppt salinity)

Growth of larval, juvenile and adult fish and shellfish;protective of threatened/endangered species.

7-day mean > 4 mg liter-1 Survival of open-water fish larvae.

Open-water fish and

shellfish use

Instantaneous minimum > 3.2 mg liter-1 Survival of threatened/endangered sturgeon species.1

Year-round

30-day mean > 3 mg liter-1 Survival and recruitment of bay anchovy eggs and larvae.1-day mean > 2.3 mg liter-1 Survival of open-water juvenile and adult fish.Instantaneous minimum > 1.7 mg liter-1 Survival of bay anchovy eggs and larvae.

June 1 - September 30 Deep-water

seasonal fish and

shellfish use

Open-water fish and shellfish designated-use criteria apply October 1 - May 31

Instantaneous minimum > 1 mg liter-1 Survival of bottom-dwelling worms and clams. June 1 - September 30

Deep-channel

seasonal refuge use Open-water fish and shellfish designated use criteria apply October 1 - May 31

1 At temperatures considered stressful to shortnose sturgeon (>29C), dissolved oxygen concentrations above an instantaneous minimum of 4.3 mg

liter-1 will protect survival of this listed sturgeon species.

Table III-10. Chesapeake Bay dissolved oxygen criteria.

Deep-channel

seasonal refuge use
LITERATURE CITED

Adelson, J. M., G. R. Helz and C. V. Miller. 2000. Reconstructing the rise of recent coastal

anoxia; molybdenum in Chesapeake Bay sediments. Geochemica et Cosmochemica Acta

65:237-252.

Anger, K., R. Dawirs, V. Anger, J. Goy and J. Costlow. 1981a. Starvation resistance in first

stage zoea of brachyuran crabs in relation to temperature. Journal of Crustacean Biology

1(4):518-525.

Anger, K., R. Dawirs, V. Anger and J. Costlow. 1981b. Effects of early starvation periods

on zoeal development of brachyuran crabs. Biological Bulletin 161:199-212.

Atlantic States Marine Fisheries Commission. 1987. Interstate fisheries management plan

for the striped bass of the Atlantic coast from Maine to North Carolina. Revised Resource

Document and Management Plan Framework. Prepared by Versar, Inc. Columbia, Maryland. 

Baden, S. P. and L. Pihl. 1996. Effects of Autumnal Hypoxia on Demersal Fish and Crustaceans in the SE Kattegat, 1984-1991, Science Symposium on the North Sea, Quality Status

Report, 1993. Danish Environmental Protection Agency. Pp. 189-196. 

Baden, S. P., L. O. Loo, L. Pihl and R. Rosenberg. 1990. Effects of eutrophication on benthic

communities including fish: Swedish west coast. Ambio 19:113-122.

Bain, M. B. 1997. Atlantic and shortnose sturgeon of the Hudson River: Common and divergent life history attributes. Environmental Biology of Fishes 48:347-358.

Baird, D. and R. E. Ulanowicz. 1989. The seasonal dynamics of the Chesapeake Bay

ecosystem. Ecological Monograms 59(4):329-364.

Boicourt, W. C. 1992. Influences of circulation processes on dissolved oxygen in Chesapeake Bay. In: Smith, D., M. Leffler and G. Mackiernan (eds.). Oxygen Dynamics in

Chesapeake Bay: A Synthesis of Research. University of Maryland Sea Grant College Publications, College Park, Maryland. Pp. 7-59.

Bogdanov, A. S., S. I. Dorschev and A. F. Korpevich. 1967. Experimental transfer of Salmo

gairdneri (Richardson) and Roccus saxatilis (Walbaum) from the USA for acclimatization in

waters of USSR. Vorposy Ikhtiologii, Akademiva Raak SSSR 7:185-187.

Boreman, J. and R. J. Klauda. 1988. Distribution of early life stages of striped bass in the

Hudson River estuary, 1974-1979. American Fisheries Society Monographs 4:53-58.

Bowker, R. G., D. J. Baumgartner, J. A. Hutcheson, R. H. Ray and T. C. Wellborn, Jr. 1969.

Striped Bass Morone saxatilis (Walbaum) 1968 Report on the Development of Essential

Requirements for Production. U.S. Fish and Wildlife Service Publication, Washington, D. C.

112 pp.

Boynton, W. R., W. M. Kemp and C. W. Keefe. 1982. A comparative analysis of nutrients

and other factors influencing estuarine phytoplankton production. In V.S. Kennedy (ed.).

Estuarine Comparisons. Academic Press, New York. Pp. 209-230.

Boynton, W. R., T. T. Polar and H. Zion. 1981. Importance of juvenile striped bass food

habits in the Potomac estuary. Transactions of the American Fisheries Society 110:56-63.

Boynton, W. R., W. M. Kemp. 2000. Influence of river flow and nutrient loading on selected

ecosystem processes and properties in Chesapeake Bay. Pp. 269-298, In: J. Hobbie (ed).

Estuarine Science: A Synthetic Approach to Research and Practice. Island Press, Washington, DC.

▼67

chapter iii • Dissolved Oxygen Criteria
Boynton, W. R., J. H. Garber, R. Summers and W. M. Kemp. 1995. Inputs, transformations,

and transport of nitrogen and phosphorous in Chesapeake Bay and selected tributaries. Estuaries 18:285- 314.

Boynton, W. R. and W. M. Kemp. 2000. Influence of river flow and nutrient loads on selected

ecosystem processes: A synthesis of Chesapeake Bay data. In: J. E. Hobbie (ed.). Estuarine

Science: A Synthetic Approach to Research and Practice. Island Press, Washington, D. C.

Bradford, A. D., J. G. Miller and K. Buss. 1968. Bio-assays on eggs and larval stages of

American shad Alosa sapidissima. In: Suitability of the Susquehanna River for Restoration

of Shad. U. S. Department of the Interior, New York Conservation Department and Pennsylvania Fisheries Commission. Pp. 52-60.

Brandt, S. B. and J. Kirsch. 1993. Spatially explicit models of striped bass growth potential

in Chesapeake Bay. Transactions of the American Fisheries Society 122:845-869.

Brandt, S. B., E. Demers, J. A. Tyler and M. A. Gerken. 1998. Fish Bioenergetics Modeling:

Chesapeake Bay Ecosystem Modeling Program (1993-1998). Report to the Chesapeake Bay

Program. U. S. Environmental Protection Agency, Chesapeake Bay Program Office,

Annapolis, Maryland.

Bratton, J. F., S. M. Colman, R. R. Seal and P. C. Baucom. 2003.In press. Eutrophication and

carbon sources in Chesapeake Bay over the last 2,700 years: Human impacts in context.

Geochimica et Cosmochimica Acta.

Breitburg, D. L. 1990. Near-shore hypoxia in the Chesapeake Bay: Patterns and relationships

among physical factors. Estuarine, Coastal and Shelf Science 30:593-609.

Breitburg, D. L. 1992. Episodic hypoxia in Chesapeake Bay: Interacting effects of recruitment, behavior and physical disturbance. Ecological Monographs 62(4):525-546.

Breitburg, D. L. 1994. Behavioral response of fish larvae to low dissolved oxygen concentrations in a stratified water column. Marine Biology 120:615-625.

Breitburg, D. L. 2002. Effects of hypoxia, and the balance between hypoxia and enrichment,

on coastal fishes and fisheries. Estuaries 25:767-781.

Breitburg, D. L., N. Steinberg, S. DuBeau, C. Cooksey and E. D. Houde. 1994. Effects of low

dissolved oxygen on predation on estuarine fish larvae. Marine Ecology Progress Series

104:235-246.

Breitburg, D. L., L. Pihl and S. E. Kolesar. 2001. Effects of low dissolved oxygen on the

behavior, ecology and harvest of fishes: A comparison of the Chesapeake Bay and BalticKattegat systems. In: Coastal Hypoxia: Consequences for living resources and ecosystems.

Coastal and Estuarine Studies 58, Rabelais, N. N. and R. E. Turner, eds. American Geophysical Union, Washington, D. C.

Breitburg, D. L., A. Adamack, S. E. Kolesar, M. B. Decker, K. A. Rose, J. E. Purcell, J. E.

Keister and J. H. Cowan, Jr. 2003 (In press). The pattern and influence of low dissolved

oxygen in the Patuxent River, a seasonally hypoxic estuary. Estuaries.

Broad, A. C. 1957. Larval development of Palaemonetes pugio Holthuis. Biological Bulletin

112(2):144-161.

Brundage, H. M. and R. E. Meadows. 1982. Occurrence of the endangered shortnose sturgeon, Acipenser brevirostrum, in the Delaware River estuary. Estuaries 5:203-208.

Burton, D. T., L. B. Richardson and C. J. Moore. 1980. Effect of oxygen reduction rate and

constant low dissolved oxygen concentrations on two estuarine fish. Transactions of the

American Fisheries Society 109:552-557.

▼68

chapter iii • Dissolved Oxygen Criteria
Caddy, J. F. 1993. Marine catchment basins effects versus impacts of fisheries on semienclosed seas. ICES Journal of Marine Science 57:628-640.

Campbell, J. G. and L. R. Goodman. 2003 (In press). Acute sensitivity of juvenile shortnose

sturgeon to low dissolved oxygen concentrations. Transactions of the American Fisheries

Society.

Carter, H. H., R. J. Regier, E. W. Schniemer and J. A. Michael. 1978. The Summertime Vertical

Distribution of Dissolved Oxygen at the Calvert Cliffs Generating Station: A Physical Interpretation. Chesapeake Bay Institute, Johns Hopkins University, Special Report 60.

Chabot, D. and J. D. Dutil. 1999. Reduced growth of Atlantic cod in non-lethal hypoxic

conditions. Journal of Fish Biology 55:472-491.

Chamberlain, N. A. 1957. Larval development of the mud crab Neopanope texana sayi

(Smith). Biological Bulletin 113:338.

Chesapeake Executive Council. 1987. Chesapeake Bay Agreement. Annapolis, Maryland.

Chesney, E. J. and E. D. Houde. 1989. Chapter 9; Laboratory studies on the effect of hypoxic

waters on the survival of eggs and yolk-sac larvae of the bay anchovy, Anchova mitchilli. In:

Houde, E. D., E. J. Chesney, T. A. Newberger, A. V. Vazquez, C. E. Zastrow, L. G. Morin, H.

R. Harvey and J. W. Gooch. 1989. Population Biology of Bay Anchovy in Mid-Chesapeake

Bay. Final report to Maryland Sea Grant. R/F-56, UMCEES Ref. No. CBL 89-141. Pp. 184-

191.

Chittenden, M. E., Jr. 1973. Effects of handling on oxygen requirements of American shad

(Alosa sapidissima). Journal of Fisheries Research Board of Canada. 30:105-110.

Chittenden, M. E., Jr. 1972. Effects of handling and salinity on oxygen requirements of the

striped bass Morone saxatilis. Journal of Fisheries Research Board of Canada. 28:1823-

1830.

Chittenden, M. E., Jr. 1971. Status of striped bass, Morone saxatilis, in the Delaware River.

Chesapeake Science 12:131-136.

Chittenden, M. E., Jr. 1969. Life history and ecology of the American shad, Alosa sapidissima, in the Delaware River. PhD. thesis, Rutgers University, New Brunswick, New Jersey.

Christie, R. W., P. T. Walker, A. G. Eversole and T. A. Curtis. 1981. Distribution of spawning

blueback herring on the West Branch of Cooper River and the Santee River, South Carolina.

Proceeding of the Annual Conference of the Southeastern Association of Fisheries and

Wildlife Agencies. 35:632-640.

Churchill, P. A. 1985. Potomac Electric Power Company 1985 Striped Bass Aquaculture

Project. Environmental Affairs Group, Potomac Electric Power Company, Washington, D.C.

Cloern, J. E. 2001. Our evolving conceptual model of the coastal eutrophication problem.

Marine Ecology Progress Series 201:223-253.

Colligan, M., M. Collins, A. Hecht, M. Hendrix, A. Kahnle, W. Laney, R. St. Pierre, R.

Santos and T. Squiers. 1998. Status Review of Atlantic Sturgeon (Acipenser oxyrinchus).

National Oceanic and Atmospheric Administration, National Marine Fisheries Service,

Silver Spring, Maryland.

Collins, M. R. and T. I. J. Smith. 1996. Bycatch of Atlantic and Shortnose Sturgeon in the

South Carolina Shad Fishery. South Carolina Department of Natural Resources, Charleston,

South Carolina. 25 pp.

Colman, S. M. and J. F. Bratton. 2003. Anthropogenically induced changes in sediment and

biogenic silica fluxes in Chesapeake Bay. Geology 31(1):71-74. 

▼69

chapter iii • Dissolved Oxygen Criteria
Colman, S. M., P. C. Baucom, J. Bratton, T. M. Cronin, J. P. McGeehin, D. A. Willard, A.

Zimmerman and P. R. Vogt. 2002. Radiocarbon dating of Holocene sediments in Chesapeake Bay. Quaternary Research. 57:58-70.

Conover, D. O. 1992. Seasonality and the scheduling of life history at different latitudes.

Journal of Fish Biology 41:161-178.

Conover, D. O. and T. M. C. Present. 1990. Countergradient variation in growth rate:

Compensation for lengths of the growing season among Atlantic silversides from different

latitudes. Oecologia 83:316-324.

Cooper, S. R. 1995. Chesapeake Bay watershed historical land use: Impact on water quality

and diatom communities. Ecological Applications 5:703-723.

Cooper, S. R. and G. S. Brush. 1991. Long-term history of Chesapeake Bay anoxia. Science

254:992- 996.

Cornwell, J. C., D. J. Conley, M. Owens and J. C. Stevenson. 1996. A sediment chronology

of the eutrophication of Chesapeake Bay. Estuaries 19:488-499.

Costlow, J. D. and C. G. Bookhout. 1961. The larval development of Eurypanopeus

depressus (Smith) under laboratory conditions. Crustaceana 2:6-15.

Coutant, C. C. 1985. Striped bass, temperature and dissolved oxygen: A speculative hypothesis for environmental risk. Transactions of the American Fisheries Society 114:31-61.

Coutant, C. C. 1987. Thermal preference: When does an asset become a liability. Environmental Biology of Fishes 18:161-172.

Coutant, C. C. 1985. Striped bass, temperature and dissolved oxygen: A speculative hypothesis for environmental risk. Transactions of the American Fisheries Society 114:31-61.

Cronin, T. M., (ed.). 2000. Initial Report on IMAGES V Cruise of the Marion-Dufresne to

Chesapeake Bay June 20-22, 1999. USGS Open-file report 00-306.

Cronin, T. M. and C. Vann. 2003. The sedimentary record of anthropogenic and climatic

influence on the Patuxent estuary and Chesapeake Bay ecosystems. Estuaries 26 (2A).

Custer, J. F. 1986. Prehistoric use of the Chesapeake estuary: A diachronic perspective.

Journal of Washington Academy of Sciences. 76:161-172.

Dadswell, M. J., B. D. Taubert, T. S. Squiers, D. Marchette and J. Buckley. 1984. Synopsis

of Biological Data on Shortnose Sturgeon, Acipenser brevirostrum LeSeur 1818. National

Oceanic and Atmospheric Administration, Washington, D.C. 45 pp.

Dalton, P. D. 1987. Ecology of bay anchovy (Anchoa mitchilli) eggs and larvae in the midChesapeake Bay. Masters thesis. University of Maryland, College Park, Maryland. 104 pp.

D’Avanzo, C. and J. N. Kremer. 1994. Diel oxygen dynamics and anoxic events in an

eutrophic estuary of Waquoit Bay, Massachusetts. Estuaries 17:131-139.

Deubler, E. E. Jr. and G. S. Posner. 1963. Response of postlarval flounders, Paralichthys

lethostigma, to water of low oxygen concentrations. Copeia 2:312-317.

Diaz, R. J. and A. Solow. 1999. Topic two report for the integrated assessment on hypoxia in

the Gulf of Mexico. Ecological and Economic Consequences of Hypoxia Decision Analysis

Series No. 16. United States Department of Commerce, NOAA.

Diaz, R. J. and R. Rosenberg. 1995. Marine benthic hypoxia: A review of its ecological

effects and the behavioral responses of benthic macrofauna. Oceanography and Marine

Biology: An Annual Review 33:245-303.

▼70

chapter iii • Dissolved Oxygen Criteria
Diaz, R. J., R. J. Neubauer, L.C. Schaffner, L. Phil and S. P. Baden. 1992. Continuous monitoring of dissolved oxygen in an estuary experiencing periodic hypoxia and the effect of

hypoxia on macrobenthos and fish. Science of the Total Environment, supplement.

Diaz, R. J. and L. C. Schaffner. 1990. The functional role of estuarine benthos. In: M. Haire

and E.C. Krome (eds.). Perspectives on the Chesapeake Bay, 1990. Advances in Estuarine

Sciences. Chesapeake Research Consortium, Gloucester Point, Virginia Report. No.

CBP/TRS41/90. Pp. 25-56.

Dovel, W. L. 1971. Fish Eggs and Larvae of the Upper Chesapeake Bay. Natural Resources

Institute, University of Maryland Special Report 4. 71 pp.

Dovel, W. L., A. W. Pekovitch and T. J. Berggren. 1992. Biology of the shortnose sturgeon

(Acipenser brevirostrum Lesuere, 1818) in the Hudson River estuary, New York. In: C. L.

Smith (ed.). Estuarine Research in the 1980s. New York State University, Stony Brook, New

York. Pp. 187-227.

Dovel, W. L. and T. J. Berggen. 1983. Atlantic sturgeon of the Hudson estuary, New York.

New York Fish and Game Journal 30:140-172.

Eby, L. A. 2001. Response of a fish community to frequent and infrequent disturbances in an

estuarine ecosystem. Ph.D. dissertation, Duke University, Durham, North Carolina.

Forbes, T. L. and G. R. Lopez. 1990. The effect of food concentration, body size, and environmental oxygen tension on the growth of the deposit feeding polychaete, Capitella species

1. Limnology and Oceanography 35:1535-1544.

Funderburk, S. L., S. J. Jordan, J. A. Mihursky and D. R. Riley (eds.). 1991. Habitat Requirements for Chesapeake Bay Living Resources, 1991 Second Edition. Living Resources

Subcommittee, Chesapeake Bay Program. Annapolis, Maryland.

Gaston, G. R. 1985. Effects of hypoxia on macrobenthos of the inner shelf of Cameron,

Louisiana. Estuarine, Coastal and Shelf Science 20:603-613.

Geoghegan, P., M. T. Mattson and R. G. Keppel. 1992. Distribution of shortnose sturgeon in

the Hudson River Estuary, 1984-1988. In: C. L. Smith (ed.). Estuarine Research in the

1980s. State University of New York, Stony Brook, New York. Pp. 217-227.

Gleason, T. R. and D. A. Bengston. 1996. Size-selective mortality of inland silversides:

Evidence from otolith microstructure. Transactions of the American Fisheries Society

125:860-873.

Grant, G. C. and J. E. Olney. 1991. Distribution of striped bass Morone saxatilis (Walbaum)

eggs and larvae in major Virginia Rivers. Fisheries Bulletin 89:187-193.

Hagy, J. D. 2002. Eutrophication, hypoxia and trophic transfer efficiency in Chesapeake Bay.

Ph.D. dissertation, University of Maryland, College Park, Maryland.

Haley, N. J. 1999. Habitat characteristics and resource use patterns of sympatric sturgeons in

the Hudson River estuary. Masters thesis, University of Massachusetts, Amherst, Massachusetts. 124 pp.

Harding, L. W. and E. S. Perry. 1997. Long-term increase of phytoplankton biomass in

Chesapeake Bay, 1950-1994. Marine Ecology Progress Series 157:39-52

Harrell, R. M. and J. D. Bayless. 1981. Effects of suboptimal dissolved oxygen concentrations on developing striped bass embryos. South Carolina Wildlife and Marine Resources

Department, Bonneau, South Carolina. 15 pp.

▼71

chapter iii • Dissolved Oxygen Criteria
Hartman, K. J. 1993. Striped bass, bluefish and weakfish in the Chesapeake Bay: Energetics,

trophic linkages and bioenergetic model application. Ph.D. dissertation, University of Maryland, College Park, Maryland.

Hartman, K. J. and S. B. Brandt. 1995. Comparative energetics and the development of

bioenergetics models for sympatric estuarine piscivores. Canadian Journal of Fisheries

Aquatic Sciences 52:1647- 1666.

Hawkins, J. N. 1979. Anadromous fisheries research program: Neuse River. North Carolina

Department of Natural Resources and Community Development, Division of Marine Fisheries, Morehead City, North Carolina.

Holland, A. F., A. T. Shaughnessy, L. C. Scott, V. A. Dickens, J. Gerritsen and J. A. Ranasinghe. 1989. Long-Term Benthic Monitoring and Assessment Program for the Maryland

Portion of Chesapeake Bay: Interpretative Report. CBRM-LTB/EST-2. Maryland Department of Natural Resources Annapolis, Maryland.

Holland, A. F., N. K. Mountford and J. A. Mihursky. 1977. Temporal variation in upper bay

mesohaline benthic communities: I. The 9-m mud habitat. Chesapeake Science 18:370-378.

Houde, E. D. 1987. Fish early life dynamics and recruitment variability. American Fisheries

Society Symposium 2:17-29.

Houde, E. D. and C. E. Zastrow. 1991. Bay anchovy: In: S. L. Funderburk, S. J. Jordan, J.

A. Mihursky and D. R. Riley (eds.) Habitat Requirements for Chesapeake Bay Living

Resources, 1991 Revised Edition. Living Resources Subcommittee, Chesapeake Bay

Program Office, Annapolis, Maryland. Pp. 8-1 to 8-14.

Howell, P. and D. Simpson. 1994. Abundances of marine resources in relation to dissolved

oxygen in Long Island Sound. Estuaries 17:394-402.

Jenkins, W. E., T. I. J. Smith, L. D. Heyward and D. M. Knott. 1993. Tolerance of shortnose

sturgeon, Acipenser brevirostrum, juveniles to different salinity and dissolved oxygen

concentrations. Proceedings of the Annual Conference of Southeastern Association of Fish

and Wildlife Agencies 47:476-484.

Johns, D. M. 1981. I. Physiological studies on Cancer irroratus larvae. II. Effects of

temperature and salinity on physiological performance. Marine Ecology Progress Series

6:309-315.

Jones, P. W., J. J. Speir, N. H. Butowski, R. O’Reilly, L. Gillingham and E. Smoller. 1988.

Chesapeake Bay Fisheries: Status, Trends, Priorities and Data Needs. Maryland Department

of Natural Resources, Annapolis, Maryland, and Virginia Marine Resources Commission,

Richmond, Virginia. 226 pp.

Jordan, S. J., C. Stenger, M. Olson, R. Batiuk and K. Mountford. 1992. Chesapeake Bay

dissolved oxygen goal for restoration of living resource habitats: A Synthesis of Living

Resource Requirements with Guidelines for Their Use in Evaluating Model Results and Monitoring Information. CBP/TRS 88/93. Chesapeake Bay Program Office, Annapolis, Maryland.

Jorgensen, B. B. 1980. Seasonal oxygen depletion in the bottom waters of a Danish fjord and

its effects on the benthic community. Okios 34:68-76.

Karlsen, A. W., T. M. Cronin, S. E. Ishman, D. A. Willard, R. Kerhin, C. W. Holmes and M.

Marot. 2000. Historical trends in Chesapeake Bay dissolved oxygen based on benthic

foraminifera from sediment cores. Estuaries 23:488-508.

Keister, J. E., E. D. Houde and D. L. Breitburg. 2000. Effects of bottom-layer hypoxia on

abundances and depth distributions of organisms in Patuxent River, Chesapeake Bay. Marine

Ecology Progress Series 205:43-59.

▼72

chapter iii • Dissolved Oxygen Criteria
Kemp, W. M. and W. R. Boynton. 1980. Influence of biological and physical processes on

dissolved oxygen dynamics in an estuarine system: Implications for measurement of community metabolism. Estuarine and Coastal Marine Science 11:407-431.

Kemp, W. M., P. A. Sampou, J. Garber, J. Tuttle and W.R. Boynton. 1992. Seasonal depletion of oxygen from bottom waters of Chesapeake Bay: Roles of benthic and planktonic

respiration and physical exchange processes. Marine Ecology Progress Series 85:137-152.

Kieffer, M. C. and B. Kynard. 1993. Annual movements of shortnose and Atlantic sturgeon

in the Merrimack River, Massachusetts. Transactions of the American Fisheries Society

122:1088-1103.

Kramer, D. L. 1987. Dissolved oxygen and fish behavior. Environmental Biology of Fishes

18:81-92.

Krouse, J. S. 1968. Effects of dissolved oxygen, temperature and salinity on survival of

young striped bass, Roccus saxatilis (Walbaum). Masters thesis, University of Maine, Orono,

Maine.

Kynard, B. 1987. Life History, latitudinal patterns, and status of the shortnose sturgeon,

Acipenser brevirostrum. Environmental Biology of Fishes 48:319-334.

Llanso, R. J. 1992. Effects of hypoxia on estuarine benthos: The lower Rappahannock River

(Chesapeake Bay), a case study. Estuarine, Coastal and Shelf Science 35:491-515.

Llanso, R. J. 1991. Tolerance of low dissolved oxygen and hydrogen sulfide by the polychaete Strepblospio benedicti (Webster). Journal of Experimental Marine Biology and

Ecology 153:165-178.

Llanso, R. J. and R. J. Diaz. 1994. Tolerance to dissolved oxygen by the tubicolous polychaete Loimia medusa. Journal of the Marine Biological Association of the United Kingdom

74:143-148.

Logan, D. T. and C. E. Epifanio. 1978. A laboratory energy balance for the larvae and juveniles of the American Lobster Homarus americanus. Marine Biology 47:381-389.

MacGregor, J. and E. D. Houde. 1996. Onshore-offshore pattern and variability in distribution and abundance of bay anchovy Anchoa mitchilli eggs and larvae in Chesapeake Bay.

Marine Ecology Progress Series 138:15-25.

Magnusson, J., O. Vadstein and G. AErtegjerg. 1998. Critical oxygen levels for demersal

fishes and invertebrates. NIVA Report SNO 3917-98.

Malone, T. C., W. M. Kemp, H. W. Ducklow, W. R. Boynton, J. H. Tuttle and R. B. Jonas.

1986. Lateral variation in the production and fate of phytoplankton in a partially stratified

estuary. Marine Ecology Progress Series 32:149-160.

Malone, T. C., L. H. Crocker, S. E. Pike and B. W. Wendler. 1988. Influences of river flow

on the dynamics of phytoplankton production in a partially stratified estuary. Marine Ecology

Progress Series 48:235-249.

Malone, T. C. 1992. Effects of water column processes on dissolved oxygen: Nutrients,

phytoplankton and zooplankton. In: Smith, D., M. Leffler, G. Mackiernan (eds.) Oxygen

Dynamics in Chesapeake Bay: A Synthesis of Research. University of Maryland Sea Grant

College Publications., College Park, Maryland. Pp. 61-112.

Mangold, M. 2003. Atlantic Sturgeon Reward Program catch data (unpublished), 1994-

March 2003. U.S. Fish and Wildlife Service, Maryland Fisheries Resource Office, Annapolis,

Maryland.

▼73

chapter iii • Dissolved Oxygen Criteria
Marcus, N. H. 2001. Zooplankton: Responses to and consequences of hypoxia. In: Rabelais,

N. N. and R. E. Turner (eds). Coastal hypoxia: Consequences for living resources and ecosystems. Coastal and Estuarine Studies 58. American Geophysical Union, Washington, D. C.

Marcy, D. C., Jr. and P. Jacobson. 1976. Early life history studies of American shad in the

lower Connecticut River and the effects of the Connecticut Yankee plant. American Fisheries

Society Monograms 1:141-168.

Maris, R. C. 1986. Patterns of diurnal vertical distribution and dispersal-recruitment mechanisms of decapod crustacean larvae and post-larvae in the Chesapeake Bay, Virginia and

Adjacent Offshore Waters. Ph.D. dissertation. Old Dominion University, Norfolk, Virginia.

McGovern, J. C. and J. E. Olney. 1996. Factors affecting survival of early life stages and

subsequent recruitment of striped bass on the Pamunkey River, Virginia. Canadian Journal

of Fisheries 53:1713-1726.

McMichael, R. H. and K. M. Peters. 1987. Early life history of the red drum, Sciaenops ocellatus (Pisces: Sciaenidae) in Tampa Bay, Florida. Estuaries 10:92-107.

McNatt, R. A. 2002. Hypoxic-induced growth rate reduction in two juvenile estuarydependent fishes. Masters thesis, North Carolina State University, Raleigh, North Carolina.

Meldrim, J. W., J. J. Gift and B. R. Petrosky. 1974. The effects of temperature and chemical

pollutants on the behavior of several estuarine organisms. Ichthyological Associates Inc.,

Bulletin. No. 11. Middletown, Delaware. 129 pp.

Miller, H.. M. 2001. Living along the “Great Shellfish Bay”: The relationship between

prehistorical peoples and the Chesapeake. In: P. D. Curtin, G. S. Brush, and G. W. Fisher

(eds.) Discovering the Chesapeake: The History of an Ecosystem. The Johns Hopkins

University Press, Baltimore. Pp. 109-126.

Miller, D. C., S. L. Poucher and L. Coiro. 2002. Determination of lethal dissolved oxygen

levels for selected marine and estuarine fishes, crustaceans and a bivalve. Marine Biology

140:287-296.

Miller, J. P., F. R. Griffins and P. A. Thurston-Regoers. 1982. The American shad (Alosa

sapidissima) in the Delaware River basin. U. S. Fish and Wildlife Service, Rosemont, New

Jersey.

Murdy, E. O., R. S. Birdsong and J. A. Musick. 1997. Fishes of Chesapeake Bay. Smithsonian Institution Press, Washington, D.C.

National Marine Fisheries Service. 1998. Recovery plan for shortnose sturgeon (Acipenser

breirostrum). Silver Spring, Maryland.

Newcombe, C. L. and W. A. Horne. 1938. Oxygen-poor waters of the Chesapeake Bay.

Science 88:80- 81.

Newcombe, C. L., W. A. Horne and B. B. Shepherd. 1939. Studies of the physics and chemistry of estuarine waters in Chesapeake Bay. Journal of Marine Research 2(2):87-116.

Niklitschek, E. J. 2001. Bioenergetics modeling and assessment of suitable habitat for juvenile Atlantic and shortnose sturgeons in Chesapeake Bay. Ph.D. thesis. University of

Maryland, College Park, Maryland.

Nixon, S. W. 1988. Physical energy inputs and the comparative ecology of lake and marine

ecosystems. Limnology and Oceanography 33:1005-1025.

North, E. 2001. Ph.D. dissertation, University of Maryland, College Park, Maryland.

▼74

chapter iii • Dissolved Oxygen Criteria
O’Malley, M. and J. Boone. 1972. Oxygen vital to normal hatching and survival in striped

bass. Maryland Fish and Wildlife News 3:2.

Officer, C. B., R. B. Biggs, J. L. Taft, L. E. Cronin, M. A. Tyler and W. R. Boynton. 1984.

Chesapeake Bay anoxia: Origin, development, and significance. Science 223:22-27.

Olney, J. E. 1983. Eggs and early larvae of the bay anchovy, Anchoa mitchilli, and the weakfish, Cynoscion regalis, in lower Chesapeake Bay with notes on associated ichthyoplankton.

Estuaries 6(1):20-35.

Olney, J. E., J. D. Field and J. C. McGovern. 1991. Striped bass egg mortality, production

and female biomass in Virginia rivers, 1980-1989. Transactions of the American Fisheries

Society 120:354-367.

Petersen, J. K. and G. I. Petersen. 1990. Tolerance, behaviour and oxygen consumption in the

sand goby, Pomatochistus minutus (Pallas), exposed to hypoxia. Journal of Fish Biology

37:921-933.

Petersen, J. K. and L. Pihl. 1995. Responses to hypoxia of plaice, Pleuronectes platessa, and

dab, Limanda limanda, in the southeast Kattegat: Distribution and growth. Environmental

Biology of Fisheries. 43:311-321.

Pihl, L., S. P. Baden and R. J. Diaz. 1991. Effects of periodic hypoxia on distribution of

demersal fish and crustaceans. Marine Biology 108:349-360.

Pihl, L., S. P. Gaden and L. C. Schaffner. 1992. Hypoxia-induced structural changes in the

diet of bottom-feeding fish and crustacea. Marine Biology 112:349-361.

Poucher, S. and L. Coiro. 1997. Test reports: Effects of low dissolved oxygen on saltwater

animals. Memorandum to D. C. Miller. U. S. Environmental Protection Agency, Atlantic

Ecology Division, Narragansett, Rhode Island.

Qureshi, N. A. and N. N. Rabalais. 2001. Distribution of zooplankton on a seasonally

hypoxic continental shelf. In: Coastal Hypoxia: Consequences of Living Resources and

Ecosystems. Coastal and Estuarine Studies, American Geophysical Union. Pp. 61-67.

Rabalais, N. N., D. E. Harper and R. E. Turner. 2001. Responses of nekton and demersal and

benthic fauna to decreasing dissolved oxygen concentrations. In: Coastal Hypoxia: Consequences of Living Resources and Ecosystems. Coastal and Estuarine Studies, American

Geophysical Union. Pp. 115-128.

Rilling, G. C. and E. D. Houde. 1999. Regional and temporal variability in distribution and

abundance of bay anchovy (Anchova mitchilli) eggs and larvae in the Chesapeake Bay. Estuaries 22(4):1096-1109.

Rogers, B. A., D. T. Westlin and S. B. Saila. 1980. Development of techniques and methodology for the laboratory culture of striped bass, Morone saxatilis (Welbaum). Report for 

U.S. EPA, National Environmental Research Center, Cincinnati, Ohio. 263 pp.

Roman, M., A. L. Gauzens, W. K. Rhinehart and J. R. White. 1993. Effects of low oxygen

water on Chesapeake Bay zooplankton. Limnology and Oceanography 38:1603-1614.

Rombough, P. J. 1988. Respiratory gas exchange, aerobic metabolism and effects of hypoxia

during early life. In: Hoar, W. S. and D. J. Randall (eds.). Fish Physiology. Vol. XI: The Physiology of Developing Fish, Part A: Eggs and Larvae. Academic press, Inc. San Diego,

California. Pp. 59-161.

Rooker, J. R. and S. A. Holt. 1997. Utilization of subtropical seagrass meadows by newly

settled red drum Sciaenops ocellatus: Patterns of distribution and growth. Marine Ecology

Progress Series 158:139-149.

▼75

chapter iii • Dissolved Oxygen Criteria
Rosenberg, R. 1977. Benthic macrofaunal dynamics, production, and dispersion in an oxygen

deficient estuary of West Sweden. Journal of Experimental Marine Biology and Ecology

26:107-113.

Rothschild, B. J. 1990. Final report. Development of a sampling expert system:

“FISHMAP”. Maryland Department of Natural Resources and U. S. Fish and Wildlife

Service Project No. F171-89-008. University of Maryland CEES Ref. No. (UMCEES) CBL

90-090; Chesapeake Biological Laboratory, Solomons, MD.

Rutherford, E. S. 1992. Relationship of larval-stage growth and mortality to recruitment of

striped bass, Morone saxatilis, in Chesapeake Bay. Ph.D. dissertation, University of Maryland, College Park, Maryland.

Rutherford, E. S. and E. D. Houde. 1995. The influence of temperature on cohort-specific

growth, survival, and recruitment of striped bass, Morone saxatilis, larvae in Chesapeake

Bay. Fisheries Bulletin 93:315-332.

Ryan, E. P. 1956. Observations on the life histories and the distribution of the xanthidae

(mud crabs) of Chesapeake Bay. American Midland Naturalist 56:138-162

Saksena, V. P. and E. B. Joseph. 1972. Dissolved oxygen requirements of newly-hatched

larvae of the striped blenny (Chasmodes bosquianus,) the naked goby (Gobiosoma bosci) and

the skilletfish (Gobiesox strumosus). Chesapeake Science 13:23-28.

Sagasti, A., L. C. Schaffner and J. E. Duffy. 2000. Epifaunal communities thrive in an estuary

with hypoxic episodes. Estuaries 23:474-487.

Sale, J. W. and W. W Skinner. 1917. The vertical distribution of dissolved oxygen and the

precipitation of salt water in certain tidal areas. Franklin Institute Journal 184:837-848.

Sandifer, P. A. and W. A. Van Engel. 1971. Larval development of the spider crab, Libinia

dubia H. Milne Edwards (Brachyura, Majidae, Pisinae) reared in laboratory culture. Chesapeake Science 12(1):18- 25.

Sandifer, P. A. 1973. Distribution and abundance of decapod crustacean larvae in the York

River estuary and adjacent lower Chesapeake Bay. Chesapeake Science 14(4):235-257.

Sanford, L. P., K. Sellner and D. L. Breitburg. 1990. Covariability of dissolved oxygen with

physical processes in the summertime Chesapeake Bay. Journal of Marine Research 48:567-

590.

Sasaki, G. C., J. M. Capuzzo and P. Biesiot. 1986. Nutritional and bioenergetic considerations in the development of the American lobster Homarus americanus. Canadian Journal

of Fisheries and Aquatic Science 43(11):2311-2319.

Sastry, A. N. and J. F. McCarthy. 1973. Diversity in metabolic adaptation of pelagic larval

stages of two sympatric species of brachyuran crabs. Netherlands Journal of Sea Research

7:434-446.

Sastry, A. N. 1977. The larval development of the rock crab, Cancer irroratus, under laboratory conditions (Decapoda brachyura). Crustaceana 32(2):155-168.

Sastry, A. N. 1970. Culture of brachyuran crab larvae using a recirculating sea water system

in the laboratory. Helgoländer Meeresuntersuchungen 20:406-416.

Savoy, T. and D. Shake. 2000. Atlantic sturgeon, Acipenser oxyrinchus, movements and

important habitats in Connecticut waters. Biology, Management, and Protection of Sturgeon

Symposium pre-print. EPRI. Palo Alto, California.

▼76

chapter iii • Dissolved Oxygen Criteria
Scholz, U. and U. Waller. 1992. The oxygen requirements of three fish species from the

German Bight: Cod Gadus morhua, plaice Pleuronectes platessa and dab Limanda limanda.

Journal of Applied Ichthyology 41: 927-934.

Schurmann, H. and J. F. Steffensen. 1992. Lethal oxygen levels at different temperatures and

the preferred temperature during hypoxia of the Atlantic cod, Gadus morhua L. Journal of

Fish Biology 41:927-934.

Secor, D. H. 2003. Review of salinity thresholds for shortnose sturgeon. Technical Report for

Chesapeake Bay Program Dissolved Oxygen Criteria Task Group. Technical Report Series

No. TS-398- 03-CBL. Solomons, Maryland. 5 pp.

Secor, D. H. 2000. Spawning in the nick of time? Effect of adult demographics on spawning

behavior and recruitment of Chesapeake Bay striped bass. ICES Journal of Marine Science

57:403-411.

Secor, D. H. and T. E. Gunderson. 1998. Effects of hypoxia and temperature on survival,

growth and respiration of juvenile Atlantic sturgeon, Acipenser oxyrinchus. Fisheries Bulletin

96:603-613.

Secor, D. H. and E. D. Houde. 1995. Temperature effects on the timing of striped bass egg

production, larval viability, and recruitment potential in the Patuxent River (Chesapeake

Bay). Estuaries 18:527-544.

Secor, D. H. and E. J. Niklitschek. 2003 (In press). Sensitivity of sturgeons to environmental

hypoxia: Physiological and ecological evidence. In: Fish Physiology, Toxicology and Water

Quality– Proceedings of the Sixth International Symposium, La Paz, Mexico, January 22-26,

2001. U. S. EPA Office of Research and Development, Ecosystems Research Division,

Athens, Georgia.

Secor, D. H. and E. J. Niklitschek. 2001. Hypoxia and Sturgeons: Report to the Chesapeake

Bay Program Dissolved Oxygen Criteria Team. University of Maryland Center for Environmental Studies, Chesapeake Biological Laboratory. Technical Report Series No.

TS-314-01-CBL.

Secor, D. H., E. Niklitschek, J. T. Stevenson, T. E. Gunderson, S. Minkkinen, B. Florence,

M. Mangold, J. Skjeveland and A. Henderson-Arzapalo. 2000. Dispersal and growth of yearling Atlantic sturgeon Acipenser oxyinchus released into the Chesapeake Bay. Fisheries

Bulletin 98(4):800-810.

Seliger, H. H., J. A. Boggs and S. H. Biggley. 1985. Catastrophic anoxia in the Chesapeake

Bay in 1984. Science 228:70-73.

Setzler-Hamilton, E. M., W. R. Boynton, J. A. Mihursky, T. T. Polgar and K. V. Wood. 1981.

Spatial and temporal distribution of striped bass eggs, larvae and juveniles in the Potomac

estuary. Transactions of the American Fisheries Society 110:121-136.

Simpson, D. G. 1995. Cooperative interagency resource assessment. In: A Study of Marine

Recreational Fisheries in Connecticut. Federal Aid to Sport Fish Recreation, F54R, final

report. Connecticut Department of Environmental Protection, Bureau of Natural Resources,

Fisheries Division.

Skjeveland, J. E., S. A. Welsh, M. F. Mangold, S. M. Eyler and S. Nachbar. 2000. A Report

of Investigations and Research on Atlantic and Shortnose Sturgeon in Maryland Waters of

Chesapeake Bay (1996-2000). U.S. Fish and Wildlife Service, Annapolis, Maryland.

Smith, D. E., M. Leffler and G. Mackiernan (eds.). 1992. Oxygen Dynamics in the Chesapeake Bay: A Synthesis of Recent Research. Maryland and Virginia Sea Grant College

Program, College Park, Maryland.

▼77

chapter iii • Dissolved Oxygen Criteria
Spells, A. 2003. Atlantic Sturgeon Reward Program catch data (unpublished), 1996. U.S.

Fish and Wildlife Service, Maryland Fisheries Resource Office, Annapolis, Maryland.

Stachowitsch, M. 1984. Mass mortality in the Gulf of Trieste: The course of community

destruction. Marine Ecology 5:243-264.

Stalder, L. C. and N. H. Marcus. 1997. Zooplankton responses to hypoxia: Behavioral

patterns and survival of three species of calanoid copepods. Marine Biology 127:599-607.

Stott, B. and B. R. Buckley. 1979. Avoidance experiments with homing shoals of minnows,

Phoxinus phoxinus in a laboratory stream channel. Journal of Fish Biology 14:135-146.

Sulkin, S. D. and K. Norman. 1976. A comparison of two diets in the laboratory culture of

the zoeal stages of the brachyuran crabs Rhithropanopeus harrissi and Neopanope sp.

Helgol. Meeresunters 28:183-190.

Taft, J. L., W. R. Taylor, E. O. Hartwig and R. Loftus. 1980. Seasonal oxygen depletion in

Chesapeake Bay. Estuaries 3:242-247.

Tagatz, M. E. 1961. Reduced oxygen tolerance and toxicity of petroleum products to juvenile American shad. Chesapeake Science 2:65-71.

Tuttle, J. H., R. B. Jonas and T. C. Malone. 1987. Origin, development and significance of

Chesapeake Bay anoxia. In: S. E. Majumdar, L. W. Hall, Jr. and K. M. Austin (eds.) Contaminant Problems and Management of Living Chesapeake Bay Resources. Pennsylvania

Academy of Science, Philadelphia, Pennsylvania. Pp. 442-472.

Tyler, M. A. 1984. Dye tracing of a subsurface chlorophyll maximum of a red-tide dinoflagellate to surface frontal regions. Marine Biology 78:285-300.

U.S. Environmental Protection Agency (EPA). 1985. Guidelines for Deriving Numerical

National Water Quality Criteria for the Protection of Aquatic Organisms and their Uses.

NTIS Publication No. PB85- 227049. U.S. Environmental Protection Agency, Washington,

D.C.

U.S. EPA. 1986. Ambient Water Quality Criteria for Dissolved Oxygen (Freshwater). EPA

440/5/86- 003. U.S. Environmental Protection Agency, Washington, D. C.

U.S. EPA. 1994. Interim Guidance on Determination and Use of Water-Effect Ratios for

Metals. EPA- 823-B-94-001. Office of Water Office of Science and Technology Washington,

D. C.

U.S. EPA. 1998. A Comprehensive List of Chesapeake Bay Basin Species 1998. EPA 903R98-013. Chesapeake Bay Program Office, Annapolis, Maryland.

U.S. EPA. 2000. Ambient Aquatic Life Water Quality Criteria for Dissolved Oxygen (Saltwater): Cape Cod to Cape Hatteras. EPA-822-R-00-012. Office of Water, Office of Science

and Technology, Washington, D.C. and Office of Research and Development, National

Health and Environmental Effects Research Laboratory, Atlantic Ecology Division, Narragansett, Rhode Island.

U.S. EPA. 2003a. Technical Support Document for Identification of Chesapeake Bay

Designated Uses and Attainability. EPA 903-R-03-004. Chesapeake Bay Program Office,

Annapolis, Maryland.

U.S. EPA 2003b. Biological Evaluation for the Issuance of Ambient Water Quality Criteria

for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and its Tidal

Tributaries–U.S. Environmental Protection Agency, Region III April 2003. Chesapeake Bay

Program Office, Annapolis, Maryland. 

▼78

chapter iii • Dissolved Oxygen Criteria
U.S. EPA, U.S. Fish and Wildlife Service and NOAA National Marine Fisheries Service. In

draft. Biological Evaluation on the CWA 304(a) Aquatic Life Criteria as Part of the National

Consultations Methods Manual.

Ulanowicz, R. E. and T. T. Polgar. 1980. Influences of anadromous spawning behavior and

optimal environmental conditions upon striped bass (Morone saxatilis) year-class success.

Canadian Journal of Fisheries and Aquatic Science 37:143-154.

Vargo, S. L. and A. N. Sastry. 1977. Interspecific differences in tolerance of Eurytemora

affinis and Acartia tonsa from an estuarine anoxic basin to low dissolved oxygen and

hydrogen sulfide. In: McCluskey, D. S. and A. J. Berry (eds.). Physiology and Behavior of

Marine Organisms, Pergamon Press. 12th European Marine Biology Symposium. Pp. 219-

226.

Vismann, B. 1990. Sulfide detoxification and tolerance in Nereis (dediste) diversicolor and

Nereis (neanthes) virens (Annelida: polychaeta). Marine Ecology Progress Series 59:229-238.

Wannamaker, C. M. and J. A. Rice. 2000. Effects of hypoxia on movements and behavior of

selected estuarine organisms from the southeastern United States. Journal of Experimental

Marine Biology and Ecology 249:145-163.

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BACKGROUND

The loss of underwater bay grasses1 from the shallow waters of the Chesapeake Bay,

which was noted in the early 1960s, is a widespread, well-documented problem.

Although other factors, such as climatic events and herbicide toxicity, may have

contributed to the decline of underwater bay grasses in the Bay, the primary causes

are nutrient over-enrichment and increased suspended sediments in the water and the

associated reduction of light. The loss of underwater bay grass beds is of particular

concern because these plants create rich animal habitats that support the growth of

diverse fish and invertebrate populations. Similar declines in underwater bay grasses

have been occurring worldwide with increasing frequency in the past several

decades.

One of the major features contributing to the high productivity of the Chesapeake

Bay has been the historical abundance of underwater bay grasses. There are more

than 20 freshwater and marine species of rooted, submerged flowering plants in

Chesapeake Bay tidal waters. These underwater bay grasses provide food for waterfowl and provide critical habitat for shellfish and fish. Underwater bay grasses also

positively affect nutrient cycling, sediment stability and water turbidity.

The health and survival of these plant communities in the Chesapeake Bay and its

tidal tributaries depend on suitable environmental conditions, which define the

quality of underwater bay grass habitat. The key to restoring these critical habitats

and food sources is to provide the necessary levels of light penetration in shallow

waters to support their survival, growth and repropagation.

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chapteriv

Water Clarity Criteria

1The term underwater bay grasses refers to submerged vascular plants often referenced in the

scientific literature as ‘seagrasses’ as well as submerged aquatic vegetation or SAV, not to be

confused with emergent wetland plants.
APPROACH

The Chesapeake Bay’s scientific and resource management communities collaborated

to produce two internationally recognized technical syntheses of information that

support the quantitative habitat requirements for Chesapeake Bay underwater bay

grasses (Batiuk et al. 1992; Batiuk et al. 2000). Key findings, the underlying light

requirements and management-oriented diagnostic tools and restoration targets have

been reported in the peer-reviewed scientific literature (Dennison et al. 1993;

Gallegos 2001; Koch 2001; Kemp et al., in review). The two technical syntheses,

along with Chesapeake Bay-specific research and field studies and recent model

simulation and data evaluation, provide the scientific foundation for the Chesapeake

Bay water clarity criteria described here. Readers are encouraged to consult these two

syntheses and the resulting published papers for further details and documentation.

The Chesapeake Bay-specific water clarity criteria were derived in four stages: first,

water column-based light requirements for underwater bay grass survival and growth

were determined; second, factors contributing to water-column light attenuation

were quantified; third, contributions from epiphytes to light attenuation at the leaf

surface were factored into methods for estimating and diagnosing the components of

total light attenuation; and fourth, a set of minimal requirements for light penetration

through the water and at the leaf surface were determined to give the water clarity

criteria values.

THE RELATIONSHIPS BETWEEN WATER QUALITY, 

LIGHT AND UNDERWATER BAY GRASSES

The principal relationships between water quality conditions and light regimes for

the growth of underwater bay grasses are illustrated in Figure IV-1. Incident light,

which is partially reflected at the water surface, is attenuated through the water

column above the underwater bay grasses by particulate matter (chlorophyll a and

total suspended solids), by dissolved organic matter and by water itself. In most

estuarine environments, the water-column light attenuation coefficient (called Kd) is

dominated by contributions from chlorophyll a and total suspended solids. 

Light that actually reaches the underwater bay grass leaves also is attenuated by the

epiphytic material (i.e., algae, bacteria, detritus and sediment) that accumulates on

the leaves. This epiphytic light attenuation coefficient (called Ke) increases exponentially with epiphyte biomass, where the slope of this relationship depends on the

composition of the epiphytic material. Dissolved inorganic nitrogen (DIN) and phosphorous (DIP) in the water column stimulate the growth of epiphytic algae (as well

as water-column algae), and suspended solids also can settle onto underwater bay

grass leaves. Because epiphytic algae also require light to grow, water depth and

water-column light attenuation constrain epiphyte accumulation on underwater bay

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chapter iv • Water Clarity Criteria
grass leaves, and light attenuation by epiphytic material depends on the mass of both

algae and total suspended solids settling on the leaves.

An algorithm was developed to compute the biomass of epiphytic algae and other

materials attached to bay grass leaves and to estimate the light attenuation associated

with these materials (Kemp et al., in review; Batiuk et al. 2000). The algorithm was

verified by applying it to Chesapeake Bay water quality monitoring data. The results

of these field verifications are documented in Chapter V, “Epiphyte Contribution to

Light Attenuation at the Leaf Surface,” in Batiuk et al. (2000).

The algorithm uses monitoring data for the water-column light attenuation coefficient (or Secchi depth), total suspended solids, dissolved inorganic nitrogen and

dissolved inorganic phosphorus concentrations to calculate the potential contribution

of epiphytic materials to total light attenuation for bay grasses at a particular depth

(Figure IV-2). Using a set of commonly monitored water quality parameters,

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chapter iv • Water Clarity Criteria

Light Transmission Light Attenuation

Surface

Reflection

Plankton

Chlorophyll a

Total

Suspended

Solids

Water-Column

Light Attenuation

(Kd)

Epiphyte

Light Attenuation

(Ke)

•Water

•Particles

•Color

•Algae

•Detritus

•Sediments

Epiphytes

Light-at-Leaf

Light-through-Water

Underwater Bay Grasses

DIN

DIP

Grazers

Figure IV-1. Availability of light for underwater bay grasses is influenced by water-column and at-the-leaf

surface light attenuation processes. DIN = dissolved inorganic nitrogen and DIP = dissolved inorganic

phosphorus.
attainment of the percent light-through-water (PLW) water clarity criteria (this

chapter) and percent light-at-the-leaf (PLL) diagnostic parameter (Chapter IV) can

be readily determined for any established restoration depth.

DETERMINING LIGHT REQUIREMENTS

Much of the published literature values for underwater bay grass PLW minimum

light requirements were derived from studies of underwater bay grass light requirements in which epiphyte accumulation on plant leaves was not controlled. Therefore,

light measurements in those studies did not account for light attenuation due to

epiphytes on the underwater bay grass leaves themselves. To determine the Chesapeake Bay water clarity criteria necessary to ensure that sufficient light reaches

underwater bay grass leaves at a defined restoration depth, three lines of evidence

were compared:

1. Applied the original 1992 underwater bay grasses habitat requirements parameter values to the new algorithm for calculating PLL (Figure IV-2), for each of

the four salinity regimes;

2. Evaluated the results of light requirement studies from areas with few or no

epiphytes; and

3. Compared median field measurements of the amount of light reaching plants’

leaves (estimated through the PLL algorithm) along gradients of underwater

bay grasses growth observed in the Chesapeake Bay and its tidal tributaries. 

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chapter iv • Water Clarity Criteria

 

Percent Light-through-Water (PLW) Percent Light-at the-Leaf (PLL)

 Inputs

Kd measured directly

 or

Kd calculated from 

 Secchi depth

 Inputs

Kd

Total suspended solids

Dissolved inorganic

 Dissolved inorganic phosphorus

nitrogen

Evaluation

*Ke = Epiphyte attenuation

*Be = Epiphyt e biomass

Evaluation

Water

 Color

Total Suspended

Solids

Algae

PLW

Epiphyte

Attenuation

Leaf Surface

PLL

100% Ambient Light of Water Surface

PLW=100exp(-K d Z)

 Calculation Calculation

PLL=100[exp(-Kd e Z)][exp(-K B )] e

Calculated PLW vs. PLW criteria PLL vs. PLL Diagnostic Requirement 

Figure IV-2. Illustration of the inputs, calculation and evaluation of the two percent-light parameters:

percent light-through-water (PLW) and percent light-at-the-leaf (PLL).
The derived minimum light requirements apply to the bottom sediment surface in

order to accommodate plants with a variety of heights and plants just emerging from

the bottom sediments.

STRENGTHS AND LIMITATIONS OF THE CRITERIA DERIVATION

PROCEDURES

Scientific Syntheses

The water clarity criteria are based on a solid scientific foundation, synthesizing

more than 20 years of Chesapeake Bay research and related worldwide findings. The

criteria address the minimum light requirements of underwater bay grasses through

the water column (this chapter) and a separate diagnostic tool addresses the plants’

minimum light requirements at the leaf surface (Chapter VII), both applied at the

depth of intended restoration necessary to support the designated use for shallowwater habitats (see U.S. EPA 2003).

The methods for determining attainment of the water clarity criteria use the Chesapeake Bay Program’s water quality monitoring data generated across all Bay tidal

waters (see Chapter VI). Management tools for diagnosing the relative contributions

of various sources of light reduction through the water column and at the leaf surface

have been developed in tandem with the PLW criteria values (see Chapter VII). The

scientific basis for the criteria, diagnostic tools and criteria-attainment methodologies have been through independent peer reviews and have been published in

peer-reviewed scientific journals (Dennison et al. 1993; Gallegos 2001; Koch 2001;

Kemp et al., in review).

Light Availability Studies

The minimum light requirements used in deriving the Chesapeake Bay water clarity

criteria were based, in part, on data and models of light availability from freshwater,

estuarine and marine environments. The EPA recognizes that relatively few studies

of underwater bay grass light requirements have been conducted in lower salinity

estuarine habitats. Most of the underwater plant species growing in the Chesapeake

Bay and its tidal tributaries are, however, the same species as those that have been

observed in light requirement studies of lakes, higher salinity estuarine and coastal

marine habitats (see Chapter III and Appendix A in Batiuk et al. 2000). The EPA is

confident that the findings of these lake, estuarine and marine studies are directly

applicable to deriving the Chesapeake Bay water clarity criteria.

Light Requirements for Sparse versus 

Dense UnderwaterBay Grass Beds

The Chesapeake Bay water clarity criteria call for sufficient light to address the

collective minimum light requirements for all these underwater plants’ growth and

reproductive stages. The minimum light requirements of underwater plants in new,

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chapter iv • Water Clarity Criteria
sparse grass beds would be similar to those of individual plants in well-established,

dense underwater bay grass beds. However, since the water clarity criteria were

based in large part on relationships between existing underwater bay grasses and

water quality conditions, the criteria are less likely to protect new or sparse grass

beds, since existing, dense grass beds can directly influence their local water quality

conditions. Water velocities, algal abundance and suspended sediment concentrations decrease inside dense, established underwater grass beds, improving water

clarity compared with adjacent open-water habitats. Established underwater bay

grass beds also are less likely to be affected by yearly fluctuations in water clarity

(Moore et al. 1995; Moore 1996). Additionally, their capacity to produce more abundant seeds and propagules would improve their chances for revegetation (Orth et al.

1994). Unvegetated areas do not have these advantages; therefore, the light requirements for establishing new underwater grass beds are likely going to be greater.

The effect of improved water clarity on the restoration of underwater bay grasses is

demonstrated by the resurgence of 12 underwater bay grass species to the upper tidal

Potomac River by 1983. In the late 1930s, underwater bay grasses had virtually

disappeared from the tidal-fresh Potomac. The decline coincided with nutrient

enrichment, increased algal concentrations and extreme storms (Carter et al.1985;

Rybicki and Carter 1986). Through the 1970s, high nitrogen and phosphorus

concentrations from municipal wastewater treatment plants and loadings from other

point and nonpoint sources fueled frequent algal blooms and decreased water clarity.

Secchi depth measurements between 1978 and 1981 averaged < 0.6 meters over the

growing season (corresponding to less than 9 percent light at the 1-meter depth).

Beginning in the early 1980s, improved treatment plant technologies and a ban on

phosphate detergents led to a reduction of nutrients and suspended solids, which

resulted in a significant improvement to water clarity by 1988. When the growing

season average Secchi depth improved to > 0.9 meters (corresponding to 20 percent

light at 1-meter depth, a value much higher than the PLW criterion of 13 percent),

water clarity had improved enough to spark a resurgence of underwater bay grasses

in the Potomac River tidal-fresh zone (Carter and Rybicki 1994; Carter et al. 1994).

Effective Depth of Photosynthesis/Application 

Depth Relationship

The ‘effective depth’ measures the water-column depth at which the active photosynthetic plant structures are located. For most plants grown from seed or from

underground tubers or rhizomes, minimum light requirements are most crucial for

newly formed leaves shortly after plants emerge from the bottom sediments. Therefore the ‘effective depth’ for newly emerging shoots is the total water depth.

Additionally, although plants in the inner, shallower sections of a bed may extend

toward the water surface, effectively reducing the ‘effective depth’ of water over the

photosynthetic tissue compared to the actual water depth there, plants at the deepest

colonizing edge of the beds are typically very short and sparse. At this point the

‘effective depth’ and the total water depth are again similar. Based on these two

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chapter iv • Water Clarity Criteria
important examples of the process of new bed formation and bed colonization, the

application depth is defined as the total water depth.

Plant Morphology’s Influence on 

Determining Light Requirements

The size of a plant’s reproductive structures and its morphology play key roles in

survival during periods when light levels fall below minimum requirements at watercolumn depths of 1 meter or less. Species that produce large reproductive structures

tolerate periods of poor water clarity better than those with small reproductive structures. Underwater plants that sprout from large reproductive structures (large tubers,

for example) have greater stored energy reserves and, regardless of light levels, may

elongate several decimeters towards the surface where light levels are more

adequate. The reserves alone may provide enough energy to sustain survival for

several weeks (Rybicki and Carter 2002).

If light levels are inadequate for short periods and become adequate thereafter, plants

from large tubers may survive and grow to heights where their minimum light

requirements are met. On the other hand, plants originating from small reproductive

structures (such as small tubers or seeds) have smaller amounts of energy reserves

and little elongation potential, and are more likely to become weak and brittle and to

evanesce. Spring, therefore, is an especially critical period for plants with small

reproductive structures.

Similarly, mature plants that are canopy-formers are more tolerant of poor water

clarity than are meadow-forming species. If minimum light requirements are met at

0.5 meters but not at 1 meter, the taller canopy-formers are more likely to have their

light requirement met than are shorter, meadow-formers growing at the same depth.

The minimum light requirements used in deriving the water clarity criteria are meant

to allow species of all growth types to survive at the desired restoration depth.

Validation of Predicted versus Actual Bay Grass Distribution

Batiuk et al. (2000) documented their validation of the PLL diagnostic requirements

by relating calculated PLL values to field data on underwater bay grass presence

(over a 13-year record) in areas adjacent to water quality monitoring stations. Underwater bay grass presence was categorized as: always abundant (AA), always some

(AS), sometimes none (SN), usually none (UN) and always none (AN). It was

assumed that PLL value would exceed the minimum requirement in the AA areas

and would be approximately equal to the requirement in the AS and SN areas. In

fact, in tidal-fresh and oligohaline waters, the median values of PLL at the 0.5-meter

and 1-meter depths were 5 to 8 percent and 1 to 3 percent in AS and SN areas,

respectively, well below the minimum PLL requirement of 9 percent. The validation

results were much closer in mesohaline and polyhaline waters.

Similar results were found in relating PLW to changes in underwater bay grass

coverage from year to year in tidal-fresh and oligohaline waters (Batiuk et al. 2000).

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chapter iv • Water Clarity Criteria
Positive increases in bay grass coverage occurred even when the median PLW was

considerably less than the minimum requirement at 1 meter (mean low water).

Finally, the authors noted that, based on light requirements alone, underwater bay

grasses often were found at depths greater than the predicted maximum. Clearly, data

must continue to be collected to ensure consistency between predicted and actual

underwater bay grass distribution.

Natural Water Color

Color, listed as ‘dissolved organic matter,’ is one factor that attenuates light (see

Figure IV-1). The quantitative role of color, accounted for directly as a component

of light attenuation in both the PLW criteria and the PLL diagnostic requirement, is

not addressed separately as a criterion, for several reasons. Color data are not

collected in the Chesapeake Bay Water Quality Program. The only color data that

exist for the Chesapeake Bay have been collected by research institutions, with

sporadic spatial and temporal coverage. Color in the Chesapeake Bay’s tidal waters

is largely of natural origin, including the few tributaries on the Eastern Shore in

which dissolved color concentrations are high, such as the Pocomoke River. Some

decline in color might accompany a reduction in chlorophyll a as nutrient inputs are

reduced, but currently there is no way to gauge the probable magnitude of such a

response.

Other Environmental Factors

Although light is the principal factor controlling the distribution of underwater bay

grasses throughout the Chesapeake Bay, other biological, physical, geological and

chemical factors may preclude their growth in particular sites even when minimum

light requirements are met (Livingston et al. 1998). These factors include the availability of propagules (e.g., seeds and vegetative reproductive structures), salinity,

temperature, water depth, tidal range, grazers, suitable sediment quality (organic

content and grain size), sediment nutrients, wave action, current velocity and chemical contaminants (Koch 2001). Some of these factors operate directly on underwater

plants, while others inhibit the interaction of underwater plants and light or their

habitat.

Very high wave energy may prevent bay grasses from becoming established (due to

the drag exerted on the plants and the constant sediment motion), even when the

minimum light requirements are met (Clarke 1987). Waves and tides alter the light

climate by changing the depth of the water through which light passes, and by resuspending bottom sediments, thereby increasing total suspended solids and associated

light attenuation (Koch 2001).

Particle sinking and other sedimentological processes alter the texture, grain-size

distribution and organic content of bottom sediments. These alterations can affect

underwater bay grass growth by modifying the availability of nutrients in the sediments (Barko and Smart 1986) and by producing reduced sulfur compounds that are

toxic to underwater plants (Carlson et al. 1994). In addition, pesticides and other

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chapter iv • Water Clarity Criteria
anthropogenic chemical contaminants tend to inhibit underwater bay grass growth.

An extensive review of the literature has revealed that certain underwater bay grass

species appear to have limited tolerance of certain physical, sedimentological and

chemical variables (Koch 2001).

Attaining the water clarity criteria in a given underwater bay grass growing season

does not guarantee the presence or return of underwater bay grasses, given the environmental factors described above. However, a wealth of scientific evidence

indicates that not attaining the water clarity criteria at the desired restoration depth

will prevent or severely reduce survival and propagation of underwater bay grasses,

regardless of the status of other environmental factors (Dennison et al. 1993).

Areas for Refinement

The process of deriving the water clarity criteria has brought areas requiring further

research and understanding into focus. Particular attention should be paid to the relationships between epiphyte biomass and nutrient concentrations and flux, and

between total suspended solids and the total mass of epiphytic material. Also, a

better understanding of the relationships between water clarity and abundance of

underwater bay grasses in lower salinity areas is needed. In addition, the published

diagnostic PLL algorithm (see Chapter VII) has been documented both to under- and

overestimate epiphyte biomass when compared with field observations. 

Although the second technical synthesis (Batiuk et al. 2000) provided an initial

consideration of physical, geological and chemical requirements for bay grass

habitat, more work is needed to develop physical, geological and chemical measures

of bay grass habitat suitability.

Finally, there is a general need for a better understanding of the minimum light

requirements for the survival and growth of underwater grass species in various

Chesapeake Bay tidal habitats, as well as the influence of other environmental

factors on minimum light requirements. Detailed field and laboratory studies are

needed to develop estimates of the minimum light required by each species, both for

the survival of existing bay grass beds and reestablishment of underwater bay grasses

in unvegetated sites. The area that remains most problematic is minimum light

requirements for turbid, low-salinity habitats (particularly estuarine turbidity

maximum zones) inhabited by canopy-forming plant species. The short-term

temporal applications of the minimum light requirements need further study to determine the critical length of time required for underwater bay grasses to recover after

short periods of extremely low light levels at various stages of the growing season.

The EPA maintains that these water clarity criteria reflect the best available science

compiled and interpreted by recognized national and international scientific experts

in this field. The criteria document recognizes and clearly documents known certainties and uncertainties, and where professional judgments have been exercised. In

cases where such judgments have been made, these judgments have led to the publication of water clarity criteria that protects the full array of underwater bay grass

species inhabiting Chesapeake Bay tidal waters. 

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chapter iv • Water Clarity Criteria
WATER CLARITY CRITERIA DERIVATION

MINIMUM LIGHT REQUIREMENTS

Determining the PLW requirements for bay grass survival and growth involved an

extensive search of the pertinent literature and examination of results from research

and monitoring conducted in the Chesapeake Bay. A detailed documentation of this

process can be found in Chapter III, “Light Requirements for SAV Survival and

Growth” in Batiuk et al. 2000. The authors interpreted the information to determine

the range of light requirements for individuals and groups of species occurring in the

four major salinity zones of the Chesapeake Bay.

They found that the information fell into four general categories: (1) physiological

studies of photosynthesis/irradiance relationships; (2) results of field observations of

the maximum depth of underwater bay grass colonization and available light at that

depth; (3) experiments involving the artificial or natural manipulation of light levels

during long- or short-term growth studies; and (4) statistical models intended to

generalize light requirements. These four categories are discussed in the order of

their perceived utility for the purpose of determining minimum light requirements,

with physiological studies considered the least useful and models and light manipulation experiments considered the most useful. The literature reviewed included lake,

estuary and coastal marine studies throughout the world.

Photosynthesis-Irradiance Measurements

Numerous studies have presented photosynthesis-irradiance curves for underwater

plants. Photosynthesis-irradiance curves are generated by exposing whole plants,

leaves or leaf or stem sections to varying light intensities and measuring the rate of

photosynthesis based on the generation of oxygen or consumption of carbon dioxide.

Most photosynthesis-irradiance measurements are made in the laboratory, although

some studies use ambient light and environmental conditions, with plants suspended

in bottles at different water depths. As suggested by Zimmerman et al. (1989), it is

questionable to use short-term photosynthesis-light experiments to estimate lightgrowth relationships and depth penetration, particularly when plants are not

acclimated to experimental conditions. In addition to the balance between photosynthesis and respiration, estimates of minimum light requirements must consider other

losses of plant organic carbon through herbivory, leaf sloughing and fragmentation

as well as reproductive requirements.

Field Observations of Maximum Depth and Available Light

Numerous studies around the world link observations of the maximum depth to

which an underwater grass species grows (Zmax) to the available light (Im) at that

depth (see Appendix A in Batiuk et al. 2000). Individual maximum-depth-of-

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chapter iv • Water Clarity Criteria
colonization studies were not particularly useful for setting up minimum light

requirements for Chesapeake Bay environments. Most studies were of freshwater

and oligohaline species in freshwater lakes, where the water was clear and the

percent of surface light in midsummer on a clear day was not indicative of the plant’s

seasonal light environment. Determinations were based on the maximum depth at

which the plants were rooted, disregarding chance fragments or propagules that

might have established outlier populations and not survive an entire growing season

(e.g., Moore 1996). Measurement frequency is a significant problem that should be

considered in these studies. However, taken in the aggregate, these field observations

serve as a basis for models that predict maximum depths of colonization or minimum

light requirements (see section titled “Light Availability Models” below). 

Light Manipulation Experiments

Light requirements for the growth and survival of underwater bay grasses have been

tested using short- to long-term studies under experimental light conditions. These

studies were done in situ, in mesocosms where plants receive a measured percentage

of ambient light, or in the laboratory where underwater plants are grown under

constant light and temperature regimes. Most field studies were done using polyhaline and mesohaline species. In the case of prolonged field experiments, recovery of

the plants was sometimes monitored. Some studies did not involve the actual manipulation of light levels; for example, Dunton (1994) involved natural shading by an

algal bloom and continuous monitoring of light in Texas coastal bays, whereas

Kimber et al. (1995) and Agami et al. (1984) suspended plants in buckets at specific

depths and observed survival rates. Laboratory and mesocosm experiments under

controlled light, temperature and flow conditions may substantially underestimate

natural light requirements because of the absence of natural light variability,

herbivory, fragmentation losses and tidal or riverine currents. 

Light Availability Models

In recent years attempts have been made to develop statistical regression models to

quantify the relationship of light availability to the depth of underwater bay grass

growth, based on the maximum depth of colonization and water-column light attenuation (Canfield et al. 1985; Chambers and Kalff 1985; Vant et al. 1986; Duarte

1991; Middleboe and Markager 1997). Models also have been developed to relate

light availability to productivity, primarily in polyhaline species (Zimmerman et al.

1994), and to show the relationships of various factors affecting underwater bay

grass survival (Wetzel and Neckles 1986). Since the models relating depth of colonization and water clarity tend to use large data sets from different habitats, they are

considered more robust than models based on single studies or sites, yet some of the

more robust models still depend on one-time observations at maximum depth or light

availability from the literature.

Figure IV-3 shows a good correspondence among models. For lake species in general,

a depth of 1 meter would be colonized when Secchi depth = 0.4 to 0.7 meters. The

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chapter iv • Water Clarity Criteria
0.4- to 0.7-meter range is comparable with the light constraints

described by Carter and Rybicki in

Batiuk et al. (1992). They

suggested that when median

seasonal Secchi depths were 

0.7 meters, underwater bay grass

beds would increase in size,

whereas at Secchi depths less than 

0.5 meters, revegetation would not

occur. Between 0.5 and 0.7 meters,

other factors, such as epiphyte

loading, available sunshine, size

and the number of tubers set in the

previous year all play a role in

determining survival. 

Models relating maximum depth of

colonization to Secchi depth or

light attenuation and percent of

surface irradiance for mesohaline

and polyhaline species are summarized in Batiuk et al. (2000).

Relationships between maximum

depth of colonization and light

attenuation coefficients indicate

that for any specific light attenuation coefficient the maximum depth

of colonization is greater for tidalfresh and oligohaline species than

for mesohaline and polyhaline

species (see Figure III-4 in Batiuk et al. 2000). These studies indicate that there is a

greater minimum light requirement for mesohaline and polyhaline species.

Examination of the four types of evidence for minimum light requirements discussed

above—photosynthesis-irradiance curves, field observations, light manipulation and

models—indicates that models were the best source of comparative information for

developing minimum light requirements for the Chesapeake Bay (Batiuk et al.

2000). The shading experiments, although they did not help to refine the minimum

light requirements, illuminated the complexity of plant success under reduced light

conditions. Although the published literature did not provide specific numbers for

Chesapeake Bay minimum light requirements, the information was used to guide

decisions and suggest limiting factors.

A considerable fraction of the total studies on light requirements for underwater bay

grasses were done in estuarine environments. Most of these were, however,

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chapter iv • Water Clarity Criteria

Figure IV-3. Relationship of maximum depth of colonization (Zmax)

to Secchi depth for freshwater SAV species as modeled by 

Canfield et al. (1985), Chambers and Kaff (1985), Duarte 

and Kalff (1987), Middleboe and Markager (1997) and 

Vant et al. (1986).
conducted in higher salinity mesohaline and polyhaline habitat areas, and virtually

none are from lower salinity oligohaline and tidal-fresh portions of estuaries. The

EPA recognizes that there is a need for continued research to improve the understanding of light requirements for underwater plants in these environments.

Although results of these studies would certainly help to refine detailed knowledge

of underwater bay grass light requirements and of how to apply these to predict plant

survival in nature, the EPA is confident they will not change the broad foundations

of the water clarity criteria. 

The present criteria are based on studies involving virtually all of the important

underwater bay grass species found in the Chesapeake Bay. Healthy populations of

the two seagrasses found in the Bay, Zostera marina and Ruppia maritima, have

been studied in environments of widely varying salinity. On the other hand, lowsalinity regions of the upper Chesapeake Bay and its tributaries have historically

provided habitat for many freshwater species that tolerate brackish conditions. There

is no evidence that the light requirements for these species would be radically

different in freshwater versus low-salinity estuarine habitats.

Chesapeake Bay Research and Monitoring Findings

Research and monitoring results from the Chesapeake Bay also contributed to the

derivation of the minimum light requirements, especially in tidal-fresh and oligohaline waters where limited scientific literature existed. Batiuk et al. (1992) established

PLW requirements by salinity regime for the restoration of underwater bay grasses

to a depth of 1 meter throughout the Chesapeake Bay: Kd = 2 m-1 in tidal-fresh and

oligohaline regimes and Kd = 1.5 m-1 in mesohaline and polyhaline segments. Light

attenuation coefficients are calculated using Beer’s Law Iz = Ioexp(-KdZ), where Io

is light (photosynthetically active radiation [PAR]) measured just below the surface

and Iz is light measured at depth Z. Using the relationship

PLW = 100exp(-KdZ) (Equation IV-1)

where Z = depth in the water column, and setting Z = 1 meter, the Chesapeake Bay

minimum seasonal percent light requirement as published in Batiuk et al. (1992) was

13.5 percent of ambient surface light in tidal-fresh and oligohaline environments and

22.3 percent of ambient surface light in mesohaline and polyhaline environments.

More specific seasonal criteria were suggested by Carter and Rybicki in Batiuk et al.

(1992) for the tidal Potomac River and estuary: Kd = 2.2 m-1 in tidal-fresh regions

and Kd = 2.7 m-1 in oligohaline regions, which translated into PLW requirements of

11 percent in tidal-fresh and 7 percent in oligohaline habitats.

Tidal-Fresh/Oligohaline Potomac River Findings. From 1983 through 1996,

underwater bay grass coverage in the tidal Potomac River varied greatly in both the

tidal-fresh and oligohaline reaches. The change in underwater bay grass coverage

from the previous year and the median PLW calculated from growing season Secchi

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chapter iv • Water Clarity Criteria
depth varied greatly, but both exhibited a general downward trend during this period.

When the change in underwater bay grass coverage from the previous year is plotted

against the median PLW at 1 meter during the underwater bay grass growing season

(April 1 through October 31), underwater bay grasses increased with increases in the

PLW. When median PLW was greater than 13 percent, underwater bay grass coverage

showed only positive increases over three years. However, positive increases occurred

even in years when median percent light at 1 meter was considerably less than 13

percent, indicating that other factors besides light also influence changes in coverage, or

that underwater bay grasses were growing at depths < 1 meter. 

A median growing season PLW of 13 percent at 1 meter is equivalent to a median

Secchi depth of 0.7 meters or median Kd =2.07, assuming Kd = 1.45/Secchi depth.

Secchi depth is only reported to 0.1 meters, so the error in the median measurements

is ± 0.05 meter, median seasonal Secchi depth ranges from 0.65 to 0.75 meters and,

therefore, Kd ranges from 1.93 to 2.23 meter-1. Carter, Rybicki and Landwehr

reported in Batiuk et al. (2000) that for the tidal-fresh and oligohaline segments of the

Potomac River, a corresponding range of PLW of 11 percent to 14.5 percent presented

a boundary condition for a net increase in growth from year to year. It should also be

noted that if other habitat conditions are favorable, underwater bay grasses may

tolerate worse light conditions for a season, but not on a protracted basis.

Tidal-Fresh Patuxent River Findings. Between 1985 and 1996, light conditions

at the tidal-fresh Patuxent River monitoring station PXT0402 (or TFI.5) improved.

Kd dropped from 6 meter-1 to about 4 meter-1 (Naylor, unpublished data reported in

Batiuk et al. 2000) and average Secchi depth increased from 0.25 to 0.4 meters.

During the last four years of this period, colonization by underwater bay grasses also

increased, primarily in the shallow areas less than 0.5 meters deep. A Kd of 4 meter1 results in 13.5 percent light at a depth of 0.5 meters. A second Patuxent River

tidal-fresh water quality monitoring station (PXT0456 or TFI.4) also showed a

significant increase in Secchi depth during the underwater bay grass growing season

of this same period.

It appears that when the seasonal Secchi depth at monitoring station PXT0456 was

greater than a threshold value of 0.35 meters, the underwater bay grass coverage

continued to increase, whereas a Secchi depth below 0.35 meters coincided with a

decrease in underwater bay grass coverage. A Secchi depth threshold of 0.35 meters

for plants colonizing a depth of less than 0.5 meters is equivalent to a 0.68-meter

Secchi depth threshold for plants colonizing a depth of less than 1 meter (as seen in

the Potomac). Thus, it appears that similar threshold light conditions are required for

successful recolonization in the tidal-fresh areas of both the Potomac and Patuxent

rivers (Batiuk et al. 2000).

Mesohaline Potomac River Findings. In the mesohaline segment of the

Potomac River, underwater bay grasses have continued to increase steadily since

1983, although the coverage remains relatively small compared to pre-1960 conditions. Colonization by underwater bay grasses has taken place primarily in areas less

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chapter iv • Water Clarity Criteria
than 1 meter deep. Midchannel light conditions are better in the mesohaline segment

of the river compared to either the tidal-fresh or oligohaline segments, with the

median seasonal Secchi depth generally never dropping below 1 meter for the period

of 1983 through 1996. Secchi depth is only reported to 0.1 meters, so the error in the

median measurements is at least ±0.05 meters. If median Secchi depth is 1 meter,

then using a conversion factor of 1.45 to calculate Kd median light conditions of 23.5

percent at 1-meter depth, with a range of 21.7 percent to 25.1 percent (Batiuk et al.

2000). Thus, the Chesapeake Bay water-column light requirements published previously by Batiuk et al. (1992) for mesohaline and polyhaline segments are consistent

with those observed in the mesohaline region of the Potomac River where underwater bay grasses are recovering.

Mesohaline/Polyhaline York River Findings. Strong positive relationships

between water clarity and the maximum depth of the growth of underwater plants have

been demonstrated (Dennison et al. 1993; Duarte 1991; Olesen 1996). Assuming that

a light requirement of approximately 22 percent of surface irradiance at the sediment

surface is necessary for the long-term growth and survival of underwater bay grasses

in high salinity regions of the Chesapeake Bay (Batiuk et al. 2000), the presence of

underwater bay grasses to a depth of 1 to 1.5 meters below mean low water in this

region would require light-attenuation coefficients of approximately 1 meter-1 or 0.7

meter-1, respectively. In the high mesohaline and polyhaline reaches of the lower York

River, field measurements of Kd have yielded long-term median values of 1 meter-1 in

the shallow littoral zone where underwater bay grasses have been consistently growing

down to depths of 1 meter (Moore 1996; Moore et al. 2001).

LIGHT-THROUGH-WATER REQUIREMENTS

Based on a thorough review of the results of shading experiments and model findings published in the scientific literature, a PLW value of greater than 20 percent is

needed for the minimum light requirement of Chesapeake Bay polyhaline and mesohaline species (Batiuk et al. 2000). Consistent with the value derived from the

scientific literature, the PLW requirement of 22 percent was determined for mesohaline and polyhaline regions of the Chesapeake Bay and its tidal tributaries by

applying the appropriate 1992 underwater bay grass habitat requirement for Kd of

1.5 meter-1 to Equation IV-1 (Batiuk et al. 1992). This PLW requirement was

confirmed by almost two decades of field observations in the mesohaline Potomac

River and mesohaline/ polyhaline York River (Batiuk et al. 1992, 2000; Moore 1996;

Moore et al. 2001) as discussed above.

Based on published model findings reviewed in detail by Carter, Rybicki and

Landwehr in Batiuk et al. (2000) and confirmed by a review of the results of recent

tidal Potomac and Patuxent River research and monitoring studies (see above), a

PLW requirement of 13 percent was determined to apply to Chesapeake Bay tidalfresh and oligohaline species. This light requirement was calculated using Equation

IV-1 and the appropriate 1992 SAV habitat requirement for Kd of 2 meter-1 (Batiuk

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chapter iv • Water Clarity Criteria
et al. 1992). The PLW requirement also is consistent with the 13.5 percent value

published by Dennison et al. (1993).

These PLW requirements were validated through a comprehensive analysis of 13

years (1985–1998) of Chesapeake Bay water quality monitoring data. The results

were published in Chapter VII of Batiuk et al. (2000).

CHESAPEAKE BAY WATER CLARITY CRITERIA

The Chesapeake Bay water clarity criteria are summarized in Table IV-1 as PLW and

Secchi depth equivalents over a range of application depths. They reflect a set of

minimum light requirements to protect underwater bay grass species found in the

two sets of salinity regimes, that have different growth and reproductive strategies

and individual light requirements. The water clarity criteria were derived to support

the propagation and growth of a wide variety of species, including meadow formers

and perennials, not just canopy formers and annuals. In tidal-fresh and oligohaline

habitats, the water clarity criteria call for sufficient light to address the minimum

requirements of meadow-forming species (e.g., Vallisneria americana, or wild

celery), which generally need more light, as well as canopy-forming species (e.g.,

Myriophyllum spicatum, or milfoil), which require less. Water clarity criteria applicable to mesohaline and polyhaline habitats call for light conditions necessary for

the survival and growth of the two principal species—widgeon grass (Ruppia

maritima) and eelgrass (Zostera marina)—inhabiting the more saline shallow-water

habitats of Chesapeake Bay and its tidal tributaries.

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chapter iv • Water Clarity Criteria

Table IV-1. Summary of Chesapeake Bay water clarity criteria for application to shallow-water bay 

grass designated use habitats (application depths given in 0.25 meter depth intervals.2

Water Clarity Criteria as Secchi Depth

Water Clarity Criteria Application Depths

0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0

 Salinity

Regime

Water Clarity

Criteria as

Percent Lightthrough-Water

Secchi Depth (meters) for above Criteria Application Depth

Temporal

 Application

Tidal-fresh 13 % 0.2 0.4 0.5 0.7 0.9 1.1 1.2 1.4 April 1 - October 31

Oligohaline 13 % 0.2 0.4 0.5 0.7 0.9 1.1 1.2 1.4 April 1 - October 31

Mesohaline 22 % 0.2 0.5 0.7 1.0 1.2 1.4 1.7 1.9 April 1 - October 31

Polyhaline 22 % 0.2 0.5 0.7 1.0 1.2 1.4 1.7 1.9 March 1 - May 31,

September 1 - November 30

2Base on application of Equation IV-1, PLW = 100exp(-KdZ), the appropriate PLW criterion value and the selected

application depth are inserted and the equation is solved for Kd. The generated Kd value is then converted to Secchi depth 

(in meters) using the conversion factor Kd = 1.45/Secchi depth.

1.1
For these reasons, these Chesapeake Bay water clarity criteria, along with the appropriate dissolved oxygen and chlorophyll a criteria, fully support the “survival,

growth and propagation of rooted underwater bay grasses necessary for the propagation and growth of balanced, indigenous populations of ecologically, recreationally

and commercially important fish and shellfish inhabiting vegetated shallow-water

habitats” (Appendix A; U.S. EPA 2003). 

When these water clarity criteria were derived, there was an insufficient scientific

basis for deriving a set of water clarity or related (e.g., total suspended solids) criteria

for protection of open-water designated use habitats. The EPA will derive and

publish criteria addressing water clarity-related impairments for open-water habitat

when the necessary scientific data becomes available.

LITERATURE CITED

Agami, M., S. Beer and Y. Waisel. 1984. Seasonal variations in the growth capacity of Najas

marina L. as a function of various water depths at the Yarkon Springs, Israel. Aquatic Botany

19:45-51. 

Batiuk, R. A., P. Bergstrom, M. Kemp, E. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V.

Carter, N. B. Rybicki, J. M. Landwehr, C. Gallegos, L. Karrh, M. Naylor, D. Wilcox, K. A.

Moore, S. Ailstock and M. Teichberg. 2000. Chesapeake Bay Submerged Aquatic Vegetation

Water Quality and Habitat-Based Requirements and Restoration Targets: A Second Technical

Synthesis. CBP/TRS 245/00 EPA 903-R-00-014. U.S. EPA Chesapeake Bay Program,

Annapolis, Maryland.

Batiuk, R. A., R. Orth, K. Moore, J. C. Stevenson, W. Dennison, L. Staver, V. Carter, N. B.

Rybicki, R. Hickman, S. Kollar and S. Bieber. 1992. Chesapeake Bay Submerged Aquatic

Vegetation Habitat Requirements and Restoration Targets: A Technical Synthesis. CBP/TRS

83/92. U.S. EPA Chesapeake Bay Program, Annapolis, Maryland.

Barko, J. W. and R. M. Smart. 1986. Sediment-related mechanisms of growth limitation in

submersed macrophytes. Ecology 67:1328-1340.

Canfield, E. D. Jr., K. A. Langeland, S. B. Linda and W. T. Haller. 1985. Relations between

water transparency and maximum depth of macrophyte colonization in lakes. Journal of

Aquatic Plant Management 23:25-28.

Carlson, P. R., L. A. Yarbro and T. R. Barber. 1994. Relationship of sediment sulfide to

mortality of Thalassia testudinum in Florida Bay. Bulletin of Marine Science 54:733-746.

Carter, V., Rybicki, N. B., Landwehr, J. M., and Turtora, M.. 1994. Role of weather and water

quality in population dynamics of submersed macrophytes in the tidal Potomac River. Estuaries 17(2):417-426.

Carter, V., Paschal, J. E., Jr., and Rybicki (Bartow), N. 1985. Distribution and abundance of

submersed aquatic vegetation in the tidal Potomac River and Estuary, Maryland and Virginia,

May 1978 to November 1981. U.S. Geological Survey Water Supply Paper 2234A. 46 pp.

Carter, V., and Rybicki, N. B. 1994. Invasions and declines of submersed macrophytes in the

tidal Potomac River and Estuary, the Currituck Sound-Back Bay system, and the Pamlico

River Estuary. Lake and Reservoir Management 10(1):39-48.

▼97

chapter iv • Water Clarity Criteria
Chambers, P. A. and J. Kalff. 1985. Depth distribution and biomass of submersed aquatic

macrophyte communities in relation to Secchi depth. Canadian Journal of Fisheries and

Aquatic Science 42:701-709.

Clarke, S. M. 1987. Seagrass-sediment dynamics in Holdfast Bay: Summary. Safish 11:4-10.

Czerny, A. B. and K. H. Dunton. 1995. The effects of in situ light reduction on the growth of

two subtropical seagrasses, Thalassia testudinum and Halodule wrightii. Estuaries 18:418-

427.

Dennison, W. C., R. J. Orth, K. A. Moore, J. C. Stevenson, V. Carter, S. Kollar, P. W.

Bergstrom and R. A. Batiuk. 1993. Assessing water quality with submersed aquatic vegetation habitat requirements as barometers of Chesapeake Bay health. Bioscience 43:86-94.

Duarte, C. M. 1991. Seagrass depth limits. Aquatic Botany 40:363-377.

Dunton, K. H. 1994. Seasonal growth and biomass of the subtropical seagrass Halodule

wrightii in relation to continuous measurements of underwater irradiance. Marine Biology

120:479-489.

Gallegos, C. L. 2001. Calculating optical water quality targets to restore and protect

submersed aquatic vegetation: Overcoming problems in partitioning the diffuse attenuation

coefficient for photosynthetically active radiation. Estuaries 24:381-397.

Kemp, W. M., R. A. Batiuk, R. Bartleson, P. Bergstrom, V. Carter, C. L. Gallegos, W. Hunley,

L. Karrh, E. Koch, J. M. Landwehr, K. A. Moore, L. Murray, M. Naylor, N. B. Rybicki, J. C.

Stevenson, and D. J. Wilcox. In review. Habitat requirements for submerged aquatic vegetation in Chesapeake Bay: Water quality, light regime and physical-chemical factors. Estuaries.

Kimber, A., J. L. Owens and W. G. Crumpton. 1995. Light availability and growth of wild

celery (Vallisneria americana) in upper Mississippi River backwaters. Regulated Rivers:

Research and Management 11:167-174.

Koch, E. W. 2001. Beyond light: Physical, geological and geochemical parameters as

possible submersed aquatic vegetation habitat requirements. Estuaries 24:1-17.

Livingston, R. J., S. E. McGlynn and X. Niu. 1998. Factors controlling seagrass growth in a

gulf coastal system: Water and sediment quality and light. Aquatic Botany 60:135-159.

Middleboe, A. L. and S. Markager. 1997. Depth limits and minimum light requirements of

freshwater macrophytes. Freshwater Biology 37:553-568.

Moore, K., D. Wilcox and B. Anderson. 2001. Analysis of historical distribution of

submerged aquatic vegetation (SAV) in the York and Rappahannock rivers as evidence of

historical water quality conditions. Special Report No. 375 in Applied Marine Science and

Ocean Engineering Virginia Institute of Marine Science, School of Marine Science, College

of William and Mary, Gloucester Point, Virginia.

Moore K. A. 1996. Relationships between seagrass growth and survival and environmental

conditions in a lower Chesapeake Bay tributary. Ph.D. dissertation. University of Maryland.

College Park, Maryland. 188 pp.

Moore, K.A., J. L. Goodman, J. C. Stevenson, L. Murray and K. Sundberg. 1995. Chesapeake Bay nutrients, light and SAV: relations between variable water quality and SAV in field

and mesocosm studies. CB003909-02. Chesapeake Bay Program Office, Annapolis, Maryland. 106 pp.

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chapter iv • Water Clarity Criteria
Olesen, B. 1996. Regulation of light attenuation and eelgrass Zostera marina depth distribution in a Danish embayment. Marine Ecology Progress Series 134:187-194.

Orth, R. J., M. Lukenback and K. A. Moore. 1994. Seed dispersal in a marine macrophyte:

Implications for colonization and restoration. Ecology 75(7):1927-1939.

Rybicki, N. B. and V. Carter. 2002. Light and temperature effects on the growth of wild celery

and hydrilla. Journal of Aquatic Plant Management 40:92-99.

Rybicki, N. B. and V. Carter. 1986. Effects of sediment depth and sediment type on the

survival of Vallisneria americana grown from tubers. Aquatic Botany 26:307-323.

U.S. Environmental Protection Agency. 2003. Technical support document for identifying

Chesapeake Bay designated uses and attainability. EPA 903-R-03-004. Chesapeake Bay

Program Office, Annapolis, Maryland.

Vant, W. N., R. J. Davies-Colley, J. S. Clayton and B. T. Coffey. 1986. Macrophyte depth

limits in North Island (New Zealand) lakes of differing clarity. Hydrobiologia 137:55-60.

Wetzel, R. L. and H. A. Neckles. 1986. A model of Zostera marina L. photosynthesis and

growth: Simulated effects of selected physical-chemical variables and biological interactions.

Aquatic Botany 26:307-323.

Zimmerman, R. C., A. Cabello-Pasini and R. S. Alberte. 1994. Modeling daily production of

aquatic macrophytes from irradiance measurements: A comparative analysis. Marine

Ecology Progress Series 114:185-196.

Zimmerman, R. C., R. D. Smith and R. S. Alberte. 1989. Thermal acclimation and wholeplant carbon balance in Zostera marina L. (eelgrass). Journal of Experimental Biology and

Ecology 130:93-109.

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chapter iv • Water Clarity Criteria
BACKGROUND

Phytoplankton are small microscopic plants, or algae, drifting in the water column

with the currents. They constitute a diverse group that contributes importantly to the

base of the Chesapeake Bay’s food web, linking nutrients and the energy of sunlight

with small planktonic animals or zooplankton, forage fish, filter feeders such as

oysters, bottom-dwelling worms and clams and fishes (Bay and Horowitz 1983;

Tuttle et al. 1987; Malone et al. 1986; Heck 1987; Malone et al. 1988). The majority

of the Chesapeake Bay’s animals feed directly on phytoplankton or secondarily on the

products of phytoplankton that support the ‘microbial loop’ (such as nonphotosynthetic flagellates, protozoa, bacteria and fungi), all of which support higher trophic

levels. The Chesapeake Bay’s ‘carrying capacity’ or its ability to support productive

and diverse populations of flora and fauna, including highly valued species, depends

largely on how well phytoplankton meet the nutritional needs both in quantity and

quality of the various consumers.

SCOPE AND MAGNITUDE OF 

NUTRIENT ENRICHMENT IN CHESAPEAKE BAY

Problems caused by nutrient over-enrichment are perhaps the longest-standing water

quality issues created by people (Vollenweider 1992). Early marine scientists considered nutrients as a resource, not a problem (Brandt 1901) and considered ways to

fertilize coastal seas to increase fisheries production. However, this was before

human populations and land use activities to support these bourgeoning populations

had reached today’s levels, especially since about the 1960s. The problem is especially challenging in the Chesapeake Bay ecosystem because the Bay ecosystem’s

variable dynamics produce large, natural fluctuations. Superimposed onto these

natural changes are those caused by human disturbance, and nutrient enrichment is

only one among many other pressures experienced by the Bay ecosystem (Breitburg

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chapterv

Chlorophyll a Criteria
et al. 1999). The scientific challenge persists because human disturbance often is

subtle, indirect and sometimes is confounded by natural changes (Cloern 1996) that

are not yet understood enough for predictive purposes. Anthropogenic nutrient

enrichment of rivers—which deliver much of their nutrient loads to estuaries and

shelf waters—has resulted, in the U.S. in nitrogen fluctuations 5 to 14 times greater

than natural rates (Jaworski et al. 1997). Phosphorus loading to estuarine systems has

increased two- to sixfold since 1900 (Conley 2000).

Nutrient over-enrichment can cause ecological symptoms in the Chesapeake Bay

that impair designated uses, as defined by the Clean Water Act. Nutrient enrichment

and changes in important grazer populations such as oysters, menhaden,

zooplankton and benthic macroinvertebrates have potentially altered the natural

equilibrium between phytoplankton production and consumption in the last century

(Kennedy and Breisch 1981; Boynton et al. 1982; Officer et al. 1984; Marshall and

Lacouture 1986; Nixon et al. 1986; Gerritsen et al. 1988; Newell 1988; Verity 1987;

Malone et al. 1991; Malone 1992; Gerritsen et al. 1994; Hartman and Brandt 1995;

and Kemp et al.1997). Phytoplankton populations currently reach very high concentrations (Filardo and Dunstan 1985; Boynton et al. 1982; Sellner et al. 1986;

Magnien et al. 1992; Malone 1992; Haas and Wetzel 1993; Lacouture et al. 1993;

Harding 1994; Glibert et al. 1995) and high production rates during the spring and

summer (Sellner et al. 1986; Magnien et al. 1992; Lacouture et al. 1993; Marshall

and Nesius 1996; Sin et al. 1999). Phytoplankton communities also are capable of

supporting several potentially toxic taxa (Seliger et al. 1975; Ho and Zubkoff 1979;

Luckenbach et al. 1993; Lewitus et al. 1995; Marshall 1995; Glibert et al. 2001). 

Excess, uneaten phytoplankton accumulate in the water column and contribute to

reduced water clarity and summer oxygen depletion in bottom waters, ultimately

stressing the food webs they support (Neilson and Cronin 1981; Boynton et al. 1982;

Harding et al. 1986; Seliger et al. 1985; Fisher et al. 1988; Malone 1992). Nutrient

enrichment had already affected underwater bay grass distributions throughout much

of the Chesapeake Bay by the early 1960s (Flemer et al. 1983; Orth and Moore

1983) and deep-channel hypoxia and anoxia has been confirmed to have been initiated during the early 1970s (Hagy 2002). Local nutrient over-enrichment problems

occurred earlier in some Bay tidal tributaries; massive blue-green algae blooms in

the upper tidal freshwater Potomac River Estuary began during the 1950s (Jaworski

et al. 1972), and Baltimore Harbor experienced a widening hypoxia problem wellestablished by the mid-1800s (Capper et al. 1983). 

CHLOROPHYLL A: KEY INDICATOR OF PHYTOPLANKTON BIOMASS

Scientific interest and practical management needs required that the quantity of

phytoplankton biomass in aquatic ecosystems be simply measured as an indicator of

water quality and ecosystem health. It was discovered many decades ago that chlorophyll a, a ubiquitous photosynthetic pigment often associated with other pigments in

freshwater and coastal marine phytoplankton, would serve as a useful indicator for

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chapter v • Chlorophyll a Criteria
both the photosynthetic potential and biomass of phytoplankton (Flemer 1970).

Thus, over the years, chlorophyll a has become a principal measure of the amount of

phytoplankton present in a water body. Chlorophyll a also plays a direct role in

reducing light penetration (Lorenzen 1972). Relatively rapid methods evolved to

measure the concentration of chlorophyll a in discrete water samples and in vivo

(Flemer 1969; U.S. EPA 1997). Methods have been developed to measure chlorophyll a using aerial surveillance techniques based on passive multispectral signals

associated with phytoplankton (Harding 1992). As Harding and Perry (1997) wrote,

“Chlorophyll a is a useful expression of phytoplankton biomass and is arguably the

single most responsive indicator of N [nitrogen] and P [phosphorus] enrichment in

this system [Chesapeake Bay].”

Compelling evidence indicates that reduced water clarity and low dissolved oxygen

conditions improve when excess phytoplankton or blooms, measured as chlorophyll

a, are substantially reduced (National Research Council 2001). Improvement in

water clarity is a major issue for the recovery of the Bay’s shallow-water underwater

grasses (see Chapter IV); correcting the low dissolved oxygen problems that occur

in the deeper waters of the mesohaline mainstem Chesapeake Bay and lower tidal

tributaries has been a challenge to Chesapeake Bay restoration for decades (see

Chapter III). High algal biomass present in small embayments may be associated

with super-saturated dissolved oxygen conditions during the day and hypoxic to

anoxic conditions during the early morning hours (D’Avanzo and Kremer 1994).

Attaining the Chesapeake Bay dissolved oxygen and water clarity criteria will

require reductions in chlorophyll a concentrations by reducing nutrient (yielding

nutrient limitation) and sediment (resulting in light saturation) loadings. 

In addition to the habitats described above that require chlorophyll a criteria, other

locations in Chesapeake Bay tidal waters experience phytoplankton blooms that may

not be directly associated with low dissolved oxygen and the shading of underwater

bay grasses due to phytoplankton. Numerous small shallow-water embayments

continue to experience inordinately high chlorophyll a concentrations. Some of these

habitats may experience early-morning hypoxia or anoxia, while others may not

have contained documented growth of underwater bay grasses before the baywide

decline. In some parts of the Chesapeake Bay and its tidal tributaries, even reducing

nutrient and sediment loadings to levels that would result in attaining the deep-water

and deep-channel dissolved oxygen and shallow-water clarity criteria will not

prevent harmful algal blooms or ensure the return of high quality food to open-water

habitats. These areas include, but are not limited to, those without low oxygen conditions for hydrologic reasons (e.g., high mixing rates) and those in which reduced

water clarity conditions are driven more by suspended sediments than by watercolumn algae. For these reasons, the EPA believes it is necessary to develop and

adopt chlorophyll a criteria in addition to water clarity and dissolved oxygen criteria

for the protection of Chesapeake Bay tidal waters.

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chapter v • Chlorophyll a Criteria
CHESAPEAKE BAY CHLOROPHYLL A CRITERIA

This chapter presents the EPA’s recommended narrative chlorophyll a criteria, along

with supporting numeric concentrations and methodological approaches to

addressing nutrient-enrichment impairments related to the overabundance of algal

biomass measured as chlorophyll a. The EPA expects states to adopt narrative

chlorophyll a criteria into their water quality standards for all Chesapeake Bay and

tidal tributary waters. The EPA strongly encourages states to develop and adopt sitespecific numerical chlorophyll a criteria for tidal waters where algal-related

impairments are expected to persist even after the Chesapeake Bay dissolved oxygen

and water clarity criteria have been attained.

The narrative chlorophyll a criteria in Table V-1, derived in part through a review of

other states’ chlorophyll a water quality standards (Appendix D), are recommended

for encompassing the full array of possible impairments, all of which may not manifest themselves within a particular water body at any one time. The site-specific

nature of impairments caused by the overabundance of algal biomass supports state

adoption of the EPA-recommended narrative criteria, with application of site-specific

numeric criteria for localized waters addressing local algal-related impairments. 

Because of the regional and site-specific nature of algal-related water quality impairments, baywide numerical criteria have not been published here. Therefore, the

chlorophyll a concentrations tabulated in this document are not numerical EPA

criteria. Along with the documented methodologies, the tabulated chlorophyll a

concentrations are provided as a synthesis of the best available technical information

for the states consideration and use in their development and adoption of more

regional and site-specific numerical chlorophyll a criteria. States can use this

information in deriving numerical translators for their narrative criteria, and use

these for their narrative criteria, target chlorophyll a concentrations in concert with

narrative criteria. 

Several different approaches were evaluated to develop relationships among chlorophyll a concentrations and tidal-water designated uses. The states also should

consider the strengths and limitations of each approach, as well as other available

scientific and technical information, when deriving site-specific numerical chlorophyll a criteria or numerical translators for their narrative criteria.

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chapter v • Chlorophyll a Criteria

Table V-1. Recommended Chesapeake Bay chlorophyll a narrative criteria.

Concentrations of chlorophyll a in free-floating microscopic aquatic plants (algae) shall not exceed

levels that result in ecologically undesirable consequences—such as reduced water clarity, low

dissolved oxygen, food supply imbalances, proliferation of species deemed potentially harmful to

aquatic life or humans or aesthetically objectionable conditions—or otherwise render tidal waters

unsuitable for designated uses. 
SUPPORTING TECHNICAL INFORMATION 

AND METHODOLOGIES

Algae play a unique role at the base of the aquatic food web. The size and composition of the phytoplankton community strike a delicate balance between supporting a

balanced, productive ecosystem and fueling severe impairments of water quality and

natural ecological relationships. Given that an overabundance or a shift in species

composition can yield diverse negative ecological consequences, the supporting

chlorophyll a concentrations and methodologies have been structured to characterize

an array of ecological conditions. They are based on decades of historical observations; scientific findings published in the international, peer-reviewed literature; field

and laboratory experiments; historic Chesapeake Bay water quality data; and extensive Chesapeake Bay-specific research, monitoring and modeling.

CONTEXT FOR THE NARRATIVE 

CHESAPEAKE BAY CHLOROPHYLL A CRITERIA

To interpret the narrative chlorophyll a criteria that will protect the designated uses

of the Chesapeake Bay and its tributaries, various ecological conditions must be

considered and different water quality impairments should be addressed. Table V-2

presents various water quality conditions along the continuum of trophic status or

ecological conditions, framing the connections between algal growth and productivity, the various ecological and water quality consequences and, ultimately,

designated uses for Chesapeake Bay tidal waters.

Estuarine scientists and managers have borrowed from the field of limnology such

terms as oligotrophic, mesotrophic, eutrophic and hypereutrophic to reflect a range

in symptoms of nutrient over-enrichment. The reality is that there is no scientific

consensus on exactly what these terms mean for nutrient enrichment in estuaries. In

the case of the Chesapeake Bay, Table V-2 establishes an ecosystem trophic status

classification scheme useful for setting the context for the narrative Chesapeake Bay

chlorophyll a criteria (see Table V-1) and supporting technical information and

methodologies.

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chapter v • Chlorophyll a Criteria

An oligotrophic status indicates conditions that are not signficantly affected by nutrient and

sediment enrichment, typically characterized with low nutrient/low organic matter input or

production. Under mesotrophic conditions, a water body is nutrient-enriched but still functions

adequately without the enhanced production of algae having an adverse impact on the aquatic

food web. When a water body reaches eutrophic conditions, excess production of algae can lead

to low dissolved oxygen conditions, reduced water clarity, harmful algal blooms and other

ecological impairments that reflect alterations of the aquatic food web. Aquatic systems that have

become so overloaded with nutrients that they are unable to assimulate available nutrients are

characterized as hyper- or highly eutrophic.
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• Chlorophyll 

a Criteria Trophic Status Oligotrophic Mesotrophic Eutrophic Highly Eutrophic Near-pristine conditions; not significantly affected by nutrient enrichment Experiencing some level of nutrient enrichment but still functioning adequately

with an enhancement of productivity and

without large impact on the structure of the

food web

Significantly impacted by nutrient

enrichment, excess primary production

leading to dissolved oxygen, harmful algal

blooms and other problems; food web

structure significantly altered

Aquatic system so overloaded with

nutrients those nutrients cannot be

assimilated by the system and therefore

nutrients are exported to adjacent waters;

all the effects listed for eutrophic, but even

more extreme

Status inChesapeakeBay Waters

No examples of oligotrophic conditions

currently in Chesapeake Bay waters

Occasionally found in some areas of the

Chesapeake Bay and its tidal tributaries

Many areas in the Chesapeake Bay and its

tidal tributaries are currently characterized

as eutrophic

Areas in the tidal waters where very large

load reductions are required to improve

water quality; Back River in Maryland is

an example

EcologicalFunctions

• Strong trophic coupling and nutrient

recycling processes (ecosystem is efficient);

• No undesirable algal blooms;

•Algal growth often is nutrient-limited, with

little accumulation of inorganic nitrogen or

phosphorus;

• Habitat goals for bay grasses,

phytoplankton, zooplankton, water-column

fishes and bottom organisms met in all

places; 

• Food pathways to fish and higher trophic

levels dominate

• Trophic coupling and nutrient recycling

processes;

• A few undesirable algal blooms;

•Algal growth often is nutrient-limited, with

little accumulation of inorganic nitrogen or

phosphorus, and light-saturated;

• Habitat goals for bay grasses,

phytoplankton, zooplankton, water column

fishes and bottom organisms met in most

places;

• Food pathways to fish and higher trophic

levels dominate

• Sporadic trophic decoupling;

• Algal growth frequently limited by light,

not nutrients; nutrient saturation occurs with

significant accumulations of inorganic

nitrogen and phosphorus;

• Transport of nutrients downstream;

• More frequent undesirable algal blooms;

• Organic matter buildup in bottom waters

leading towards anoxia/hypoxia;

• Phytoplankton biomass is a frequent

cause of water clarity impairment;

•Bay grasses stressed by reduced water

clarity due to algal shading in the water

column and epiphytic algae growth on leaf

surfaces

• Persistent, significant trophic decoupling

(inefficient ecosystem);

• Nutrient saturation /light limitation of

algae growth with significant

accumulations of nitrogen and phosphorus;

• Significant transport of nutrients

downstream;

• More frequent undesirable algal blooms;

• Significant organic matter buildup in

bottom waters leading towards

anoxia/hypoxia;

• Phytoplankton biomass is a major cause

of water clarity impairment;

•Bay grasses decline or are lost due to

turbidity in water column and epiphytic

algal growth on leaf surfaces;

• Food pathways to bacteria (microbial

loop) dominate

Bay CriteriaAttainment

Meets all water clarity and dissolved

oxygen criteria

Meets water clarity and dissolved oxygen

criteria most of the time

Often does not meet water clarity and

dissolved oxygen criteria infrequently

Very often does not meet water clarity and

dissolved oxygen criteria

Never meets water clarity and 

dissolved oxygen criteria

Table V-2. Trophic status, water quality, phytoplankton community and ecological function along a trophic continuum.
The analogy equating oligotrophic with pristine is somewhat forced, because even

before European contact, the Chesapeake Bay probably was never poor in nutrients

(in the sense of an oligotrophic lake, for example, where likely a small watershed

and a relatively impervious geology supplied very low nutrient loads). Proximity to

terrestrial nutrient inputs, long residence times for nutrient recycling and generally

shallow (8 meters average depth) conditions allowing fairly significant benthicpelagic coupling are all factors that would prevent the Chesapeake Bay from ever

being truly oligotrophic. 

So, in a relative sense, the Chesapeake Bay might have been considered mesotrophic

during these earlier times and became eutrophic as changes in land uses resulted in

increased nutrient supplies. This is based on a definition of eutrophic as having

excess algae, leading to the observed more frequent, persistent and intense periods

of low to no dissolved oxygen and substantial reductions in water clarity. Tidal

waters surrounded by intensely developed lands have become hyper-eutrophic. In a

reference condition context, if a majority of Chesapeake Bay tidal waters are considered eutrophic now, a management goal might be to reduce nutrient loadings and,

therefore, chlorophyll a concentrations, to achieve a more mesotrophic condition, in

contrast to the present eutrophic to hypereutrophic situations.

CHLOROPHYLL A CONCENTRATIONS CHARACTERISTIC OF

VARIOUS ECOLOGICAL CONDITIONS

Described and documented below are the chlorophyll a concentrations characteristic

of various ecological conditions within Chesapeake Bay tidal-water habitats.

Historical Chlorophyll a Concentrations

Chlorophyll a concentrations that historically reflected a more balanced Bay

ecosystem were quantified through reviews and evaluations of 1950s through 2000

data (Harding 1994; Harding and Perry 1997; Olson 2002). The chlorophyll a

concentrations derived through this detailed analysis of historically observed

concentrations are characteristic of a mesotrophic estuarine system.

1950s to 1990s Concentration Trends. Harding and Perry (1997) documented

significantly increasing trends in chlorophyll a concentrations during the past several

decades in the Chesapeake Bay mainstem. Surface mixed-layer concentrations

increased five- to tenfold in the higher salinity mesohaline and polyhaline regions,

with 1.5- to twofold increases observed in the tidal-fresh to oligohaline regions of

the Bay. During this 50-year period, they documented three major patterns in freshwater flow to the Chesapeake Bay: a long period of low river flows during the 1960s,

followed by a series of high flow years throughout most of the 1970s, with a mix of

river flow levels in the following two decades, and the extreme droughts (1989) and

near-record river flows (1993, 1994) reported toward the end of the data record.

Harding and Perry (1997) applied an autoregressive moving-average procedure to

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chapter v • Chlorophyll a Criteria
explain possible chlorophyll a concentrations over time strictly on the basis of

observed freshwater inflow, salinity and temperature. When compared with observed

concentration trends over decades, the significant increases in chlorophyll a could

not be accounted for strictly by the variability of freshwater flow, salinity and

temperature. The resulting trends could be explained by increased nutrient enrichment of the estuarine ecosystem.

Taking into account the effects of variable annual river flows, chlorophyll a concentrations were shown to respond to changes in nutrient loadings over the period of

record. These historically observed chlorophyll a concentrations were more representative of mesotrophic conditions.

In oligohaline to tidal-fresh reaches of the Chesapeake Bay mainstem (regions V and

VI, respectively), Harding and Perry (1997) documented an increasing trend in

chlorophyll a concentrations from the 1950s to the 1970s, followed closely by a

decreasing trend that has carried through into the 1990s (Table V-3; Figure V-1). The

decreasing trends are likely due to significant decreases in phosphorus loadings to

the Bay, resulting from widespread upgrades in wastewater treatment for phosphorus. Bans on phosphates in detergents also were enacted in states surrounding the

Bay during the mid- to late 1980s. The phytoplankton in lower salinity systems

where phosphorus has been limited have responded positively, and this has led to

lower chlorophyll a concentrations, whereas comparable reductions in nitrogen

loads have not yet been achieved, limiting opportunities for reduced phytoplankton

biomass in the higher salinity regions of the mainstem Bay.

In the 1950s, recognizing limitations in the temporal and spatial coverage of the

available data, regional mean chlorophyll a concentrations were 3.19 and 2.51 μg

liter-1in the tidal-fresh to low- salinity regions between the Susquehanna Flats and

the Bay Bridge and between the Bay Bridge and the South River, respectively

(regions VI and V, respectively, Harding and Perry 1997; see Figure V-1). Concentrations peaked at 15.59 μg liter-1 (1960s) and 13.12 μg liter-1 (1970s) in these two

regions, respectively, and were recorded as regional means of 5.57 μg liter-1 and

10.86 μg liter-1during the 1985-1994 period.

In the higher salinity mesohaline regions—Region IV-South River down to the

Patuxent River and Region III-Patuxent River south to the Rappahannock River—

chlorophyll a concentrations increased 1.5- to twofold from the 1950s through the

mid-1990s (Figure V-1; Harding and Perry 1997). Regional mean chlorophyll a

concentrations ranged from 4.33 μg liter-1 in the 1950s up to 8.20 μg liter-1 for the

period of 1985- 1994 in the mainstem Bay between the South and Patuxent rivers.

At the same time, regional mean chlorophyll a concentrations were 3.58 μg liter-1and

8.03 μg liter-1, respectively, in the mainstem Bay between the Patuxent and Rappahannock rivers.

Harding and Perry (1997) reported the largest trends in the polyhaline regions of the

mainstem Bay, where chlorophyll a concentrations increased five- to tenfold in

nearly 50 years. In the mainstem Bay from the Rappahannock River down to

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chapter v • Chlorophyll a Criteria
▼109

Mobjack Bay (Region II; Figure V-1), regional chlorophyll a concentrations averaged 1.21 μg liter-1 in the 1950s, but increased to 7.40 μg liter-1 from 1985 to 1994.

The regional mean chlorophyll a concentration of 0.46 μg liter- 1observed in the

1950s increased tenfold through the 1990s to 5.57 μg liter-1in the mainstem Bay

from Mobjack Bay to the mouth of the Bay.

Benchmark Levels Derived from Analysis of the CBP Water-Quality

Database. Evaluating a similar time period of data using different methodologies,

chapter v • Chlorophyll a Criteria

Table V-3. Chesapeake Bay mainstem surface chlorophyll a concentration (μg liter-1) 

annual means for 1950 to 1994.

Time Period Region Chlorophyll a

Annual Mean

Number of

Observations

Percent

Difference1

1950-1959

I 0.46 41 -

II 1.21 18 -

III 3.58 108 -

IV 4.33 7 -

V 3.19 15 -

VI 2.51 18 -

1960-1969

I 1.89 8 310

II 2.61 9 115

III 7.09 28 98

IV 7.48 58 73

V 7.79 97 144

VI 15.59 295 521

1970-1979

I 4.39 101 853

II 6.89 31 468

III 7.95 100 122

IV 7.29 206 68

V 13.12 324 311

VI 12.90 845 414

1985-1994

I 5.49 1862 1093

II 7.40 2350 510

III 8.03 1261 124

IV 8.20 1022 89

V 10.86 1164 240

VI 5.57 1005 122

1 Percent difference of annual mean chlorophyll a concentration for each region is based upon a

comparison with the corresponding chlorophyll a concentration in 1950-1959.

Source: Harding and Perry 1997. Source: Harding and Perry 1997.
Olson (2002) reported a series of benchmark concentrations for chlorophyll a as well

as for nitrogen, phosphorus and total suspended solids. Benchmark concentrations,

derived from a 1985 to 1990 benchmark data set, were applied to the entire 1950s

through late 1990s data set (Table V-4). Tabular summaries of decadal spring,

summer and annual median chlorophyll a concentrations across five decades are

documented in Appendix E, tables E-1 and E-2. Table V-5 summarizes the results of

these reviews and evaluations of the extensive historical and recent chlorophyll a

concentration data records.

Strengths and Limitations. Consideration of the historical chlorophyll a

concentrations reflecting a more balanced, mesotrophic Chesapeake Bay ecosystem

must be tempered by a recognition of the limited spatial and temporal coverage of

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chapter v • Chlorophyll a Criteria

Figure V-1. The Chesapeake Bay showing locations of the six regions chosen

to represent major salinity provinces of the estuary, the principal rivers draining into the Chesapeake Bay and major metropolitan areas.

Source: Harding and Perry 1997. 
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chapter v • Chlorophyll a Criteria

the available data for the 1950s and 1960s, as well as the different living resource

communities present in the Bay’s tidal habitats more than 50 years ago. The data

limitations of the 1950s and 1960s data are particularly of concern in the lower

portion of the Chesapeake Bay. The large reduction in filter-feeder (e.g., oysters,

menhaden) populations has reduced the capacity of the Chesapeake Bay’s living

resources to assimilate nutrient loads and to maintain lower chlorophyll a concentrations. Thus, the changes in living resources may have affected chlorophyll a

Table V-4. Historical chlorophyll a concentrations (μg liter-1) derived through applying relative

status benchmark data.

Season Salinity

Zone

Chlorophyll a

Median

Chlorophyll a

Mean

Chlorophyll a

90th Percentile

Number of 

Observations

Annual Tidal-Fresh 3.1 4.2 10.2 972

Oligohaline 4.7 6.0 10.8 910

Mesohaline 7.3 7.2 10.9 4192

Polyhaline 4.4 4.3 7.0 1132

Spring Tidal-Fresh 3.1 3.7 4.2 488

Oligohaline 5.1 5.9 9.8 279

Mesohaline 6.9 7.2 11.0 708

Polyhaline 3.4 4.1 12.9 91

Summer Tidal-Fresh 7.3 7.0 8.7 423

Oligohaline 8.0 7.6 10.8 566

Mesohaline 8.4 7.9 11.1 1677

Polyhaline 4.3 3.7 6.0 341

Source: Olson 2002. Sources: Olson 2002.

Table V-5. Summary of historical Chesapeake Bay chlorophyll a concentrations (μg liter-1).

Salinity

Regime

Harding and Perry

(1997)-1950s

Chesapeake Bay

Mainstem Annual

Mean

Concentrations

Olson (2002)-1950s

Chesapeake Bay and

Tidal Tributaries

Spring/Summer/

Annual Mean

Concentrations

Olson (2002)–Relative

Status Spring/Summer/

Annual Benchmark

Concentrations

Tidal-fresh 2.51 1.1 / 1.1 / - 3.7 / 7.0 / 4.2

Oligohaline 2.51-3.19 2.3 / 2.0 / 3.1 5.9 / 7.6 / 6.0

Mesohaline 3.58-4.33 3.7 / 4.4 / 3.1 7.2 / 7.2 / 7.9

Polyhaline 0.46-1.21 3.9 / - / 3.2 4.1 / 3.7 / 4.3

Sources: Harding and Perry 1997; Olson 2002.
concentrations as much as or more than the reverse. It should be noted that temporal

trends alone do not demonstrate causal relations between chlorophyll a concentrations and specific ecological conditions.

Literature Values Related to Trophic Status

Several influential scientific papers, synthesizing data from many different aquatic

systems, describe conditions that were judged to reflect the trophic status of different

water bodies (e.g., Wetzel 2001; Ryding and Rast 1989; Smith 1998). Chlorophyll a

is the principal parameter quantified in these literature reviews. The information is

drawn from a diversity of systems across the spectrum of healthy (oligotrophic) to

severely stressed (eutrophic) water bodies.

Several papers in the literature synthesize data from many aquatic systems and focus on

conditions that reflect different trophic states of water bodies. R. G. Wetzel’s Limnology

presents a table of phytoplankton-related trophic states based on hundreds of studies in

freshwater systems (Wetzel 2001). His text defines eutrophic systems as having the same

four dominant phytoplankton species as those currently found in most of the Chesapeake

Bay system’s tidal-fresh or oligohaline habitats and chlorophyll a concentrations greater

than 10 μg liter-1. A system is defined as eutrophic when it has: 1) very high productivity

but mostly occurring in the lower trophic levels (e.g., algae, bacteria); 2) a simplified

structure of biological components; and 3) reduced ability to withstand severe stresses

and return to pre-stress conditions. In a eutrophic condition, “excessive inputs commonly

seem to exceed the capacity of the ecosystem to be balanced, but in reality the systems

are out of equilibrium only with respect to the freshwater chemical and biotic characteristics desired by man for specific purposes” (Wetzel 2001). Mesotrophic freshwater

systems are defined by Wetzel (2001) as having chlorophyll a concentrations in the range

of 2-15 μg liter-1 (Table V-6).

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Table V-6. Summary of aquatic system trophic status as characterized by mean chlorophyll a 

concentrations (μg liter-1).

Aquatic

System

Trophic

Status

Wetzel

(2001)

Ryding

and Rast

(1989)

Smith 

et al.

(1999)

Molvaer

et al.

(1997)

Novotny

and Olem

(1994)

Freshwater

Eutrophic >10 6.7-31 9-25 - >10

Mesotrophic 2-15 3-7.4 3.5-9 - 4-10

Oligotrophic 0.3-3 0.8-3.4 <3.5 - <4

Marine

Eutrophic - - 3-5 >7 -

Mesotrophic - - 1-3 2-7 -

Oligotrophic - - <1 <2 -

Sources: Molvaer et al. 1997, Novotny and Olem 1994, Ryding and Rast 1989, Smith et al 1999, Wetzel 2001.

chapter v • Chlorophyll a Criteria
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Ryding and Rast (1989) also deal with characteristics of eutrophication in lakes,

based on surveys of hundreds of temperate lakes globally. In Table 4.2, they give the

following boundary values for mean and peak chlorophyll a values (μg liter-1), as

follows:

Mean range Peak Range

Oligotrophic 0.8-3.4 2.6-7.6 

Mesotrophic 3.0-7.4 8.9-29 

Eutrophic 6.7-31 16.9-107

The peak range is for occasional blooms, and the mean ranges are those for annual

geometric means, with outliers removed (see Table 4.2 in Ryding and Rast 1989).

The ranges overlap slightly, and in fact the authors recommend using multiple

parameters, including total phosphorus, total nitrogen, chlorophyll a and Secchi

depth to classify the lakes. Using their criteria, much of the Chesapeake Bay would

clearly be classified as ‘eutrophic.’

In a review of lake and marine systems, Smith et al. (1999) equated mesotrophic

status in lake systems to mean chlorophyll a concentrations ranging from 3.5 to 9 μg

liter-1. A chlorophyll a concentration range of 1 to 3 μg liter-1 was equated with

mesotrophic status in marine systems (assumed here to be principally polyhaline in

terms of salinity). Smith et al. (1999) also published values characteristic of hypereutrophic lake (>25 μg liter-1) and marine systems (>5 μg liter-1) .

The Norwegian Environmental Protection Agency has constructed a system for classifying estuaries and coastal waters with respect to water quality and eutrophication

using five classes of water quality (Molvaer et al. 1997). For salinities above 20 ppt,

chlorophyll a concentrations below 2 μg liter-1 are considered Class I or ‘very good,’

whereas concentrations above 20 μg liter-1are classified as “very bad” or Class V

waters. Sweden has adopted similar chlorophyll a water quality standards for its

estuarine (1.3 to 2.0 μg liter-1) and marine (1.0 to 1.5 μg liter-1) waters that reflect the

lower end of these concentration ranges (Sweden Environmental Protection Agency

2002).

Strengths and Limitations. The trophic classifications should be used with

caution since the majority of the scientific literature-based values were developed for

lake, coastal or marine systems, not temperate, partially mixed estuaries such as the

Chesapeake Bay. In particular, marine ecosystems should not be considered directly

comparable to polyhaline estuarine areas. The polyhaline areas of the Chesapeake

Bay are in much closer proximity to land-based freshwater and nutrient inputs.

Therefore, they should be expected to have higher nutrient concentrations and associated chlorophyll a concentrations than marine systems.

chapter v • Chlorophyll a Criteria
Trophic classifications are useful, general ecological concepts. However waters classified strictly by chlorophyll a concentrations may or may not experience all or any

of the ecological conditions characteristic of that category (see Table V-2).

Phytoplankton Growth-Limiting Water Quality Conditions 

and Related Chlorophyll a Concentrations

Biological communities found in pristine or minimally affected habitats provide

essential information on how restoration efforts might improve ecosystem structures

and functions. They also serve as references for measuring restoration progress.

Chesapeake Bay water quality and phytoplankton data collected at Chesapeake Bay

Program phytoplankton monitoring stations between 1984 and 2001 were analyzed

to identify reference phytoplankton communities for Chesapeake tidal waters. The

seasonal and salinity-specific reference communities were used to quantify chlorophyll a concentrations in the least-impaired water quality conditions currently found

in the Chesapeake Bay. 

For the purposes of deriving the reference communities, least-impaired water quality

conditions were defined as the co-occurrence of high light penetration, low dissolved

inorganic nitrogen and low dissolved inorganic phosphorus concentrations. Low

dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (PO4)

concentrations are below the threshold concentrations shown to limit phytoplankton

growth in Chesapeake Bay waters (Fisher et al. 1999), whereas high light penetrations are the Secchi depth values identified by the Relative Status, or benchmark,

method as ‘good’ (Olson 2002). The high light penetration levels are approximately

the same as those necessary for restoring underwater bay grasses (Batiuk et at.

2000). Thresholds for DIN, PO4 and Secchi depth for spring and summer across four

salinity zones (tidal-fresh, oligohaline, mesohaline and polyhaline) were applied to

the 1984 through 2001 Chesapeake Bay water quality monitoring database to bin the

data records into six water quality categories. Reference communities were derived

from the least impaired water quality categories found in each season-salinity

regime. 

Water quality conditions that met all three reference criteria (‘better’/‘best’) between

1984 and 2001 occurred in 1.6 percent (spring) and 5.8 percent (summer) of the

mesohaline biomonitoring samples, and 21.1 percent (spring) and 10.4 percent

(summer) of the polyhaline water quality monitoring samples, so reference communities could be characterized directly from the data. Water quality conditions that met

all the reference criteria rarely occurred in tidal-fresh and oligohaline salinities. In

these cases, the ‘mixed better light’ category (see Appendix F for definition) was

used as a surrogate, since values of most phytoplankton parameters (e.g., chlorophyll

a, biomass, pheophytin and species composition) in this category closely resembled

those in ‘better’/‘best’ in mesohaline and polyhaline waters. For the spring mesohaline reference community, ‘better’/‘best’ data were augmented with ‘mixed better

light’ data to increase the number of data records. Chlorophyll a concentrations 

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chapter v • Chlorophyll a Criteria
observed in the phytoplankton reference communities are shown in Table V-7. The

water quality binning method and identification of the phytoplankton reference

communities are described in more detail in Appendix F.

It is important to realize that the chlorophyll a concentrations in Table V-7 reflect

phytoplankton reference communities in the absence of robust grazer populations.

There are no undisturbed sites in the Chesapeake Bay with a full complement of

natural grazers. Harvesting and disease have significantly decreased Chesapeake

oyster abundances (Newell 1988). Menhaden populations have declined to approximately 5 percent of 1970s levels (data from the Maryland Department of Natural

Resources). Comparisons of historic and contemporary populations of mesozooplankton and benthos indicate that declines may also have occurred in these grazers.

Median chlorophyll a concentrations in the reference communities are significantly

lower than those in impaired waters, and algal blooms are absent. Reference community chlorophyll a concentrations are slightly higher than historic Chesapeake Bay

concentrations and are typical of mesotrophic conditions. They indicate the chlorophyll a concentrations that could be attained in the present-day Chesapeake Bay with

significant nutrient and sediment reductions, in the absence of robust populations of

grazers. If key grazer populations are at least partially restored to historical levels, it

is possible that the phytoplankton reference community chlorophyll a concentrations

will approach 1950s levels (see Table V-3).

Strengths and Limitations. It is important to realize that these values were

selected from samples subject with least-improved water quality, and they came

from a larger data set obtained from generally nutrient- and sediment-enriched

Chesapeake Bay. Under better water quality conditions (lower annual nutrient load-

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chapter v • Chlorophyll a Criteria

Table V-7. Chlorophyll a concentrations in the salinity- and season-based Chesapeake phytoplankton

reference communities (μg liter-1). The median and range (5%–95%) are shown. 

Reference community values are derived from samples with the least improved water

quality conditions in the 1984–2001 Chesapeake Bay Program phytoplankton and water

quality monitoring station data.

Salinity Regime Spring Summer

Tidal-Fresh 4.3 (1.0 - 13.5) 8.6 (3.2 - 15.9)

Oligohaline 9.6 (2.4 - 24.3) 6.0 (2.5 - 25.2)

Mesohaline 5.6 (2.2 - 24.6) 7.1 (4.4 - 14.0)

Polyhaline 2.9 (1.1 - 6.7) 4.4 (1.7 - 8.7)

Source: Chesapeake Bay Water Quality and Phytoplakton Monitoring Programs Databases.

http://www.chesapeakebay.net/data
ings, more zooplankton grazing and better trophic coupling), these chlorophyll a

values might be even lower than those obtained under low current nutrient loadings

due to the carryover of nutrients from previous high load conditions.

The phytoplankton reference community approach does not demonstrate any direct

relationship between chlorophyll a concentrations and designated use impairments.

However, this method does provide solid insights into how chlorophyll a concentrations will likely respond in estuarine systems as water quality improves, leading to

more nutrient-limited, light saturated conditions.

Chlorophyll a concentrations do not always show a high correlation with algal

biomass because after a bloom, some species of nonchlorophyll-bearing phytoplankton can feed on organic material (Livingston 2001).

Research Needs. Further analysis of the Chesapeake Bay monitoring databases

could help determine if nitrogen, phosphorus or suspended sediment reductions or a

combination thereof will be most effective in minimizing the occurrence of harmful

algal blooms. 

Chlorophyll a Concentrations Characteristic of 

Potentially Harmful Algal Blooms

The scientific literature indicates that certain phytoplankton community taxonomic

groups produce poor quality food and even toxins that impair the animals that feed

on them (Roelke et al. 1999; Roelke 2000). Phytoplankton assemblages can become

dominated by poor quality food taxonomic groups to an extent that the overall food

quality of that phytoplankton assemblage becomes significantly reduced. Chlorophyll a concentrations were identified that corresponded to an increased probability

that potentially harmful algal taxa would exceed specific impairment thresholds.

Several of the more than 700 phytoplankton species in the Chesapeake Bay are

known to be harmful to consumers. Approximately 2 percent of these species have

shown evidence of producing toxins (Marshall 1996). Some species, however, form

blooms and can dominate the community at particular locations during specific

times of the year. Some of these species are even capable of producing toxins. 

The dinoflagellates, Prorocentrum minimum and Cochlodinium heterolobatum,

which commonly bloom in spring and summer, respectively, in certain mesohaline

areas of the estuary, have been shown to harm various life stages of the Eastern

oyster, Crassostrea virginica (Ho and Zubkoff, 1979; Luckenbach et al. 1993; Wickfors and Smolowitz 1995). The dinoflagellate Karlodinium micrum has been

associated with numerous fish kills in the Chesapeake Bay (Goshorn et al. 2003). In

tidal-fresh regions, a colonial cyanophyte, Microcystis aeruginosa, forms surface

blooms that cover the upper reaches of certain Bay tributaries for miles during the

summer. This species has been documented to affect zooplankton communities

under bloom conditions (Lampert 1981; Fulton and Paerl 1988). 

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chapter v • Chlorophyll a Criteria
The occurrence of harmful algal blooms is a complex, incompletely-understood

phenomenon. Many harmful blooms cannot effectively be predicted or modeled at

this time, and the physical, chemical and biological controls on many such blooms

are not known. Nutrient concentrations or loads are only one of many environmental

parameters that can potentially affect harmful algal blooms. For example, some

harmful blooms may respond more to nutrient ratios than absolute concentrations, or

may be regulated by top-down controls (e.g., grazer dynamics) more than by nutrient

availability. This section represents a valuable compilation of information, focusing

on several Chesapeake Bay species that have been observed to correlate with chlorophyll a concentrations. As illustrated below using the four previously cited species,

the likelihood of bloom conditions being produced by some harmful or nuisance

algal species tends to be associated with elevated chlorophyll a levels. Future monitoring and research is expected to provide more insight into the practicality and

methodology for managing blooms of these and other species. 

Microcystis aeruginosa. A substantial body of literature deals with the negative

effects of toxic cyanobacteria on the feeding, growth, behavior and survival of

micro- and mesozooplankton. Numerous studies have documented the avoidance of

ingestion of toxic and nontoxic strains of Microcystis aeruginosa by specific taxa of

zooplankton (Clarke 1978; Lampert 1981; Gilbert and Bogdan 1984; Fulton and

Paerl 1987, 1988; DeMott and Moxter 1991) while others indicate physiological and

behavioral problems associated with its ingestion (Lampert 1981, 1982; Nizan et al.

1986; Fulton and Paerl 1987; DeMott et al. 1991; Henning et al. 1991).

From laboratory studies, 10,000 cells milliliter-1 was determined to be the threshold

above which zooplankton communities can be adversely altered by the poor food

quality, large particle size of the colonies, increased density of particles in the water

column or directly by the toxin (Lampert 1981; Fulton and Paerl 1987; Smith and

Gilbert 1995). (See Appendix G for more detailed descriptions of the determination

of the effects threshold.)

Upon matching the chlorophyll a concentrations to samples containing M. aeruginosa, normalized frequency distribution plots were constructed for M. aeruginosa

bloom frequency and the frequency of both bloom and non-bloom abundances

versus chlorophyll a concentrations (figures V-2 and V-3, respectively). Chlorophyll

a concentrations <15 μg liter-1 characterize M. aeruginosa concentrations less

<10,000 cells milliliter-1 (Figure V-2). Increasing concentrations of chlorophyll a

above 15 μg liter-1 leads to increasing frequencies of bloom samples > 10,000 cells

milliliter-1 (Figure V-3).

Colonies of M. aeruginosa vary in their cell counts but colony counts provide an

additional measure of bloom conditions (Figure V-4). The ratio of cells per colony is

approximately 17:1, providing an estimate of 588 colonies containing 10,000 cells

as a translation to threshold levels for zooplankton community impacts.

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chapter v • Chlorophyll a Criteria

0-5 5-10 10-15 15-20 20-25

0%

2%

4%

6%

8%

Normalized Frequency as Percent 10%

12%

14%

16%

18%

25-30 30-35 35-40 >40

Chlorophyll a Concentration (μg liter-1)

Figure V-2. Normalized frequency of Microcystis aeruginosa abundances above-threshold

(i.e., >10,000 cells milliliter-1) versus summer tidal fresh chlorophyll a concentration. The

number of above-threshold Microcystis aeruginosa abundances in each chlorophyll a interval is divided by the total number of phytoplankton records in that interval. For summer

tidal fresh, there were 16 above-threshold occurrences in a total of 266 samples.

Source: Chesapeake Bay Phytoplankton Monitoring Program Database

http://www.chesapeakebay.net/data 

0-5 5-10 10-15 15-20

70%

60%

50%

40%

30%

20%

10%

0%

20-25 25-30 30-35 35-40 >40

Chlorophyll a Concentration (μg liter-1)

Microcystis aeruginosa 0-10,000 cells millilter-1

Microcystis aeruginosa >10,000 cells millilter-1

Normalized Frequency as Percent

Figure V-3. Normalized frequency of above- and below-threshold Microcystis aeruginosa

abundances versus summer tidal fresh chlorophyll a concentration. The number of aboveand below-threshold Microcystis aeruginosa abundances in each chlorophyll interval is divided by the total number of phytoplankton records in that interval. For summer tidal fresh,

there were 62 total occurrences of Microcystis aeruginosa in a total of 266 samples. The

increasing trend in total occurrences of Microcystis aeruginosa identify it as an indicator

species of eutrophication. 

Source: Chesapeake Bay Phytoplankton Monitoring Program Database

http://www.chesapeakebay.net/data 
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chapter v • Chlorophyll a Criteria

Cell counts = 16.97 x Colony Counts

R2

 = 0.66, n=20

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

0 500 1000 1500 2000 2500 3000

Microcystis aeruginosa Colony counts

Microcystis aeruginosa Cell Counts

(cells milliliter-1)

Figure V-4. Relationship of Microcystis aeruginosa colony counts versus cell counts. 

Cell counts=16.97 x colony counts; r2=0.66; n=20.

Source: Maryland Department of Natural Resources unpublished data.

0

1000

2000

3000

4000

5000

6000

7000

0 20 40 60 80 100 120

Microcystis aeruginosa Colony Count

(colonies milliliter-1)

Lack of Blooms Bloom region

Chlorophyll a Concentration (μg liter-1)

Figure V-5. Microcystis aeruginosa colony counts versus a gradient of chlorophyll a concentrations illustrating the boundary between bloom and non-bloom conditions.

Source: Maryland Department of Natural Resources unpublished data.
M. aeruginosa counts were made from water samples collected by the Maryland

Department of Natural Resources through a separate water quality monitoring

program from the tidal-fresh and oligohaline waters of Maryland’s Chesapeake Bay.

Between 1985 and 2000, M. aeruginosa colony counts showed low concentrations

(<588 colonies milliliter-1) and low variance between 0-33 μg liter-1 chlorophyll a

(Figure V-5). Beyond 33 μg liter-1 chlorophyll a, the variance of colony counts

increases significantly and counts exceeding the 588 colonies milliliter-1 threshold

increase to 42 percent beyond 33 μg liter-1 chlorophyll a, providing a threshold and

probability for potentially harmful blooms of this cyanobacteria with respect to

chlorophyll a measures. The chlorophyll a range of 15-33 μg liter-1 provides a

threshold region between levels that protect against M. aeruginosa blooms versus

conditions with a high likelihood for blooms. 

One of the primary locations for M. aeruginosa blooms in the Chesapeake Bay

estuary is the tidal-fresh Potomac River. Extensive blooms of M. aeruginosa were

documented over the period of 1965-1983, before the initiation of the coordinated

Chesapeake Bay monitoring program. During the period of 1965-1974, summer

chlorophyll a concentrations in the vicinity of Indian Head (near monitoring station

TF2.3) were typically above 50 μg liter-1 and often exceeded 100 μg liter-1 in the

surface layer (Pheiffer 1975). During the same period, cyanobacteria blooms were

extensive in this portion of the river in summer, although there are very few data

reflecting cell densities. Total cyanobacteria densities ranged from 20,000–120,000

cells milliliter-1 in the summer of 1971 near Possum Point (Simmons et al. 1974). 

In 1983, a massive bloom of M. aeruginosa was documented in this portion of the

Potomac River (mile 12 - mile 46) (Thomann et al. 1985). Chlorophyll a concentrations averaged over 200 μg liter-1 for the Indian Head area in August 1983. Again,

little species composition data is documented for this bloom. 

With the initiation of the Chesapeake Bay phytoplankton monitoring program in

August 1984, a steady flow of phytoplankton species composition and chlorophyll a

data was recorded for a station in the tidal-fresh Potomac River near Indian Head

(TF2.3). Figure V-6 summarizes these data for M. aeruginosa during the summer

months of 1985–2002. The data show that the threshold is rarely exceeded during

this period after 1988, but one can assume that during the severe blooms of the 1970s

and early 1980s, this threshold may have been surpassed on a regular basis. The fact

remains that this taxon is an impairment to zooplankton assemblages above a

specific threshold and this threshold density has been surpassed on a number of

occasions in the tidal-fresh Potomac River during the past several decades.

Strengths and Limitations. The strength of this line of evidence for establishing a

chlorophyll a threshold for the tidal-fresh and oligohaline regions of the estuary lies

in the evidence provided in the many laboratory and field studies that indicate

adverse affects on zooplankton populations caused by cyanobacteria in general and,

more specifically, by M. aeruginosa. M. aeruginosa has been found in many of the

tidal-fresh locations sampled as part of the Chesapeake Bay water quality

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chapter v • Chlorophyll a Criteria
▼121

monitoring program, implying that this ‘indicator’ species is ubiquitous to this

particular tidal-fresh habitat during the summer under certain hydrodynamic conditions and with a given set of nutrient requirements. 

Numerous field studies have documented changes in zooplankton community structure associated with blooms of cyanobacteria in general (Infante and Riehl 1984;

Orcutt and Pace 1984; Threlkeld 1986; Burns et al. 1989; Gilbert 1990; Fulton and

Jones 1991). These studies most frequently cite the inability of many zooplankton

taxa in using cyanobacteria as a nutritive food source. Therefore, it can reasonably

be stated that high chlorophyll a concentrations in tidal-fresh and oligohaline regions

of the Chesapeake Bay estuary in summer often are associated with high densities of

cyanobacteria, which can adversely alter the zooplankton community structure in

these areas.

Colony counts have a lower variance than, and a positive relationship to, M. aeruginosa cell counts, providing a robust indicator to describe bloom conditions. Both

data sets in these analyses independently define a relatively narrow range of conditions that separate the bloom from non-bloom regions of the chlorophyll a gradient

based on threshhold level effects on living resources of 10,000 cells milliliter-1.

chapter v • Chlorophyll a Criteria

Figure V-6. Mean summer Microcystis aeruginosa cell densities from 1985–2002 from

the surface mixed layer of the Potomac River tidal fresh phytoplankton monitoring 

station TF2.3.

Source: Chesapeake Bay Phytoplankton Monitoring Program Database

http://www.chesapeakebay.net/data 
The threshold value for the cell density that affects zooplankton populations was

derived from two laboratory studies citing impairment thresholds at very different

cell densities (see Appendix G). A third study has been identified that documented

negative effects on zooplankton at M. aeruginosa cell densities of 50,000 cells milliliter-1, which is an intermediate value compared to the two previously cited studies

(Smith and Gilbert 1995). 

Some of the detrimental effect of M. aeruginosa on zooplankton assemblages is

related to the toxin content of a particular strain of this cyanobacterium (one reason

that the threshold density of the two laboratory studies is so different). The toxin

content of the strains of M. aeruginosa found in the Chesapeake Bay has not been

determined, which forced the extrapolation of the threshold for this document to be

chosen as a midpoint between the thresholds of the two laboratory studies.

Colony counts are not interchangeable with cell counts, since the variance increases

as the counts increase. The risks have been stated based on a threshold for

zooplankton effects using an abundance of cells, while the risks to toxin production

or toxic effects are less well understood in relation to cell or colony concentrations.

Research Needs. Two obvious research studies would strengthen this line of

evidence. The first would be to assess the toxin content in the populations of M.

aeruginosa found in various tidal-fresh regions of the Chesapeake Bay. The second

would be to use some strains of the cyanobacterium in specific laboratory experiments that studied effects on zooplankton feeding, reproduction and survival at

specific cell densities and associated chlorophyll a concentrations.

The estimate of colony counts as a threshold can be refined using the conversion

with cell counts through results of the Chesapeake Bay phytoplankton monitoring

program. Additional work is needed to correlate the concentration data with levels

associated with detrimental levels of microcystin toxins in the ecosystem. Spatial

and temporal resolution of M. aeruginosa levels in relation to cell and colony

concentration would provide valuable information for any reassessment of the

density driven thresholds being proposed.

Prorocentrum minimum. P. minimum effects may be a function of bloom

density or toxicity. In Japan in 1942, P. minimum was attributed as the cause of a

shellfish poisoning in Japan in which 114 people died (Nagazima 1965, 1968). P.

minimum isolated from a 1998 bloom in the Choptank River and subsequently grown

in the laboratory was found to be toxic to scallops (G. H. Wickfors, personal communication). Blooms of P. minimum in the source intake waters to Virginia and

Maryland oyster hatcheries were suspected to have caused oyster larvae mortality at

the two hatcheries in 1998 (Mark Luckenbach and Don Merritt, personal communication). There has been no documented case of shellfish toxicity or mortality as a

result of the 1998 P. minimum bloom in the Chesapeake Bay, but clearly the potential exists for toxic repercussions to shellfish and other organisms as a result of this

bloom.

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chapter v • Chlorophyll a Criteria
The P. minimum density of 3,000 cells milliliter-1 was chosen as a threshold for the

chlorophyll a criteria analysis based on laboratory analyses (Wickfors and

Smolowitz 1995; Luckenbach et al. 1993; see Appendix G). When the threshold is

applied to Chesapeake Bay phytoplankton monitoring program data, the normalized

frequency distribution of chlorophyll a concentrations associated with bloom densities (>3,000 cells milliliter-1) illustrates that concentrations > 5 μg liter-1 can generate

densities that may impair the survival of various life stages of oysters (Figure V-7).

The likelihood of bloom level events tends to increase with increasing chlorophyll a

concentrations (Figure V-8). 

When the threshold is applied to Chesapeake Bay phytoplankton monitoring

program data, the normalized frequency distribution of chlorophyll a concentrations

associated with the P. minimum bloom densities (greater than 3,000 milliliter-1) indicates a large increase at chlorophyll a concentrations of 25 to 30 μg liter-1 (Figure

V-9). More than 19 percent of samples containing P. minimum in mesohaline waters

in spring are characterized by densities that exceed the threshold whereby oyster life

stages are impaired and fall within the chlorophyll a range of 25 to 30 μg liter-1. In

addition, more than 70 percent of the above-threshold data for P. minimum occur at

chlorophyll a concentrations greater than 25 μg liter-1 (Figure V-10). These normalized frequency distributions thus indicate that chlorophyll a concentrations of greater

than 25 μg liter-1 in spring in mesohaline waters often are associated with densities

of P. minimum that may impair the survival of various life stages of oysters.

In an analysis of a separate Maryland Department of Natural Resources database

from 1985-2000, a probability analysis illustrated that no blooms of P. minimum

occurred at or below chlorophyll a concentrations of 4 μg liter-1 (Figure V-11). This

analysis of an independent data set complements the previously described Chesapeake Bay Phytoplankton Monitoring Program database analysis confirming the low

target chlorophyl a concentration needed to eliminate conditions for blooms of P.

minimum in the mesohaline Chesapeake Bay. Figure V-11 shows that as the chlorophyll a concentration increases, the probability of detecting a P minimum bloom

level above the 3,000 cells milliliter-1 threshold in a sample increases in a non-linear

fashion. The possibility increases rapidly at first above 4 μg liter-1 and then slows as

the maximum potential detection of 11 percent of samples is reached at high chlorophyll a concentrations. Maximum bloom probability was 11 percent in the spring,

or 1 in every 9 samples when conditions are optimal. Protecting against the conditions for 50 percent of maximum bloom potential occurred at approximately 25-30

μg liter-1 (Figure V-11). 

Currently, the impairment thresholds are usually reached in spring in mesohaline

waters, but P. minimum commonly occurs in both spring and summer in oligohaline,

mesohaline and polyhaline habitats.

Strengths and Limitations. P. minimum blooms occur in many mesohaline portions

of the estuary. The appearance of the major bloom events in these areas occur on

regular seasonal basis. Therefore this would be a useful indicator species to monitor.

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chapter v • Chlorophyll a Criteria

0-5 5-10 10-15 15-20

100%

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

20-25 25-30 30-35 35-40 >40

Chlorophyll a Concentration (μg liter-1)

Occurrence as Percent

Mesohaline

Oligohaline

Combined CFD

Figure V-8. All occurrences of Prorocentrum minimum abundances above threshold

versus combined spring and summer, mesohaline and oligohaline Chesapeake Bay and 

tidal tributary chlorophyll a concentration. The number of above threshold Prorocentrum

minimum densities in each chlorophyll a interval is divided by the total number of abovethreshold densities (n=44).

Source: Chesapeake Bay Phytoplankton Monitoring Program Database 

http://www.chesapeakebay.net/data

0-5 5-10 10-15 15-20

30%

25%

20%

15%

10%

5%

0%

20-25 25-30 30-35 35-40 >40

Chlorophyll a Concentration (μg liter-1)

Normalized Frequency as Percent

Figure V-7. Normalized frequency of Microcystis aeruginosa abundances abovethreshold (i.e., >3,000 cells milliliter-1) versus spring mesohaline Chesapeake Bay and 

tidal tributary chlorophyll a concentration. The number of above-threshold Prorocentrum

minimum abundances in each chlorophyll a interval is divided by the total number of

phytoplankton records in that interval. For spring mesohaline stations, there were 

35 above-threshold occurrences out of a total of 648 sampling records.

Source: Chesapeake Bay Phytoplankton Monitoring Program Database

http://www.chesapeakebay.net/data
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chapter v • Chlorophyll a Criteria

Figure V-9. Prorocentrum minimum cell densities and associated chlorophyll a

concentrations in the Chesapeake Bay, 1985-2000. Cell density threshold associated 

with impacts on the oyster community is indicated by the vertical black line at 

3,000 cells milliliter-1.

Source: Chesapeake Bay Phytoplankton Monitoring Program Database 

http://www.chesapeakebay.net/data

Figure V-10. Enlarged version of Figure V-9 focused on the Prorocentrum minimum cell

densities and associated chlorophyll a concentrations above the 3,000 cells milliliter-1

threshold. Over 73 percent of the 64 observed chlorophyll a concentrations are greater

than 25 μg milliliter-1.

Source: Chesapeake Bay Phytoplankton Monitoring Program Database

http://www.chesapeakebay.net/data
This taxon’s effects are fairly well-documented, although the toxin content of

different strains seems to be variable. The consumer organism that has been tested,

Crassostrea virginica, the Eastern oyster, is important economically and ecologically as a filter-feeder. The associated chlorophyll a threshold is well-defined based

upon the historic data from the Chesapeake Bay water quality monitoring program.

Both data sets used in these analyses independently defined a relatively narrow range

of conditions that separate the bloom from non-bloom regions of the chlorophyll a

gradient. 

Toxin content of different strains of P. minimum varies. Although widespread anecdotal evidence suggests that oyster larvae are negatively affected by blooms of P.

minimum in the Chesapeake Bay, no direct evidence supports this hypothesis. The

value chosen as a threshold for impairment is extrapolated from several laboratory

studies and does not pertain directly to the strains of P. minimum found in the Chesapeake Bay.

Research Needs. Clearly it is necessary to determine the toxin content and conditions conducive to toxin production of the Chesapeake Bay P. minimum strain. In

addition, grazing studies using water from different bloom sites or cultures isolated

from various bloom sites in the Bay would provide pertinent information on the

potential effects of this dinoflagellate on oyster larvae and other filter-feeding organisms. These studies thus would be aimed at determining not only a threshold for cell

densities but also an associated range of chlorophyll a concentrations.

Links between toxicity and density deserve further work as well as a determination

of the frequency with which toxic strains of P. minimum occur in the Chesapeake

▼126

chapter v • Chlorophyll a Criteria

0

2

4

6

8

10

12

0 20 40 60 80 100 120 140

Chorophyll a threshold

Bloom probability

(percent)

Bloom Prob <Threshold

Figure V-11. Spring (March-May) Prorocentrum minimum bloom probability, 1985–2002,

measured as a percent of the samples exceeding the 3,000 cells milliliter-1 threshold

plotted against each sample’s measured chlorophyll a concentration.

Source: Chesapeake Bay Phytoplankton Monitoring Program Database 

http://www.chesapeakebay.net/data
Bay. Defining density relationships to light field requirements is likely to be a fertile

area of analysis with this species, since its distribution coincides with spring growth

of underwater bay grass beds in the mesohaline portion of the Chesapeake Bay.

Additional studies also are needed to determine if adverse effects of P. minimum

occur in mixed algal diets. Finally, research is needed to determine effective management strategies for P. minimum. This will require a better understanding of the

physical, biological and chemical controls on blooms of this taxon.

Cochlodinium heterolobatum. This species forms intense blooms in warm

months at the mouth of the York River and in the lower Chesapeake Bay 

(Mackiernan 1968; Zubkoff and Warriner 1975; Zubkoff et al. 1979; Marshall 1995).

The bloom appears to begin at the mouth of the York River and is dispersed into the

lower Chesapeake Bay from this point of origin and has been documented to affect

~ 215 km2 in this part of the estuary (Marshall 1995). In this bloom area, cell densities were generally >1,000 cells milliliter-1. Laboratory studies indicated a threshold

concentration of ~ 500 cells milliliter-1 resulted in mortality of oyster larvae (Ho and

Zubkoff 1979). Further analysis of these data published by Zubkoff et al. (1979)

yielded a chlorophyll a concentration of approximately 50 μg liter-1 at the threshold

concentration of 500 cells milliliter-1.

Karlodinium micrum. K. micrum, synonymous with Gyrodinium galatheanum

Braarud and Gymnodinium micrum, and historically reported as Gyrodinium estuariale in Maryland, is a common and widespread estuarine dinoflagellate in the

Chesapeake Bay. Recent work by Deeds et al. (2002) has demonstrated that Maryland isolates of the dinoflagellate produced toxins with hemolytic, cytotoxic and

ichthyotoxic properties. Initial studies indicate K. micrum may produce sufficient

toxin to result in fish mortality in the field at cell densities of 10,000 to 30,000 cells

milliliter-1 and above (Deeds et al. 2002; Goshorn et al. 2003). 

K. micrum is present year-round in the water column of the Chesapeake Bay. Peak

monthly average abundances occur between April and September, favoring mesohaline salinities and elevated concentrations showing a preferred temperature of

21.5-27.5oC (Goshorn et al. 2003). Between 1985 and 2002, there were 1,312

samples from approximately 7,000 collected from Maryland’s Chesapeake Bay that

contained K. micrum. Mean density of the cell counts when present was 589 cells

milliliter-1, with nine samples (0.7 percent) exceeding the potential lethal threshold

of 10,000 cells milliliter-1 (Goshorn et al. 2003).

A historical review of a fish kill database maintained by the Maryland Department

of the Environment showed eight events where kills were associated with the presence of potential acutely lethal concentrations of K. micrum (Goshorn et al. 2003).

Cell concentrations in these near-shore creek environments not sampled in routine

monitoring provided a range in concentrations from 10,270 to 322,968 cells

milliliter-1. Deeds et al (2002) however, also report on fish kills in aquaculture ponds

on the lower Eastern Shore of Maryland that implicate K. micrum in fish kill events

▼127

chapter v • Chlorophyll a Criteria
▼128

chapter v • Chlorophyll a Criteria

with densities > 10,000 cells milliliter-1. Kempton et al. (2002) related K. micrum to

a South Carolina fish kill in a brackish water retention pond with evidence of toxicity and concentrations of 64,000-68,000 cells milliliter-1. Nielsen (1993) showed

that juvenile cod exposed to 100,000 cells milliliter-1 of K. micrum resulted in death

within 2 days. 

A subset of the K. micrum (n=684) database had chlorophyll a-associated data. K.

micrum was more likely to exceed 2,000 cells milliliter-1 when chlorophyll a concentrations exceeded 10 μg liter-1 in open-water habitat (Figure V-12). One count

exceeded the 10,000 cells milliliter-1 boundary and the associated chlorophyll a was

75 μg liter-1. Kempton et al. (2002) found chlorophyll a concentrations of 117 μg

liter-1 in association with acutely lethal concentrations (64,000-68,000 cells

milliliter-1) of K. micrum at the South Carolina fish kill site. Variance in K. micrum

cell counts increases with increasing chlorophyll a measures suggesting the risk of

acutely toxic levels coincidentally increasing with the rise in chlorophyll a out to 

75 μg liter-1. However, the present Maryland data set does not presently demonstrate

a clear threshold level for chlorophyll a with acutely toxic boundary conditions of 

K. micrum densities. 

Strengths and Limitations. K. micrum represents an abundant, relatively easy to

identify potential harmful algal bloom species in the Chesapeake Bay. Maryland

isolates from fish kill events have generated toxicity at many levels from cytotoxicity to hepatotoxicity and ichythyotoxicity. Lab results demonstrated acutely lethal

levels of K. micrum. The aquatic life impairment associated with fish kills is clear.

Sublethal effects are essentially unknown. Concentration alone does not imply toxicity but co-occurring conditions that induce disintegration of the cells may be needed

0

2000

4000

6000

8000

10000

12000

0 50 100 150 200

Chlorophyll a (μg liter)

Karlodinium micrum Count

(cells milliliter)

Figure V-12. Karlodinium micrum cell counts versus chlorophyll a concentrations in the

Maryland portions of the Chesapeake Bay. A total of 684 samples are illustrated.

Source: Maryland Department of Natural Resources unpublished data.
in order for the toxins to be released (Deeds et al. 2002). The habitats where fish kills

have been most commonly associated with potentially lethal densities of 

K. micrum are shallow-water and near-shore habitats, and small tributary systems

(Goshorn et al. 2003), aquaculture facilities (Deeds et al. 2002) and brackish retention ponds (Kempton et al. 2002). To date, these habitats are not typical of areas

routinely sampled by water quality monitoring programs of the Chesapeake Bay.

Thus far, while the risk of acutely lethal concentrations increases with increasing

chlorophyll a, only two instances are noted with chlorophyll a data at 75 μg liter-1

(this chapter), 117 μg liter-1 (Kempton et al. 2002) and > 10,000 cells milliliter-1.

Although the probability of elevated densities is higher when chlorophyll a exceeds

10 μg liter-1, above this concentration there is no strong correlation between cell

density and chlorophyll a concentration.

Research Needs. More detailed knowledge of the relationship between densities

above the acute threshold boundary and chlorophyll a levels is needed from nearshore monitoring and fish kill responses to refine the critical range of chlorophyll a

levels that we should avoid in managing for reducing levels of harmful algal blooms

in the Chesapeake Bay. The sublethal effects of K. micrum on the environment is in

an obvious area for further study. Understanding toxin concentration relationships of

K. micrum under field conditions that result in cell disintegration enhancing the likelihood of toxin interaction with living resources also needs additional research. And

it is necessary to better understand the physical, chemical and biological processes

that control K. micrum blooms in order to develop even more effective management

strategies.

CHLOROPHYLL A CONCENTRATIONS CHARACTERISTIC 

OF TROPHIC-BASED CONDITIONS

Table V-8 categorizes, by trophic status, chlorophyll a concentrations that characterize desired (oligotrophic and mesotrophic) and stressed (eutrophic) ecological

conditions in Chesapeake Bay open-water tidal habitats. These concentrations were

drawn from scientific literature values related to trophic status, historically observed

concentrations in the Chesapeake Bay and those characteristic of reference phytoplankton communities versus potentially harmful algal blooms.

Chlorophyll a concentrations characteristic of oligohaline conditions published by

Ryding and Rast (1989), Wetzel (2001), Smith et al. (1999), Molvaer et al. (1997)

and Novotny and Olem (1994) are listed first in Table V-8 in each salinity-regime

specific row under the heading ‘oligohaline conditions.’ Seasonal mean chlorophyll

a concentrations derived from Olson’s analysis (2002) of the 1950s Chesapeake Bay

mainstem chlorophyll a conditions using the same historical data set as Harding and

Perry (1997) are listed next in each ‘oligohaline conditions’ salinity-regime specific

row.

Mesotrophic conditions expressed as ranges of chlorophyll a concentrations characterized in the scientific literature by several authors (Ryding and Rast 1989; Wetzel

▼129

chapter v • Chlorophyll a Criteria
▼130

chapter v • Chlorophyll a Criteria

Table V-8. Summary of chlorophyll a concentrations reflecting trophic-based water quality,

phytoplankton community and ecological conditions.

Salinity

Regime

Chlorophyll a Concentrations (g liter-1)

Oligotrophic Conditions Mesotrophic

Conditions

Eutrophic 

Conditions

Average or

General

Range

Peak Range

Average or

General

Range

Peak Range

Average or

General

Range

Peak Range

Spring (March - May)

Tidal-Fresh 0.8 - 3.4a

0.3 - 3b

<3.5c

<4e

1.1f

2.6 - 7.6a 3.0 - 7.4a

2 - 15b

3.5 - 9c

4-10e

4.3g

8.9 - 29a

13.5g

15i

6.7 - 31a

10-500b

9-25c

>10e

6.7h

16.9 - 107a

42.9h

<33i

Oligohaline 2.3f 9.6g 24.3g

15i

5.0h 29.8h

<33i

Mesohaline 3.7f 5.6g

5j

24.6g 11.1h 44.9h

<25-30j

Polyhaline <1c

<2d

3.9f

1-3c

2 - 7d

2.9g

5j

6.7g 3-5c

>7d

9.1h

18.0h

<25-30j

Summer (July - September)

Tidal-Fresh 0.8 - 3.4a

0.3 - 3b

<4e

1.1f

2.6 - 7.6a 3.0 - 7.4a

2 - 15b

3.5 - 9c

4-10e

8.6g

8.9 - 29a

15.9g

15i

6.7 - 31a

10-500b

>10e

25.3h

16.9 - 107a

62.1h

33i

Oligohaline 2.0f 6.0g 25.2g

15i

17.1h 60.5h

33i

Mesohaline 4.4f 7.1g

5j

14g 12.2h 52.5h

<25-30j

Polyhaline <1c

<2d

1 - 3c

2 - 7d

4.4g

5j

8.7g 3-5c

>7dc

6.1h

25.8h

<25-30j

a

Ryding and Rast, 1989; bWetzel, 2001; cSmith, 1998; dMolvaer et al., 1997; eNovotny and Olem, 1994; fOlson 2002; gTable V-7 this chapter; hAppendix F, Figure F-3 this volume; i

Microcystis aeruginosa section this chapter; 

j

Prorocentrum minimum section this chapter.

Chlorophyll a Concentrations (μg liter -1

Salinity

Regime 

Oligotrophic Conditions

15i

15i
▼131

2001; Smith et al. 1999 and Novotny and Olem 1994) are listed first in each salinityregime specific row first in Table V-8 under the ‘mesotrophic conditions’ column

heading. The trophic status data shows a narrow range of chlorophyll a concentrations that characterize mesotrophic aquatic ecosystems (Table V-8). For freshwater

areas, seasonal average chlorophyll a concentrations in mesotrophic systems should

fall in the range of 2 to 15 μg liter-1 with a mean around 7 μg liter-1. In high-salinity

marine ecosystems, mesotrophic status is characterized by seasonal average chlorophyll a concentrations from 1 to 7 μg liter-1 with a mean around 3 μg liter-1

The paired general and peak values that follow are the median and 95th percentile

concentrations of chlorophyll a in waters supportive of the phytoplankton reference

community. These chlorophyll a concentrations reflect conditions in which water

clarity is sufficient for healthy algae and bay grasses growth and the concentrations

of one or both of the critical nutrients are low enough to limit excess algal growth

(e.g., ‘best,’ ‘better’ and sometimes the ‘mixed better light’ categories). The range of

chlorophyll a concentrations that follow in the mesotrophic conditions’ peak range

column are those characteristic of algal communities not containing cell densities of

Microcystis aeruginosa and Prorocentrum minimum exceeding thresholds above

which adversely impact zooplankton and oyster communities, respectively.

The spring and summer chlorophyll a concentrations characterizing each of these

salinity-based phytoplankton reference communities provide the most direct water

quality measures of a more balanced phytoplankton assemblage (see Table V-7).

Chlorophyll a concentrations characteristic of the phytoplankton reference communities, which straddle the boundary between mesotrophic and eutrophic (Table V-8)

conditions, are higher than those observed in the 1950s (see Table V-5) which reflect

oligotrophic conditions.

Ryding and Rast (1989); Wetzel (2001); Smith et al. (1999); Molvaer et al. (1997)

and Novotny and Olem (1994) have all published ranges of chlorophyll a concentrations characterizing eutrophic conditions listed first in Table V-8 under the ‘eutrophic

conditions’ in each salinity regime specific row. The paired general and peak range

values listed next in each row are the median and 95th percentile concentrations,

respectively, of chlorophyll a in waters categorized as ‘poor’ during the process for

characterizing the reference phytoplankton communities (Appendix F, Figure F-1).

These chlorophyll a concentrations reflect water quality conditions in which both

critical nutrients (nitrogen and phosphorus) exceed the empirically determined

growth-limiting thresholds for algae, and water clarity is not sufficient for healthy

algae or underwater bay grasses growth. The range of chlorophyll a concentrations

that follow in the eutrophic conditions’ peak range column are those characteristic of

harmful algal blooms exceeding cell density thresholds derived from literature-based

values for M. aeruginosa and P. minimum.

Trends in chlorophyll a concentrations observed over the past fifty years indicate

that water quality in many tidal habitats of the Chesapeake Bay has changed from

oligotrophic-mesotrophic to eutrophic and even highly eutrophic. Chlorophyll a

chapter v • Chlorophyll a Criteria
▼132

concentrations in the highly saline waters at the mouth of the Chesapeake Bay were

characteristic of oligotrophic marine conditions in the 1950s (<2 μg liter-1). They

now reflect mesotrophic conditions, with a mean concentration of 5.6 μg liter-1 and

maxima exceeding 18 μg liter-1. Chlorophyll a concentrations in the middle and

upper Chesapeake Bay mainstem were indicative of mesotrophic conditions during

the 1950s, with mean concentrations well below 7 μg liter-1. They now reflect

eutrophic conditions, with mean chlorophyll a concentrations above 7 μg liter-1 in

mid-Bay waters and above 10 μg liter-1 in the tidal-fresh, upper Chesapeake Bay

waters. Peak concentrations often exceed 30 μg liter-1.

Eutrophic conditions also characterize all the major Bay tidal tributaries. Smaller,

urbanized watershed tidal tributaries with poor flushing, such as the Back River,

experience highly eutrophic conditions. Excessive nutrient and sediment loadings

are the cause of the shift towards eutrophic conditions in the Chesapeake Bay’s tidal

waters. The results are more deep-water habitats prone to anoxia, further losses of

underwater bay grasses and more extensive harmful algal blooms. 

Decisions on what chlorophyll a value should be applied to protect a designated use

against a specific impairment should be made at local or regional water-body scales.

More specific implementation procedures and guidelines are provided in Chapter VI.

CHLOROPHYLL A CONCENTRATIONS PROTECTIVE 

AGAINST WATER QUALITY IMPAIRMENTS

Contributions to Reduced Light Levels

Phytoplankton attenuate or reduce the amount of light reaching the leaves of bay

grasses by absorbing or scattering the light (see Chapter IV). Additional reductions

in light occur at the leaf surface, as the remaining light must pass through algal

epiphytes and suspended solids settled there (see Appendix J). Chesapeake Bay

scientists have developed a diagnostic tool to calculate the relative contributions of

chlorophyll a versus total suspended solids to reducing light penetration through the

water column (Batiuk et al. 2000; Gallegos 2001).

Water-Column Diagnostic Tool. Water-column attenuation of light measured by

the light attenuation coefficient Kd can be divided into contributions from four

sources: water, dissolved organic matter (color), chlorophyll a and total suspended

solids. The basic relationships can be expressed in a series of simple equations,

which were combined to produce the equation for the percent water-column diagnostic tool (Gallegos 2001). The resulting equation calculates linear combinations of

chlorophyll a and total suspended solids concentrations that just meet the percent

light-through-water (PLW) criteria for a particular water-column depth at any site or

season in the Chesapeake Bay and its tidal tributaries. This diagnostic tool can also

be used to consider management options for improving water quality conditions

when the water clarity criteria are not currently met (see Chapter VII). 

chapter v • Chlorophyll a Criteria
▼133

Derived Chlorophyll a Concentrations. A finite yet significant number of

possible chlorophyll a concentrations exist that support attainment of the percent

light-through-water criteria, depending on the ambient total suspended solids

concentration and water-column application depth. For the purpose of deriving

chlorophyll a criteria applicable across a wide array of tidal habitats, total suspended

solids concentrations were assumed to range from 5 to 20 mg liter-1 (Table V-9). The

water-column application depths were set at 0.5, 1 and 2 meters to reflect the range

of shallow-water designated use boundary depths (U.S. EPA 2003).

Chlorophyll a concentrations of 16 μg liter-1(tidal-fresh and oligohaline) and 8 μg

liter-1 (mesohaline and polyhaline) were identified as protective against negative

water clarity effects. Values were selected as they corresponded with total suspended

solids concentrations in the range of 10-15 mg liter-1, which were previously identified as habitat requirements for underwater bay grasses (Batiuk et al. 1992;

Dennison et al. 1993; Stevenson et al. 1993) and the 1-meter shallow-water application depth (mid-depth between 0.5 and 2 meters; U.S. EPA 2003).

Strengths and Limitations. The assignment of water clarity criteria application

depths and the selection of appropriate total suspended solids ambient concentration

assumptions should be made on a Chesapeake Bay Program segment by segment

basis. These values will vary on temporal and spatial scales. In some regions, chlorophyll a/algal biomass is a negligible component of the total light attenuation,

compared with non-algal solids. In such regions, chlorophyll a reductions would not

be expected to significantly improve water clarity.

Contribution to Low Dissolved Oxygen Conditions

Algae that are not consumed by zooplankton, oysters and fish becomes fuel, through

its breakdown by the microbial community, for reducing dissolved oxygen levels.

Seasonal chlorophyll a concentrations (e.g., algal biomass) that lead to desired

chapter v • Chlorophyll a Criteria

Table V-9. Chlorophyll a concentrations (μg liter-1) that reflect attainment of 

the Chesapeake Bay water clarity criteria given a range of total

suspended solids concentrations and shallow-water application

depths. Areas in gray indicate exceedance of the water clarity criteria.

Total

Suspended

Solids

(mg liter-1)

Tidal-Fresh and Oligohaline Mesohaline and Polyhaline

Water-Column Depth (meters)

0.5 m 1 m 2 m 0.5 m 1 m 2 m

5 199 71 9 122 34

10 171 43 95 8

15 144 16 68

20 116 42
dissolved oxygen conditions can be estimated using the Chesapeake Bay water

quality model.

The Chesapeake Bay watershed model and the 13,000-cell version of the Chesapeake Bay water quality model can be used to determine the seasonal average

chlorophyll a concentrations associated with estimated nutrient and sediment reductions needed to attain the Chesapeake Bay dissolved oxygen criteria.

The model-simulated chlorophyll a levels were extracted from the nutrient and sediment loading reduction allocation scenario which attained the Chesapeake Bay

dissolved oxygen criteria across all designated uses and tidal waters. The simulated

chlorophyll a concentrations were compiled for spring (March-May) and summer

(July-September) by salinity regime—tidal-fresh, oligohaline, mesohaline and polyhaline. The seasonal mean chlorophyll a concentration for each season and salinity

regime combination was then calculated (Table V-10). See Chapter VI for details on

how the Chesapeake Bay water quality model and Chesapeake Bay water quality

monitoring results have been integrated for assessing criteria attainment under

various management scenarios in support of setting loading allocations.

Strengths and Limitations. Like the water clarity criteria, the chlorophyll a

concentrations that are needed to attain the dissolved oxygen criteria are expected to

vary over temporal and spatial scales. Table V-10 shows the general relationship

between chlorophyll a concentrations and attainment of the dissolved oxygen

criteria. Depending on their location in the Chesapeake Bay system and hydrologic

and hydrodynamic factors, individual segments or tributaries may exceed these

concentrations without experiencing dissolved oxygen-related impairments.

METHODOLOGIES FOR DERIVING WATERBODY-SPECIFIC 

CHLOROPHYLL A CRITERIA 

Water Clarity Impairment-Based Methodology

Regional and segment-specific chlorophyll a criteria can be derived to protect

against water clarity impairments by applying the water-column diagnostic tool

described previously. When applied to local and regional tidal waters, more sitespecific assumptions about existing or anticipated ambient total suspended solids

▼134

chapter v • Chlorophyll a Criteria

Table V-10. Model-simulated seasonal mean and salinity regime-specific chlorophyll a

concentrations (μg liter-1) estimated to characterize conditions supporting

attainment of the Chesapeake Bay dissolved oxygen criteria. 

Season Tidal-Fresh Oligohaline Mesohaline Polyhaline

Spring 4565

Summer 12 7 5 4
concentrations and the shallow-water bay grasses designated use boundary depths

can be factored into the derivation of the chlorophyll a criteria.

Dissolved Oxygen Impairment-Based Methodology

Region-specific chlorophyll a concentrations can be derived by applying the Chesapeake Bay water quality model and analyzing the segment-specific results.

Confidence in the derived chlorophyll a criteria can be increased by focusing on

those Chesapeake Bay Program segments that are the principal contributors to low

dissolved oxygen conditions due to an excess production of unconsumed algae.

Nuisance Bloom-Based Methodology

Regional and segment-specific chlorophyll a targets can be derived using studies—

either user perception surveys or algal condition assessments—to identify

chlorophyll a concentrations that protect against nuisance blooms.

User Perception Surveys. User perception surveys can be conducted to rate a

user’s satisfaction with a water body’s color, clarity and overall appearance. Surveys

have been successfully applied in lake settings by several states, including Vermont

and Minnesota. User perception surveys require careful design and their form

depends on the type of water body and its uses. All such studies should include

certain elements:

1. Surveys should be conducted in conjunction with water quality and phytoplankton monitoring to allow correlation of user perceptions with ambient

conditions.

2. Commercial and recreational users should be targeted for the survey.

3. Questions should be worded to avoid bias.

4. Questions should focus on present, specific conditions rather than on general

perceptions of the Chesapeake Bay’s water quality.

5. Surveys should be conducted under a variety of water quality and sky conditions, and under a range of chlorophyll a and clarity conditions.

6. Surveys should be conducted in conjunction with objective, scientific assessments of algal conditions in the water body, as described below.

Vermont and Minnesota used lake user surveys to identify specific total phosphorus,

chlorophyll a or Secchi disk values at which algal nuisances and impairment of

recreation were perceived by the public (Heiskary and Walker 1988; Smeltzer and

Heiskary 1990; North American Lake Management Society 1992). Using the results

of a survey on physical appearance and recreation potential, Smeltzer and Heiskary

(1990) defined the statistical relationships between eutrophication-related water

▼135

chapter v • Chlorophyll a Criteria
quality variables (Secchi and chlorophyll a) and user perceptions of lake quality in

Minnesota and Vermont.

In Minnesota, surveyors calibrated user response by determining Secchi depth and

chlorophyll a levels that correspond to perceived nuisance conditions or impairment

of water uses. A nonparametric procedure was used to cross-tabulate the water

quality measurements against the user categories. Results showed a distinct contrast

between observations of ‘definite algae’ and ‘high algae’ for chlorophyll a measurements. Also, ‘impaired swimming’ and ‘no swimming’ ratings generally had

chlorophyll a levels exceeding 20-40 μg liter-1 (Heiskary and Walker 1988). In

Minnesota some distinct ecoregional patterns in user perception emerged, whereby

expectations were much greater in the deeper lakes of the northern forested region

(similar to Vermont) as compared to the shallow prairie lakes of southern Minnesota.

The final steps necessary for setting chlorophyll a criteria include specifying the

nuisance criterion (e.g., extreme chlorophyll a >30 μg liter-1) or recreation potential

and the acceptable risk level (i.e., probability that nuisance condition will be encountered 1 percent). Although Minnesota has not yet adopted total phosphorus or

chlorophyll a criteria into water quality standards, the state used the methodology as

a basis for setting lake management goals for total phosphorus. The various chlorophyll a and Secchi depth thresholds can be related to total phosphorus based on

empirical relationships (e.g., total phosphorus and frequency of various levels of

chlorophyll a) as noted in Heiskary and Walker (1988). 

Algal Condition Assessments. Algal condition assessments involve qualitative

descriptions and ordinal ratings of algal conditions by monitoring personnel. These

constitute the ‘scientific’ version of the user perception survey. Qualitative information to be recorded includes the presence or absence of floating algae, its color, odor,

etc. As with user perception surveys, algal condition assessment should be

performed in conjunction with water quality and phytoplankton monitoring. It is

highly recommended that states develop and apply standard indices for use with

algal condition assessments. For example, Table V-11 provides an example developed for coastal waters in Oregon.

Algal condition assessments should be conducted by trained scientists or technicians. The more highly trained the personnel, the more detailed information can be

collected on the size, texture and density of blooms. States that decide to pursue this

approach should consider adding algal assessments to their existing Chesapeake Bay

and tidal tributary monitoring programs. Ideally, user perception surveys and algal

condition assessments would be conducted in tandem. However, algal condition

assessments will have some utility for setting chlorophyll a targets independent of

user perception surveys. It is expected that surveys and assessments would result in

different chlorophyll a targets for different salinity regimes. For example, brightgreen algae that form surface scums (e.g., M. aeruginosa) in some tidal freshwater

segments might be more perceptible at lower chlorophyll a concentrations than

brownish, more dispersed blooms.

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chapter v • Chlorophyll a Criteria
LITERATURE CITED

Batiuk, R. A., P. Bergstrom, M. Kemp, E. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V.

Carter, N. B. Rybicki, J. M. Landwehr, C. Gallegos, L. Karrh, M. Naylor, D. Wilcox, K. A.

Moore, S. Ailstock and M. Teichberg. 2000. Chesapeake Bay Submerged Aquatic Vegetation

Water Quality and Habitat-Based Requirements and Restoration Targets: A Second Technical

Synthesis. CBP/TRS 245/00 EPA 903-R-00-014. U.S. EPA Chesapeake Bay Program,

Annapolis, Maryland.

Batiuk, R. A., R. Orth, K. Moore, J. C. Stevenson, W. Dennison, L. Staver, V. Carter, N.

Rybicki, R. Hickman, S. Kollar and S. Bieber. 1992. Submerged Aquatic Vegetation Habitat

Requirements and Restoration Targets: A Technical Synthesis. CBP/TRS 83/92. U.S. EPA

Chesapeake Bay Program, Annapolis, Maryland.

Boynton, W. R., W. M. Kemp and C. W. Keefe. 1982. A comparative analysis of nutrients

and other factors influencing estuarine phytoplankton production. In: V. S. Kennedy (ed.).

Estuarine Comparisons. Academic Press, New York. Pp. 69-90. 

Breitburg, D. L., J. G. Sanders, C. C. Gilmour, C. A. Hatfield, R. W. Osman, G. F. Riedel, S.

P. Seitzinger, K. G. Sellner. 1999. Variability in responses to nutrients and trace elements, and

transmission of stressor effects through an estuarine food web. Limnology and Oceanography

44(3,2):837-863.

Burns, C. W., D. J. Forsyth, J. F. Haney, M. R. James, W. Lampert and R. D. Pridmore. 1989.

Coexistence and exclusion of zooplankton by Anabaena minutissima var. attenuata in Lake

Rotongaio, New Zealand. Archiv füür Hydrobiologie Beiheft Ergebnisse der Limnologie

32:63- 82.

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chapter v • Chlorophyll a Criteria

Table V-11. Example of an algal condition index.

Algal Index

Value Category Description

0 Clear Conditions vary from no algae to small populations

visible to the naked eye.

1 Present Some algae visible to the naked eye but present at

low to medium levels.

2 Visible Algae sufficiently concentrated that filaments

or balls of algae are visible to the naked eye.

May be scattered streaks of algae on water surface.

3 Scattered

Surface

Blooms

Surface mats of algae scattered. May be more

abundant in localized areas if winds are calm. Some

odor problems.

4 Extensive

Surface

Blooms

Large portions of the water surface covered by mats

of algae. Windy conditions may temporarily

eliminate mats, but they will quickly redevelop as

winds become calm. Odor problems in localized

areas.

.

Source: Coastnet, 1996, Sampling Procedures: A Manual for Estuary Monitoring, prepared for the

Coastnet Water quality Monitoring Project administered by the Oregon State University Extension Sea

Grant Program, http://secchi.hmsc.orst.edu/coastnet/manual/index.html.
Capper, J., G. Power, F. R. Shivers, Jr. 1983. Chesapeake Waters: Pollution, Public Health,

and Public Opinion, 1607-1972. Tidewater Publishers. Centreville, Maryland.

Clarke, N. V. 1978. The food of adult copepods from Lake Kainji, Nigeria. Freshwater

Biology 8:321-326.

D’Avanzo, C. and J. N. Kremer. 1994. Diel oxygen dynamics and anoxic events in an

eutrophic estuary of Waquoit Bay, Massachusetts. Estuaries 17:131-139.

Deeds, J. R., D. E. Terlizzi, J. E. Adolf, D. K. Stoecker and A. R. Place. 2002. Toxic activity

from cultures of Karlodinium micrum (=Gyrodinium galatheanum) (Dinophyceae) – a

dinoflagellate associated with fish mortalities in an estuarine aquaculture facility. Harmful

Algae 1:169-189.

DeMott, W. R. and F. Moxter. 1991. Foraging on cyanobacteria by copepods: Responses to

chemical defenses and resource abundance. Ecology 72:1820-1834.

DeMott, W. R., Q. Z. Zhang and W. W. Carmichael. 1991. Effects of toxic cyanobacteria and

purified toxins on the survival and feeding of a copepod and three species of Daphnia.

Limnology and Oceanography 36:1346-1357.

Dennison, W. C., R. J. Orth, K. A. Moore, J. C. Stevenson, V. Carter, S. Kollar, P. W.

Bergstrom and R. A. Batiuk. 1993. Assessing water quality with submersed aquatic vegetation habitat requirements as barometers of Chesapeake Bay health. Bioscience 43:86-94.

Filardo, M. J. and W. M. Dunstan. 1985. Hydrodynamic control of phytoplankton in low

salinity waters of the James River estuary, Virginia. Estuarine, Coastal and Shelf Science

21:653-667.

Fisher, T. R., A. B. Gustafson, K. Sellner, R. Lacouture, L. W. Haas, R. L. Wetzel, R.

Magnien, D. Everitt, B. Michaels and R. Karrh. 1999. Spatial and temporal variation of

resource limitation in Chesapeake Bay. Marine Biology 133:763-778.

Fisher, T. R., L. W. Harding, D. W. Stanley and L. G. Ward. 1988. Phytoplankton, nutrients

and turbidity in the Chesapeake, Delaware and Hudson estuaries. Estuarine, Coastal and

Shelf Science 27:61-93.

Flemer, D. A. 1970. Primary production in Chesapeake Bay. Chesapeake Science 11:117-

129.

Flemer, D. A., G. B. Mackiernan, W. Nehlsen, V. K. Tippie (technical coordinators) and R.

B. Biggs, D. Blaylock, N. H. Burger, L. C. Davidson, D. Haberman, K. S. Price, J. L. Taft

(contributing authors). 1983. Chesapeake Bay: A Profile of Environmental Change. U.S. EPA

Region III, Philadelphia, Pennsylvania.

Fulton R. S. III and H. W. Paerl. 1987. Toxic and inhibitory effects of the blue-green alga

Microcystis aeruginosa on herbivorous zooplankton. Journal of Plankton Research 9

(5):837-855.

Fulton, R. S. III and H. W. Paerl. 1988. Effects of the blue-green alga Microcystis aeruginosa

on zooplankton competitive relations. Oecologia 76:383-389.

Fulton, R.S. III and R.C. Jones. 1991. Growth and reproductive responses of Daphnia to

cyanobacterial blooms on the Potomac River. Internationale Review gesamten Hydrobiologie

76:5-19.

Gallegos, C. L. 2001. Calculating optical water quality targets to restore and protect

submersed aquatic vegetation: Overcoming problems in partitioning the diffuse attenuation

coefficient for photosynthetically active radiation. Estuaries 24:381-397.

▼138

chapter v • Chlorophyll a Criteria
Gerritsen, J. J., A. F. Holland and D. E. Irvine. 1994. Suspension-feeding bivalves and the

fate of primary production: An estuarine model applied to Chesapeake Bay. Estuaries 17-

2:403-416.

Gerritsen, J. J., A. Ranasinghe and A. F. Holland. 1988. Comparison of three strategies to

improve water quality in the Maryland portion of Chesapeake Bay. Unpublished report,

Versar Inc. Columbia, Maryland.

Gilbert, J. J. 1990. Differential effects of Anabaena affinis on cladocerans and rotifers: Mechanisms and implications. Ecology 71:1727-1740.

Gilbert, J. J. and K. G. Bogdan. 1984. Rotifer grazing: In situ studies on selectivity and rates.

In: Meyers, D. G. and J. R. Strickler (eds.). Trophic Interactions Within Aquatic Ecosystems

85:97- 133. American Association for the Advancement of Science Selected Symposium.

Boulder, Colorado.

Glibert, P. M., R. Magnien, M. W. Lomas, J. Alexander, C. Fan, E. Haramoto, M.Trice and

T. M. Kana. 2001. Harmful algal blooms in the Chesapeake Bay and coastal bays of Maryland, USA: Comparison of 1997, 1998 and 1999 events. Estuaries 24:875-883.

Goshorn, D., J. R. Deeds, P. J. Tango, C. Poukish, A. R. Place, M. McGinty, W. Butler, C.

Luckett and R. E. Magnien. 2003 (In review). Occurrence of Karlodinium micrum and its

association with fish kills in Maryland estuaries. Proceedings of the Tenth International

Conference on Harmful Algae, St. Petersburg, Florida.

Haas, L. W. and R. L. Wetzel. 1993. Nutrient limitation in the Chesapeake Bay: Nutrient

bioassays in the Virginia Bay system. Final report to Virginia Coastal Resources Management Program, Virginia Institute of Marine Science, School of Marine Science, College of

William and Mary, Gloucester Point, Virginia.

Hagy, J. D. 2002. Eutrophication, hypoxia and trophic transfer efficiency in Chesapeake Bay.

Ph.D. dissertation, University of Maryland, College Park, Maryland.

Harding, L. W., Jr, E. C. Itsweire and W. E. Esais. 1992. Determination of phytoplankton

chlorophyll concentrations in the Chesapeake Bay with aircraft remote sensing. Remote

Sensing of Environment 40:79-100.

Harding, L. W., Jr. 1994. Long-term trends in the distribution of phytoplankton in Chesapeake Bay: Roles of light, nutrients and streamflow. Marine Ecology Progress Series

104:267-291. 

Harding, L. W. Jr. and E. S. Perry. 1997. Long-term increase of phytoplankton biomass in

Chesapeake Bay, 1950-1994. Marine Ecology Progress Series 157:39-52.

Harding, L. W. Jr., B. W. Meeson and T. R. Fisher. 1986. Phytoplankton production in two

East Coast estuaries: Photosynthesis-light functions and patterns of carbon assimilation in

Chesapeake and Delaware bays. Estuarine, Coastal and Shelf Science 23:773-806. 

Hartman, K. J. and S. B. Brandt. 1995. Trophic resource partitioning, diets and growth of

sympatric estuarine predators. Transactions of the American Fisheries Society 124:520-537.

Heck, K. L. Jr. (ed.). 1987. Ecological Studies in the Middle Reach of Chesapeake

Bay–Calvert Cliffs. Springer-Verlag. Berlin, Heidelberg and New York.

Heiskary, S. A. and W. W. Walker. 1988. Developing phosphorus criteria for Minnesota lakes.

Lake and Reservoir Management 4:1-10.

▼139

chapter v • Chlorophyll a Criteria
Henning, M., H. Hertel, H. Wall and J. G. Kohl. 1991. Strain-specific influence of Microcystis aeruginosa on food ingestion and assimilation of some cladocerans and copepods.

Internationale Review gesamten Hydrobiologie 76:37-45.

Ho, M. S. and P. L. Zubkoff. 1979. The effects of a Cochlodinium heterolobatum bloom on

the survival and calcium uptake by larvae of the American oyster, Crassostrea virginica. In:

Taylor, F. J. R. and H. H. Seliger (eds.). Toxic Dinoflagellate Blooms. Elsevier North Holland,

New York.

Infante, A. and W. Riehl. 1984. The effect of cyanophyta upon zooplankton in a eutrophic

tropical lake (Lake Valencia, Venezuela). Hydrobiologia 113:293-298.

Jaworski, N. A., R. W. Howarth, L. J. Hetling. 1997. Atmospheric deposition of nitrogen

oxides onto the landscape contributes to coastal eutrophication in the northeast United States.

Environmental Science and Technology 31:1995-2004.

Kemp, W. M., E. M. Smith, M. Marvin-Dipasquale and W. R. Boynton. 1997. Organic

carbon balance and net ecosystem metabolism in Chesapeake Bay. Marine Ecology Progress

Series 150:229-248.

Kempton, J. W., A. J. Lewitus, J. R. Deeds, J. M. Law and A. R. Place. 2002. Toxicity of

Karlodinium micrum (Dinophyceae) associated with a fish kill in a South Carolina brackish

retention pond. Harmful Algae 1:233-241.

Kennedy, V. S. and L. Breisch. 1981. Review of literature pertaining to the American oyster,

Crassostrea virginica, in Chesapeake Bay. Maryland Sea Grant College Program, College

Park, Maryland.

Lacouture, R. V., J. H. Sniezek and K. G. Sellner. 1993. Level I Report: Maryland Chesapeake Bay Water Quality Monitoring Program–Phytoplankton and Microzooplankton

Component. Academy of Natural Sciences, Philadelphia, Benedict Estuarine Research Laboratory, Benedict, Maryland. 84 pp.

Lampert, W. 981. Inhibitory and toxic effects of blue-green algae on Daphnia. Internationale

Review gesamten Hydrobiologie 66:285-298.

Lampert, W. 1982. Further studies on the inhibitory effect of the toxic blue-green Microcystis

aeruginosa on the filtering rate of zooplankton. Archives of Hydrobiology . 95:207-220.

Lewitus, A. J., R. V. Jesien, T. M. Kana, J. M. Burkholder, H. B. Glasgow, Jr. and E. May.

1995. Discovery of the “phantom” dinoflagellate in Chesapeake Bay. Estuaries 18-2:373-

378.

Livingston, R. 2001. Eutrophication processes in coastal systems. CRC Press, Boca Raton,

Florida. 327 pp.

Lorenzen, C. J. 1972. Extinction of light in the ocean by phytoplankton. Journal of Conservation 34:262-267.

Luckenbach, M. W., K. G. Sellner, S. E. Shumway and K. Greene. 1993. Effects of two

bloom- forming dinoflagellates, Prorocentrum minimum and Gyrodinium uncatenum, on the

growth and survival of the Eastern oyster, Crassostrea virginica (Gmelin 1791). Journal of

Shellfish Research 12(2):411-415.

Mackiernan, G. B. 1968. Seasonal distribution of dinoflagellates in the lower York River,

Virginia. Unpublished thesis. School of Marine Science, College of William and Mary,

Gloucester Point, Virginia. 104 pp.

▼140

chapter v • Chlorophyll a Criteria
Magnien, R. E., R. M. Summers and K. G. Sellner. 1992. External nutrient sources, internal

nutrient pools and phytoplankton production in Chesapeake Bay. Estuaries 15:497-516.

Malone, T. C. 1992. Effects of water column processes on dissolved oxygen, nutrients, phytoplankton and zooplankton. In: Smith, D. E., M. Leffler and G. Mackiernan (eds.). Oxygen

Dynamics in the Chesapeake Bay, a Synthesis of recent research. Pp. 61-112. Maryland Sea

Grant College. UM-SG-TS-92-01. College Park, Maryland. 

Malone, T. C., L. H. Cocker, S. E. Pike and B. W. Wendler. 1988. Influences of river flow on

the dynamics of phytoplankton production in a partially stratified estuary. Marine Ecology

Progress Series 48:235-249.

Malone, T. C., H. W. Ducklow, E. R. Peele and S. E. Pike. 1991. Picoplankton carbon flux in

Chesapeake Bay. Marine Ecology Progress Series 78:11-22.

Malone, T. C., W. M. Kemp, H. W. Ducklow, W. R. Boynton, J. H. Tuttle and R. B. Jonas.

1986. Lateral variation in the production and fate of phytoplankton in a partially stratified

estuary. Marine Ecology Progress Series 32:149-160.

Marshall, H. G. 1995. Succession of dinoflagellate blooms in the Chesapeake Bay, U.S.A. In:

Lassus, P., G. Arzul, E. Erard, P. Gentien and C. Marcaillou (eds.). Harmful Marine Algal

Blooms. Lavoisier, Intercept Ltd., Paris. 

Marshall, H. G. and K. K. Nesius. 1996. Phytoplankton composition in relation to primary

production in Chesapeake Bay. Marine Biology 125:611-617.

Marshall, H. and R. Lacouture. 1986. Seasonal patterns of growth and composition for phytoplankton in the lower Chesapeake Bay. Estuarine, Coastal and Shelf Science 23:115-130.

Molvaer, J., J. Knutzen, J. Magnusson, B. Rygg, J. Skei and J. Sorensen. 1997. Environmental quality classification in fjords and coastal areas. Statens Forurensningstilsyn TA-1467,

Norway. 36 pp.

Nagazima, M. 1965. Studies on the source of shellfish poison in Lake Hamana I. Relation

of the abundance of a species of dinoflagellate, Prorocentrum sp. to shellfish toxicity.

Bulletin of Japanese Society of Science and Fisheries 31:198-203.

Nagazima, M. 1968. Studies on the source of shellfish poison in Lake Hamana IV. Identification and collection of the noxious dinoflagellate. Bulletin of Japanese Society of Science

and Fisheries 43:130-131.

National Research Council. 2001. Assessing the TMDL Approach to Water Quality Management. Committee to Assess the Scientific Basis of the Total Maximum Daily Load Approach

to Water Pollution Reduction, Water Science and Technology Board, Division on Earth and

Life Studies. National Academy Press, Washington, D. C.

Nielsen, M. V. 1993. Toxic effect of the marine dinoflagellate Gymnodinium galatheanum on

juvenile cod Gadus morhua. Marine Ecology Progress Series 95:273-277.

Neilson, B. and L. Cronin (eds.). 1981. Estuaries and Nutrients. Humana, Clifton, New

Jersey. 643 pp.

Newell, R. I. E. 1988. Ecological changes in Chesapeake Bay: Are they the result of overharvesting the American oyster, Crassostrea virginica? In: Understanding the Estuary:

Advances in Chesapeake Bay Research. Conference proceedings, March 1988. Publication

number 129. Chesapeake Research Consortium, Gloucester Point, Virginia.

▼141

chapter v • Chlorophyll a Criteria
▼142

chapter v • Chlorophyll a Criteria

Nixon, S. W., C. A. Oviatt, J. Frithsen and B. Sullivan.1986. Nutrients and the productivity

of estuarine and coastal marine systems. Journal of the Limnology Society of South Africa

12:43-71.

Nizan, S., C. Dimentman and M. Shilo. 1986. Acute toxic effects of the cyanobacterium

Microcystis aeruginosa on Daphnia magna. Limnology and Oceanography 31:497-502.

North American Lake Management Society. 1992. Developing eutrophication standards for

lakes and reservoirs. Report prepared by the Lake Standards Subcommittee. Alachua,

Florida. 51 pp.

Novotny V. and Olem H. 1994. Water Quality: Prevention, Identification and Management of

Diffuse Pollution. Van Nostrand Reinhold. New York, New York. 1054pp.

Officer, C. G., R. B. Biggs, J. L. Taft, L. E. Cronin, M. A. Tyler and W. R. Boynton. 1984.

Chesapeake Bay anoxia: Origin, development and significance. Science 23:22-27.

Olson, M. 2002. Benchmarks for nitrogen, phosphorus, chlorophyll and suspended solids in

Chesapeake Bay. Chesapeake Bay Program Technical Report Series, Chesapeeake Bay

Program, Annapolis, Maryland.

Orcutt, J.D. and M.L. Pace. 1984. Seasonal dynamics of rotifer and crustacean zooplankton

populations in a eutrophic, monomictic lake with a note on rotifer sampling techniques.

Hydrobiologia 119:73-80.

Orth, R. J. and K. A. Moore. 1983. Chesapeake Bay: an unprecedented decline in submerged

aquatic vegetation. Science 222:51-53.

Pheiffer, T. H. 1975. Current nutrient assessment–upper Potomac Estuary. Paper No. 1.

ICPRB Symposium, Biological Resources of the Potomac Estuary. Annapolis field office,

Region III. U.S. EPA. 22 pp.

Roelke, D. L., P. M. Eldridge and L. A. Cifuentes. 1999. A model phytoplankton competition

for limiting and non-limiting nutrients: Implications for management of dynamic environments. Estuaries 22:92-104.

Roelke, D. L. 2000. Copepod food-quality threshold as a mechanism influencing phytoplankton succession and accumulation of biomass and secondary productivity: A modeling

study with management implications. Ecological Modeling 134:245-274.

Ryding, S. O. and W. Rast. 1989. The control of eutrophication of lakes and reservoirs. Man

and the Biosphere Series, Volume 1, UNESCO, Parthenon Publication Group, Park Ridge,

New Jersey. 314 pp.

Seliger, H. H., J. A. Boggs and W. H. Biggley. 1985. Catastrophic anoxia in the Chesapeake

Bay in 1984. Science 228:70-73.

Seliger, H. H., M. E. Loftus and D. V. Subba Rao. 1975. Dinoflagellate accumulations in

Chesapeake Bay. In: Proceedings of the First International Conference on Toxic Dinoflagellate Blooms. LoCicero, V. R. (ed.). Pp.181-205. Massachusetts Science and Technology

Federation, Wakefield, Massachusetts. 

Sellner, K. G., Magnien, R. E, Boynton, W. R. and W. M. Kemp 1986. Relationships between

nutrients and plankton in Chesapeake Bay: II: Phytoplankton dynamics and fate of primary

production. ASLO_PSA. Abstract and presentation June 1986, University of Rhode Island,

Kingston, Rhode Island.
▼143

chapter v • Chlorophyll a Criteria

Simmons, G.M. Jr., B.J. Armitage and J.C. White. 1974. An ecological evaluation of heated

water discharge on phytoplankton blooms in the Potomac River. Hydrobiologia 45:441-465.

Sin,Y., R. L. Wetzel and I. C. Anderson. 1999. Spatial and temporal characteristics of nutrient

and phytoplankton dynamics in the York River estuary, Virginia: Analyses of long-term data.

Estuaries 22-2A:260-275.

Smeltzer, E. and S. A. Heiskary. 1990. Analysis and Applications of Lake User Survey Data.

Lake and Reservoir Management 6(1):109-118.

Smith, V. H. 1998. Cultural eutrophication of inland, estuarine and coastal waters. In: Pace,

M. L. and P. M. Groffman (eds.). Successes, Limitation and Frontiers in Ecosystem Science.

Springer-Verlag, New York, New York. Pp. 7-49.

Smith, A. D. and J. J. Gilbert. 1995. Relative susceptibilities of rotifers and cladocerans to

Microcystis aeruginosa. Archives of Hydrobiology 132:309-336.

Stevenson, J. C., L. W. Staver and K. W. Staver. 1993. Water quality associated with survival

of submersed aquatic vegetation along an estuarine gradient. Estuaries 16:346-361.

Sweden Environmental Protection Agency. 2002. Environmental Quality Criteria: Coasts

and Seas. Stockholm, Sweden.

Thomann, R. V., N. J. Jaworski, S. W. Nixon, H. W. Paerl and J. Taft. 1985. Blue-green algal

blooms on the Potomac River: The bases for considering them a nuisance. In: The 1983 

Algal Bloom in the Potomac Estuary. Potomac Strategy–State/EPA Management Committee. 

353 pp.

Threlkeld, S. T. 1986. Resource-mediated demographic variation during the midsummer

succession of a cladoceran community. Freshwater Biology 16:673-683.

Tuttle, J. H., T. C. Malone, R. B. Jonas, H. W. Ducklow and D. Cargo. 1987. NutrientDissolved Oxygen Dynamics: Role of Phytoplankton and Microheterotrophs Under Summer

Conditions, 1985. CBP/TRS 3/87. U.S. EPA, Chesapeake Bay Office, Annapolis, Maryland.

U.S. Environmental Protection Agency. 1997. Methods for the Determination of Chemical

Substances in Marine and Estuarine Environmental Matrices, 2nd Edition. Method 446.0.

EPA/600/R-97/072. U.S. EPA, Office of Research and Development, Washington, D. C.

U.S. Environmental Protection Agency. 2003. Technical support document for identifying

Chesapeake Bay designated uses and attainability. EPA 903-R-03-004 Chesapeake Bay

Program Office, Annapolis, Maryland.

Verity, P. G. 1987. Factors driving changes in the pelagic trophic structure of estuaries, with

implications for the Chesapeake Bay. In: Lynch, M. P. and E. C. Krome (eds.). Perspectives

on the Chesapeake Bay: Advances in Estuarine Sciences. Publication No. 127. Chesapeake

Research Consortium, Gloucester Point, Virginia. 

Vollenweider, R. A. 1992. Coastal marine eutrophication: principles and control. Science of

the Total Environment (Supplement):1-20.

Wetzel, R. G. 2001. Limnology–Lake and River Ecosystems, 3rd Edition. Academic Press,

New York, New York.
▼144

chapter v • Chlorophyll a Criteria

Wickfors, G. H. and R. M. Smolowitz. 1995. Experimental and histological studies of four

life- history stages of the Eastern oyster, Crassostrea virginica, exposed to a cultured strain

of the dinoflagellate, Prorocentrum minimum. Biology Bulletin 188:313-328.

Zubkoff, P. L., J. C. Munday, R. G. Rhodes and J. E. Warriner. 1979. Mesoscale features of

summer (1975 to 1977) dinoflagellate blooms in the York River, Virginia (Chesapeake Bay

Estuary). In: F. J. R. Taylor and H. H. Seliger (eds.). Toxic Dinoflagellate Blooms. Elsevier

North Holland, Inc. New York, New York.

Zubkoff, P. L. and J. E. Warriner. 1975. Synoptic sightings of red waters of the lower Chesapeake Bay and its tributary rivers (May 1973 to September 1974). In: Proceedings of the First

International Conference on Toxic Dinoflagellate Blooms, LoCicero, V. R. (ed.). Massachusetts Science and Technology Foundation. Wakefield, Massachusetts. 
This chapter presents implementation procedures as regional guidance to the Chesapeake Bay watershed states and other agencies, institutions, groups or individuals

applying the criteria to determine the degree of attainment. In accordance with

Section 117(b)(2)(B)(iii) of the Clean Water Act, these procedures accompany the

regional criteria to promote their consistent, baywide application in common tidalwater designated uses across jurisdictional boundaries.

The Chesapeake Bay criteria, as presented in the previous three chapters, will protect

designated uses if they are applied strictly following current EPA national guidelines. The regional implementation procedures described in this chapter are tailored

to the Chesapeake Bay and its tidal tributaries, the refined tidal-water designated

uses and the current and anticipated enhancements to the baywide coordinated monitoring program. Adoption and application of the Chesapeake Bay–specific

implementation procedures across jurisdictions will give the states and other partners

a greater degree of confidence in assessing the attainment of criteria and protection

of designated uses. The extensive shared tidal waters should be assessed consistently

across the four jurisdictions using these recommended procedures that account for

natural conditions and processes, highlight the magnitude and extent of remaining

impairments and provide up-front diagnostics of possible reasons for criteria nonattainment. The EPA strongly encourages states to adopt these implementation

procedures into their water quality standards.

The chapter includes:

• A brief review of the criteria, defining the spatial and temporal boundaries

within which criteria attainment will be measured;

• A method for quantifying and visualizing the degree of criteria attainment or

exceedance that incorporates the amount of area or volume of a region that

meets or exceeds a criterion and how often a criterion is met or exceeded; 

• A description of successful criteria attainment recognizing that 100 percent

attainment is not necessary to protect designated and existing uses;

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chapter vi • Recommended Implementation Procedures

chaptervi

Recommended 

Implementation Procedures
• A practical description of how monitoring information may be used to assess

attainment, including statistical estimation methods for addressing assessment

of the short-interval criteria, such as the 7-day mean, 1-day mean and instantaneous minimum dissolved oxygen criteria; and

• A description of how mathematical model-simulated information may be used

to assess the effect on future criteria attainment under various nutrient/sediment

reduction scenarios, which support decisions on load reductions and caps on

loadings to maximize the beneficial effect on attainment.

DEFINING CRITERIA ATTAINMENT

DISSOLVED OXYGEN CRITERIA

The Chesapeake Bay dissolved oxygen criteria were derived to protect species and

communities in the five tidal-water designated uses during specific seasons (Table

VI-1). See Chapter III for detailed information on the designated use-specific criteria

and appropriate periods for applying them. Refer to Appendix A and the Technical

Support Document for the Identification of Chesapeake Bay Designated Uses and

Attainability (U.S. EPA 2003) for details on the five designated uses and their boundaries. The Chesapeake Bay dissolved oxygen criteria should not be applied to a

designated use or during a period of the year for which they were not specifically

derived (see Chapter III).

The EPA expects the states to adopt the full set of dissolved oxygen criteria that will

protect the refined tidal-water designated uses, presented in Table VI-1. Given recognized limitations in direct monitoring at the temporal scales required for assessing

attainment of the instantaneous minimum, 1-day mean and 7-day mean criteria (see

section titled “Monitoring to Support the Assessment of Criteria Attainment” for

more details), states can waive attainment assessments for these criteria until monitoring at the required temporal scales is implemented or apply statistical methods to

estimate probable attainment. Where sufficient data at these temporal scales exist for

specific regions or local habitats, states should assess attainment of the full set of

applicable dissolved oxygen criteria.

WATER CLARITY CRITERIA

The Chesapeake Bay water clarity criteria were derived based on the minimum

percent light-through-water (PLW) requirements of underwater bay grasses (Table

VI-2). These criteria apply only to shallow-water designated use habitats. The water

clarity criteria are not intended to apply in areas where underwater bay grasses are

precluded from growing by non-water clarity-related factors such as excessive wave

action or at depths where natural and other physical habitat factors will prevent

sufficient light penetration required by the plants. See Chapter IV for a discussion of

the salinity regime-specific criteria and time periods for application. Refer to

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chapter vi • Recommended Implementation Procedures

Table VI-1. Chesapeake Bay dissolved oxygen criteria.

Designated Use Criteria Concentration/Duration Protection Provided Temporal Application

7-day mean > 6 mg liter-1(tidal habitats with 0-0.5 ppt salinity)

Survival/growth of larval/juvenile tidal-fresh resident fish;protective of threatened/endangered species.

Instantaneous minimum > 5 mg liter-1 Survival and growth of larval/juvenile migratory fish;

protective of threatened/endangered species.

February 1 - May 31 Migratory fish

spawning

and

nursery use

Open-water fish and shellfish designated use criteria apply June 1 - January 31

Shallow-water bay

grass use

Open-water fish and shellfish designated use criteria apply Year-round

30-day mean > 5.5 mg liter-1(tidal habitats with 0-0.5 ppt salinity)

Growth of tidal-fresh juvenile and adult fish; protective ofthreatened/endangered species.

30-day mean > 5 mg liter-1(tidal habitats with >0.5 ppt salinity)

Growth of larval, juvenile and adult fish and shellfish;protective of threatened/endangered species.

7-day mean > 4 mg liter-1 Survival of open-water fish larvae.

Open-water fish and

shellfish use

Instantaneous minimum > 3.2 mg liter-1 Survival of threatened/endangered sturgeon species.1

Year-round

30-day mean > 3 mg liter-1 Survival and recruitment of bay anchovy eggs and larvae.1-day mean > 2.3 mg liter-1 Survival of open-water juvenile and adult fish.Instantaneous minimum > 1.7 mg liter-1 Survival of bay anchovy eggs and larvae.

June 1 - September 30 Deep-water

seasonal fish and

shellfish use

Open-water fish and shellfish designated-use criteria apply October 1 - May 31

Instantaneous minimum > 1 mg liter-1 Survival of bottom-dwelling worms and clams. June 1 - September 30

Deep-channel

seasonal refuge use Open-water fish and shellfish designated use criteria apply October 1 - May 31

1 At temperatures considered stressful to shortnose sturgeon (>29C), dissolved oxygen concentrations above an instantaneous minimum of 4.3 mg

liter-1 will protect survival of this listed sturgeon species.

Deep-channel 

seasonal refuge use
Appendix A and U.S. EPA (2003) for broad and detailed descriptions, respectively,

of the shallow-water designated use and its boundaries.

The Chesapeake Bay water clarity criteria should not be applied to a designated use

or in a period during the year for which they were not derived. The March 1 through

May 31 and September 1 through November 30 temporal application for the polyhaline water clarity criteria was originally established for protection of eelgrass

(Zostera marina) beds (Batiuk et al. 1992). Widgeon grass (Ruppia maritima) cooccurs with eelgrass in polyhaline habitats. In shallow-water habitats where both

species currently or historically co-occur1, states and other users should assess water

clarity criteria attainment using a March 1 through November 30 or April 1 through

October 31 temporal application period. 

When the water clarity criteria were derived, there was an insufficient scientific basis

for deriving a set of water clarity or related (e.g., total suspended solids) criteria for

protection of open-water designated use habitats. 

The EPA expects the states to adopt the salinity regime-specific water clarity criteria

to protect their shallow-water designated uses, presented in Table VI-2. States are

expected to measure the achievement of the shallow-water designated use at the

Chesapeake Bay Program segment scale by achieving an established acreage of

underwater bay grasses, attainment of the applicable water clarity criteria at an

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Table VI-2. Summary of Chesapeake Bay water clarity criteria for application to shallow-water 

bay grass designated use habitats.

1Maps of the potential and recent distributions of both species were published by Batiuk et al. (1992);

see page 125 for eelgrass and page 128 for widgeon grass. Further information on underwater bay grass

aerial survey findings on the distribution of these two species can also be found at the Virginia Institute

of Marine Science’s website at http://www.vims.edu/bio/sav. 

1Based on application of Equation IV-1, PLW = 100exp(-KdZ), the appropriate PLW criterion value and the selected application

depth are inserted and the equation is solved for Kd. The generated Kd value is then converted to Secchi depth (in meters) using

the conversion factor Kd = 1.45/Secchi depth.

Water Clarity Criteria as Secchi Depth

Water Clarity Criteria Application Depths

0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0

 Salinity

Regime

Water Clarity

Criteria as

Percent Lightthrough-Water

Secchi Depth (meters) for above Criteria Application Depth

Temporal

 Application

Tidal-fresh 13 % 0.2 0.4 0.5 0.7 0.9 1.1 1.2 1.4 April 1 - October 31

Oligohaline 13 % 0.2 0.4 0.5 0.7 0.9 1.1 1.2 1.4 April 1 - October 31

Mesohaline 22 % 0.2 0.5 0.7 1.0 1.2 1.4 1.7 1.9 April 1 - October 31

Polyhaline 22 % 0.2 0.5 0.7 1.0 1.2 1.4 1.7 1.9 March 1 - May 31,

September 1 - November 30

1.1
established application depth or attainment of the applicable water clarity criteria

throughout an established potential shallow-water habitat acreage. The available

supporting technical information on segment-specific underwater bay grass

acreages, application depths and potential shallow-water habitat acreages are

described in the “Monitoring to Support the Assessment of Criteria Attainment,”

section of this chapter and published in detail in the Technical Support Document for

the Identification of Chesapeake Bay Designated Uses and Attainability (U.S. EPA

2003).

CHLOROPHYLL A CRITERIA

Because of the regional and site-specific nature of algal-related water quality impairments, only narrative chlorophyll a criteria have been published here. The

chlorophyll a concentrations tabulated in Chapter V are not numerical EPA criteria.

Along with the documented methodologies, they are provided as a synthesis of the

best available technical information supporting the states’ development and adoption

of site-specific numerical chlorophyll a criteria or the derivation of numerical translators for their narrative chlorophyll a criteria. 

The narrative Chesapeake Bay chlorophyll a criteria were derived to address the full

array of possible impairments, all of which may not manifest themselves within a

particular water body at a given time (Table VI-3). The site-specific nature of impairments caused by the overabundance of algal biomass supports the states’ adoption of

the EPA-recommended narrative criteria, with application of site-specific numeric

criteria only for localized waters addressing local algal-related impairments. 

The EPA expects states to adopt narrative chlorophyll a criteria into their water

quality standards for all Chesapeake Bay and tidal tributary waters. The EPA

strongly encourages states to develop and adopt site-specific numerical chlorophyll

a criteria for tidal waters where algal-related impairments persist after the Chesapeake Bay dissolved oxygen and water clarity criteria have been attained.

The formulation and ultimately the assessment of numerical chlorophyll a criteria

should be based upon seasonal dynamics and concentrations of chlorophyll a in the

Chesapeake Bay and its tributaries. Spring and summer were chosen for these

purposes. Any site-specific numerical impairment-based chlorophyll a criteria

should be applied as salinity regime-based spring (March through May) and summer

(July through September) seasonal mean concentrations.

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chapter vi • Recommended Implementation Procedures

Table VI-3. Recommended Chesapeake Bay chlorophyll a narrative criteria.

Concentrations of chlorophyll a in free-floating microscopic aquatic plants (algae) shall 

not exceed levels that result in ecologically undesirable consequences-such as reduced

water clarity, low dissolved oxygen, food supply imbalances, proliferation of species

deemed potentially harmful to aquatic life or humans or aesthetically objectionable 

conditions-or otherwise render tidal waters unsuitable for designated uses.
ADDRESSING MAGNITUDE, DURATION, 

FREQUENCY, SPACE AND TIME

To define and measure criteria attainment, a number of factors are taken into

account. According to a recent National Research Council (2001) review, establishing the “magnitude, duration and frequency” of a condition is crucial for

successful development and application of state water quality standards. Equally

important is the spatial extent of a condition, and the spatial and temporal dimensions of attainment assessment must be defined. 

Magnitude refers to how much of the pollutant—or a given quantifiable measure of

condition—can be allowed while still achieving the designated uses. Magnitude is

assessed through a direct comparison of ambient concentrations with the appropriate

Chesapeake Bay criterion value. The magnitude of nonattainment of a criterion value

also provides information useful to making management decisions on taking corrective actions.

Attainment of all three Chesapeake Bay criteria should be assessed by Chesapeake

Bay segment (Figure VI-1; Table VI-4), separately for each designated use habitat.

Therefore, each designated use habitat in an individual Chesapeake Bay Program

segment is considered a spatial assessment unit. This is consistent with the scale of

data aggregation and reporting for Chesapeake Bay tidal-water quality monitoring

and the physical scale of the designated use areas.

Criteria attainment should be presented in terms of spatial extent, i.e., the percentage

of the volume (dissolved oxygen) or surface area (water clarity, chlorophyll a) of the

particular designated use habitat in each Chesapeake Bay Program segment that

meets or exceeds the applicable criteria. Measuring spatial extent will be enabled

through the use of spatial interpolation methods, which are described later in this

chapter. Such ‘interpolators’ work by dividing a water body into a three-dimensional

grid, with cell size depending on data density and the application’s resolution

requirements, among other factors.

Duration is defined as the period over which exposure to the constituent of concern

is to be averaged within the assessment period (see below) to prevent detrimental

effects. Duration can also be thought of as the allowable time of exposure before

effects occur. For example, the open-water dissolved oxygen criteria includes a criterion with a magnitude of 5 mg liter-1 evaluated as a 30-day mean; another criterion

has a magnitude of 4 mg liter-1 evaluated as a 7-day mean.

The dissolved oxygen, water clarity and chlorophyll a criteria are season-specific,

and attainment should be measured only over the applicable season. For example,

attainment of the dissolved oxygen criteria for the migratory fish spawning and

nursery designated use should be assessed and reported for the period of February 1

through May 31; attainment of the open-water fish and shellfish designated use

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chapter vi • Recommended Implementation Procedures
criteria, as applied to both open- and shallow-water bay grass designated uses,

should be assessed and reported seasonally, in winter (December, January and

February), spring (March, April and May), summer (June, July, August and

September) and fall (October and November). Tables VI-1 and VI-2 define ‘seasons’

and applicable criteria for dissolved oxygen and water clarity, respectively. Numerical chlorophyll a criteria should be applied to the spring and summer seasons

defined previously.

The assessment period refers to the most recent three consecutive years for which

relevant monitoring data are available. In circumstances where three consecutive

years of data are not available, a minimum of three years within the most recent five

years should be used.

A three-year period is consistent with the water quality status assessment period

used for over a decade by the Chesapeake Bay Program partners (e.g., Alden and

Perry 1997). A three-year period includes some natural year-to-year variability

largely due to climatic events, and it also addresses residual effects of one year’s

conditions on succeeding years. Two years is not enough time to assess central

tendency, and four or more years delay response to problems that may be detected.

Longer periods are more appropriate for detecting trends than for characterizing

current water quality conditions. 

A comparison of criteria attainment across one-, three- and five-year assessment

periods confirmed the selection of three years as the appropriate temporal averaging

period. Attainment levels were highly variable using single-year periods. The fiveyear period smoothed much of the variability and resulted in little difference

between one assessment period and the next.

The allowable frequency at which the criterion can be violated without a loss of the

designated use also must be considered. Frequency is directly addressed through

comparison of the generated cumulative frequency distribution with the applicable

criterion reference curve. All values falling below the reference curve are considered

biologically acceptable exceedances of the applicable Bay criteria. Through its derivation, the reference curve directly incorporates a biologically acceptable frequency

of exceedances of the applicable Chesapeake Bay criteria.

By combining these factors to measure attainment, the spatial extent of violation or

attainment of the criterion can be determined for each designated use within each

Chesapeake Bay Program segment at temporal increments defined by the criterion.

As the next section describes, the frequency of these occurrences is tallied for each

season over the assessment period.

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chapter vi • Recommended Implementation Procedures

Figure VI. The geographical location of the 78 Chesapeake Bay Program segments.

Source: Chesapeake Bay Program 1999.
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chapter vi • Recommended Implementation Procedures

Table VI-4. Chesapeake Bay Program segmentation scheme segments.

Northern Chesapeake Bay . . . . . . . . . CB1TF

Upper Chesapeake Bay . . . . . . . . . . . CB2OH

Upper Central Chesapeake Bay . . . . CB3MH

Middle Central Chesapeake Bay . . . CB4MH

Lower Central Chesapeake Bay . . . . CB5MH

Western Lower Chesapeake Bay . . . . CB6PH

Eastern Lower Chesapeake Bay . . . . CB7PH

Mouth of Chesapeake Bay . . . . . . . . CB8PH

Bush River . . . . . . . . . . . . . . . . . . . . BSHOH

Gunpowder River . . . . . . . . . . . . . . GUNOH 

Middle River . . . . . . . . . . . . . . . . . . . MIDOH

Back River . . . . . . . . . . . . . . . . . . . . BACOH

Patapsco River . . . . . . . . . . . . . . . . . PATMH 

Magothy River . . . . . . . . . . . . . . . . MAGMH

Severn River . . . . . . . . . . . . . . . . . . . SEVMH

South River . . . . . . . . . . . . . . . . . . . SOUMH

Rhode River . . . . . . . . . . . . . . . . . . RHDMH

West River . . . . . . . . . . . . . . . . . . . . WSTMH

Upper Patuxent River . . . . . . . . . . . . PAXTF

Western Branch Patuxent River . . . . WBRTF

Middle Patuxent River . . . . . . . . . . . PAXOH

Lower Patuxent River . . . . . . . . . . . PAXMH

Upper Potomac River . . . . . . . . . . . . POTTF

Anacostia River . . . . . . . . . . . . . . . . . ANATF

Piscataway Creek . . . . . . . . . . . . . . . . PISTF

Mattawoman Creek . . . . . . . . . . . . . . MATTF

Middle Potomac . . . . . . . . . . . . . . . . POTOH

Lower Potomac . . . . . . . . . . . . . . . . POTMH

Upper Rappahannock River . . . . . . . . RPPTF

Middle Rappahannock River . . . . . . RPPOH

Lower Rappahannock River . . . . . . . RPPMH

Corrotoman River . . . . . . . . . . . . . . CRRMH

Piankatank River . . . . . . . . . . . . . . . . PIAMH

Upper Mattaponi River . . . . . . . . . . . MPNTF

Lower Mattaponi River . . . . . . . . . . MPNOH

Upper Pamunkey River . . . . . . . . . . PMKTF

Lower Pamunkey River . . . . . . . . . . PMKOH

Middle York River . . . . . . . . . . . . . . YRKMH

Lower York River . . . . . . . . . . . . . . . YRKPH

Mobjack Bay . . . . . . . . . . . . . . . . . . MOBPH

Upper James River . . . . . . . . . . . . . . . JMSTF

Appomattox River . . . . . . . . . . . . . . . APPTF

Middle James River . . . . . . . . . . . . . JMSOH

Chickahominy River . . . . . . . . . . . . CHKOH

Lower James River . . . . . . . . . . . . . . JMSMH

Mouth of the James River . . . . . . . . . JMSPH

Western Branch Elizabeth River . . . WBEMH

Southern Branch Elizabeth River . . . SBEMH

Eastern Branch Elizabeth River . . . . EBEMH

Lafayette River . . . . . . . . . . . . . . . . . LAFMH

Mouth to mid-Elizabeth River . . . . . . ELIPH

Lynnhaven River . . . . . . . . . . . . . . . . LYNPH

Northeast River . . . . . . . . . . . . . . . . . NORTF

C&D Canal . . . . . . . . . . . . . . . . . . . C&DOH

Bohemia River . . . . . . . . . . . . . . . . . BOHOH

Elk River . . . . . . . . . . . . . . . . . . . . . . ELKOH

Sassafras River . . . . . . . . . . . . . . . . . SASOH

Upper Chester River . . . . . . . . . . . . . CHSTF

Middle Chester River . . . . . . . . . . . . CHSOH

Lower Chester River . . . . . . . . . . . . CHSMH

Eastern Bay . . . . . . . . . . . . . . . . . . . EASMH

Upper Choptank River . . . . . . . . . . . CHOTF

Middle Choptank River . . . . . . . . . . CHOOH

Lower Choptank River . . . . . . . . . CHOMH1

Mouth of the Choptank River . . . . CHOMH2

Little Choptank River . . . . . . . . . . . LCHMH

Honga River . . . . . . . . . . . . . . . . . . HNGMH

Fishing Bay . . . . . . . . . . . . . . . . . . . FSBMH

Upper Nanticoke River . . . . . . . . . . . NANTF

Middle Nanticoke River . . . . . . . . . NANOH

Lower Nanticoke River . . . . . . . . . . NANMH

Wicomico River . . . . . . . . . . . . . . . . WICMH

Manokin River . . . . . . . . . . . . . . . . MANMH

Big Annemessex River . . . . . . . . . . . BIGMH

Upper Pocomoke River . . . . . . . . . . . POCTF

Middle Pocomoke River . . . . . . . . . . POCOH

Lower Pocomoke River . . . . . . . . . . POCMH

Tangier Sound . . . . . . . . . . . . . . . . . TANMH

Source: Chesapeake Bay Program 1999.
DEVELOPING THE CUMULATIVE 

FREQUENCY DISTRIBUTION

The use of cumulative frequency distributions (CFDs) is recommended for assessing

spatial and temporal water quality criteria exceedance in the Chesapeake Bay. CFDs

offer a number of advantages over other techniques that are applied for this purpose.

First, the use of CFDs is well established in both statistics and hydrologic science.

CFDs have been used for much of the past century to describe variations in hydrologic assessments (Haan 1977). For example, the U.S. Geological Survey has

traditionally used CFDs to describe patterns in historical streamflow data for the

purpose of evaluating the potential for floods or droughts (Helsel and Hirsch 1992). 

Second, the application of the CFD for evaluating water quality criteria attainment

in the Chesapeake Bay allows for the evaluation of both spatial and temporal variations in criteria exceedance. Methods currently used for the assessment of criteria

attainment are based only on temporal variations because measurements are usually

evaluated only at individual monitoring station locations. One of the limitations of

this approach is that it is often difficult to determine whether an individual sampling

location is representative, and there is always potential for bias. In a water body the

size of the Chesapeake Bay, accounting for spatial variation can be very important

and in that respect, the CFD approach represents a significant improvement over

methods used in the past. 

A CFD is developed first by quantifying the spatial extent of criteria exceedance for

every monitoring event during the assessment period. Compiling estimates of spatial

exceedance through time accounts for both spatial and temporal variation in criteria

exceedance. Assessments are performed within spatial units defined by the intersection of Chesapeake Bay Program segments (see Figure VI-1) and the refined

tidal-water designated uses (see U.S. EPA 2003 for specific boundaries), and

temporal units of three-year periods. Thus, individual CFDs will be developed for

each spatial assessment unit over three-year assessment periods. Details on the steps

involved in developing CFDs are described below.

STEP 1. INTERPOLATION OF WATER QUALITY MONITORING DATA

The Chesapeake Bay Program partners collect monitoring data over a range of spatial

scales and frequencies. Much of the water quality monitoring data collected in the

Chesapeake Bay and its tidal tributaries is drawn from a limited number of fixed

stations that are visited on a monthly (or more frequent) basis. Other types of data are

collected at different spatial frequencies. For example, some chlorophyll a data are

collected in a spatially continuous in-situ manner along the cruise tracks of monitoring vessels. All of the different types of data are useful for assessing criteria

attainment; however, they must be connected to a single spatial framework in order to

provide a common basis for interpretation. Assessment of criteria attainment requires

that conclusions be drawn for all locations within a spatial unit and not just the loca-

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tions where data may have been collected. Thus, the data must be extrapolated in

order to evaluate criteria attainment for the larger spatial unit that the data represent. 

For the Chesapeake Bay and its tidal tributaries, using a grid-based spatial interpolation software provides a common spatial framework and spatial extrapolation. Spatial

interpolation provides estimates of water-quality measures for all locations within a

spatial assessment unit. This is accomplished at any single location by linear interpolation of the data of all its nearest neighbors. This approach provides an estimate of

the water quality measure at all locations within the spatial unit being considered. 

An example of the use of spatial interpolation is illustrated in Figure VI-2, which

displays the monitoring segment boundaries and fixed-station locations in the area

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chapter vi • Recommended Implementation Procedures

Figure VI-2. Chesapeake Bay Program segment boundaries, fixed monitoring station

locations and summer chlorophyll a concentration (μg liter-1) distribution in the Tangier

Sound area of the Eastern Shore of Maryland and Virginia. Summer chlorophyll a

concentration distribution is defined by spatial interpolation.
around Tangier Sound and the adjacent portion of the Eastern Shore of Maryland and

Virginia. Using spatial interpolation, chlorophyll a concentrations were estimated

for all locations in the Tangier Sound area. Based on those estimates, the spatial

distribution of chlorophyll a is illustrated by shading the area according to the estimated concentration (darker shading represents higher chlorophyll a

concentrations). The results illustrate the spatial gradients that tend to occur

throughout an area of this size. Those gradients need to be accounted for in order to

accurately assess the extent of criteria exceedance.

The Chesapeake Bay Program spatial-interpolation software (or ‘CBP interpolator’)

computes water quality concentrations throughout the Chesapeake Bay and its tidal

tributaries from measurements collected at point locations or along cruise tracks

(Bahner 2001). It estimates water quality concentrations at all locations in a twodimensional area or in a three-dimensional volume. The CBP interpolator is

cell-based. Fixed cell locations are computed by interpolating the nearest number (n)

of neighboring water quality measurements, where n is normally 4, but is adjustable.

Typically an interpolation is performed for the entire Chesapeake Bay for a single

monitoring event (e.g., a monthly cruise). In this way all monitoring stations are used

to develop a baywide picture of the spatial variation of the parameter being considered. Segment and designated use boundaries can then be superimposed over the

baywide interpolation to assess the spatial variation of the parameter in any one

segment’s designated use(s).

Cell size in the Chesapeake Bay was chosen to be 1 kilometer (east-west) by 1 kilometer (north-south) by 1 vertical meter, with columns of cells extending from the

surface to the bottom of the water column, thus representing the three-dimensional

volume as a group of equal-sized cells. The tidal tributaries are represented by

various cell sizes, depending on the geometry of the tributary, since the narrow

upstream portions of the tidal rivers require smaller cells to represent the river’s

dimensions accurately. This configuration results in a total of 51,839 cells for the

mainstem Chesapeake Bay and a total of 238,669 cells for the Chesapeake Bay and

its tidal tributaries.

The CBP interpolator is tailored for use in the Chesapeake Bay in that the code is

optimized to compute concentration values that closely reflect the physics of stratification. The Chesapeake Bay is very shallow despite its width and length; hence

water quality varies much more vertically than horizontally. The CBP interpolator

uses a vertical filter to select the vertical range of data for each calculation. For

instance, to compute a model cell value at 5-meters deep, monitoring data at 5 meters

are preferred. If fewer than n (4) monitoring data values are found at the preferred

depth, the depth window is widened to search up to d (normally ± 2 m) meters above

and below the preferred depth, with the window being widened in 0.5-meter increments until n monitoring values have been found for the computation. The user is

able to select the smallest n value that is acceptable. If fewer than n values are

located, a missing value (normally a -9) is calculated for that cell. 

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chapter vi • Recommended Implementation Procedures
A second search radius filter is used to limit the horizontal distance of monitoring data

from the cell being computed. Data points outside the radius selected by the user

(normally 25,000 meters) are excluded from calculation. This filter is included so that

only data near a specific location are used for interpolation. In the current version of

the CBP interpolator, segment and region filters have been added (Bahner 2001).

The Chesapeake Bay Program segments are geographic limits for interpolation. For

instance, the mainstem Chesapeake Bay is composed of eight segments (see Figure

VI-1 and Table VI-4). The tidal tributaries are composed of 70 additional segments,

using the Chesapeake Bay Program 1998 segmentation scheme (CBP 1999). Each

segment represents a geographic area that has somewhat homogeneous environmental conditions. Segmentation enables users to report findings on a

segment-by-segment basis, which can reveal localized changes compared to the

entire Chesapeake Bay ecosystem. 

As stated above, the CBP interpolator uses monitoring data to fill in the three-dimensional space of the Chesapeake Bay. The CBP interpolator assumes a linear

distribution of the data between points. Given the dynamic nature of estuaries, this

is obviously a conservative assumption. However, the spatial limitations of the data

make the simplest approach the most prudent. The strength of the CBP interpolator’s

output is directly related to the quality and spatial resolution of the input data. As

sample size increases, interpolation error decreases. For more detailed documentation on the Chesapeake Bay Program interpolator and access to a downloadable

version, refer to the Chesapeake Bay Program web site at http://www.

chesapeakebay.net/tools.htm.

STEP 2. COMPARISON OF INTERPOLATED WATER QUALITY

MONITORING DATA TO THE APPROPRIATE CRITERION VALUE 

To quantify the spatial extent of criteria exceedance, the interpolated water quality

monitoring data must be compared to the appropriate criteria value. In all cases, the

water quality criteria are defined within specific spatial limits and with varying spatial

values. In order to define the spatial extent of criteria exceedance, the appropriate

criteria values must be aligned with the water quality measures throughout the spatial

assessment unit. Accordingly, the spatial definition of each criterion is superimposed

on the interpolator grid structure to assign a criteria value to each cell. Criteria assessments can then be made on a cell-by-cell basis using the water quality estimate from

the interpolator and the criteria value defined for each cell. Figure VI-3 illustrates a

schematic of the process for spatially defined criteria assessment. Chlorophyll a estimates generated from the interpolator (such as that for Tangier Sound, Figure VI-2)

are combined with the grid-based definition of criteria values. The integration of those

two layers allows the comparison of ‘measured’ chlorophyll a to the applicable

criteria value in each cell to determine if that cell exceeds the criterion for the time

period for which data were collected (Figure VI-3).

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chapter vi • Recommended Implementation Procedures
STEP 3. IDENTIFICATION OF INTERPOLATOR CELLS 

THAT EXCEED THE CRITERION VALUE 

When the appropriate criterion value has been assigned to each interpolator cell,

comparisons can be made on a cell-by-cell basis to determine if the estimated water

quality values met or exceeded the criteria at the time of the monitoring event. Evaluation of criteria exceedance is performed for each cell in a spatial unit (Figure

VI-4a), enabling the entire spatial unit to be characterized. The percentage of cells

that exceed the criteria represents the spatial extent of exceedance in that spatial unit

and for that sampling event. The same process is repeated for every sampling event

(Figure VI-4b) and the compilation of the estimates of the extent of spatial

exceedance provides an indication of the frequency of exceedance.

STEP 4. CALCULATATION OF THE CUMULATIVE PROBABILITY 

OF EACH SPATIAL EXTENT OF EXCEEDANCE 

The spatial extent of exceedance (represented by the colored cells in Figure VI-4) is

calculated as the percentage of area or volume exceeding the criteria. This is accomplished by simply dividing the area or volume of all the cells exceeding the criteria

by the total area or volume of the spatial assessment unit and multiplying by 100.

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chapter vi • Recommended Implementation Procedures

Figure VI-3. Chlorophyll a concentration values estimated for each interpolator cell

are compared to the appropriate criterion value on a cell-by-cell basis to determine the

spatial extent of exceedance.
The development of CFD is based on the estimates of spatial exceedance percentages for all monitoring events conducted during the assessment period (Figure VI-5).

CFDs are based on the concept of ‘cumulative frequency,’ where each observed

value is assigned a probability that represents the potential for observing a lower

value. To calculate cumulative frequency, data are sorted in ascending order and then

ranked. This approach is typically used for evaluating streamflow data (Helsel and

Hirsch 1992). It is similar to that used in assessing water quality criteria except that

the values are ranked in descending order (Figure VI-5), because the interest lies in

the potential for observing a spatial exceedance rate greater, not less, than the one

observed. 

Once the data are sorted and ranked, the cumulative probability is calculated using a

‘plotting position’ formula (Helsel and Hirsch 1992). The Weibull formula,

rank/(n+1), developed by Weibull (1939) was chosen as the simplest and most

commonly used; there is a strong precedent for the use of this formula in the hydrologic literature (Helsel and Hirsch 1992). 

Figure VI-6 summarizes the results of the calculations for the development of the

CFD. Cumulative probability represents the frequency of occurrence of each value

of spatial exceedance or a greater value. For example, more than 50 percent spatial

exceedance was observed 46 percent of the time. At the lower end of the plot, the

point (100, 0) is included because more than 100 percent of the area or volume will

be in exceedance 0 percent of the time. At the upper end of the plot, the point (0,

100) was included because 0 percent of the area or volume will be in exceedance

more that 100 percent of the time.

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chapter vi • Recommended Implementation Procedures

Figure VI-4. For a given sampling event, cells that exceed the criterion are determined by comparing the interpolator estimated water quality value in each cell (e.g., chlorophyll a) to the appropriate criterion value (a) as in

Figure VI-3. The same process is repeated for each sampling event through the assessment period (b).
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chapter vi • Recommended Implementation Procedures

Figure VI-5. To develop a CFD for an area/volume, estimates of spatial extent of criteria exceedance for all of the

sampling events conducted over a three-year assessment period (See Figure VI-4b) are compiled (a). To prepare for

developing the CFD the estimates of spatial extend of exceedance are sorted in descending order (b) and ranked.

Figure VI-6. To develop a CFD, estimates of spatial extent of criteria exceedance for all of the sampling events

conducted over a three-year assessment period (see Figure VI-4) are compiled, sorted in descending order and

ranked (a). Cumulative probability is calculated using the formula ‘rank/(n+1)’ (b).

Percent Area/

Month Volume

Percent Area/

Month Volume Rank

Percent Area/

Month Volume Rank

Percent Area/ Cumulative Probability

Month Volume Rank [Rank/(n+1)]
STEP 5. PLOT OF SPATIAL EXCEEDANCE VS. 

THE CUMULATIVE FREQUENCY

The CFD is a graphical illustration that summarizes criteria exceedance by plotting

the temporal and spatial exceedance values listed in Figure VI-6. Temporal

frequency of exceedance is plotted on the vertical axis and spatial extent of

exceedance on the horizontal axis (Figure VI-7). The resulting figure can be used to

draw conclusions about the extent and pattern of criteria exceedance. Each point on

the curve represents the cumulative amount of space and time in which the criteria

were exceeded. The potential for observing a spatial extent of exceedance greater

than the one observed is indicated by the temporal frequency. The curve in Figure

VI-7 shows two examples of the interpretations of individual points. In addition to

the interpretation of individual point, the area beneath the curve represents a spatial

and temporal composite index of criteria exceedance. This area is recommended as

the basis for defining criteria attainment for all Chesapeake Bay segments and designated uses.

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chapter vi • Recommended Implementation Procedures

0.0

20.0

40.0

60.0

80.0

100.0

0.0 20.0 40.0 60.0 80.0 100.0

39% or more of the area/volume 

exceeds the criteria 

in 62% of the sampling events 

during the 

three-year assessment period 

65% or more of the area 

exceeds the criteria 

in 31% of the sampling events 

during the 

three-year assessment period 

Percentage of Time a Specified Percentage or 

More of Area/Volume Exceeds the Criteria

Percentage of Area/Volume Exceeding the Criteria

Figure VI-7. The horizontal axis is the spatial extent of criteria exceedance based on monitoring data

extrapolated using spatial interpolation. The vertical axis is the cumulative frequency of criteria exceedance 

for the monitoring events conducted during the assessment period.
The shape of the curve also indicates the spatial and temporal pattern of criteria

exceedance. Figure VI-8 illustrates three potentially observable CFD plots. Curve (a)

indicates a situation in which the water quality criteria are chronically exceeded in a

relatively small amount of a given segment. Managers could use this information to

target segments for further monitoring and assessment and to identify chronic problems and tailor management plans to address them. Curve (b) illustrates a situation

where criteria are exceeded on a broad spatial scale, but relatively infrequently. Such

broad-scale acute problems should be evaluated individually. If the frequency and

duration of broad-scale criteria exceedances were low enough, ecological impacts

could be limited. On the other hand, some short-term exceedances can have significant ecological effects. Curves (a) and (b) reflect a similar degree of overall criteria

exceedance; however, the exceedance of curve (a) is primarily temporal, and the

exceedance of curve (b) is primarily spatial. Curve (c) reflects broad-scale criteria

exceedance in both space and time. The shape of the curves should be used for diagnostic purposes only. Decisions regarding full attainment should be based on the

overall amount of criteria exceedance indicated by the area under the curve.

As discussed above, it is possible that some spatial and temporal criteria exceedances

could be observed, without necessarily having significant effects on ecological

health or on the designated use of a portion of the Chesapeake Bay. Such

exceedances are referred to as ‘allowable exceedances.’ Such exceedances have been

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(a)

(b)

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Percentage or More of Area/Volume

Exceeds the Criteria 

Figure VI-8. Use of cumulative frequency distribution to characterize patterns of water

quality criteria exceedance. Curve (a) indicates that criteria are chronically exceeded in a

relatively small portion of the spatial unit. Curve (b) indicates that criteria are exceeded

over a large portion of the spatial unit on a relatively infrequent basis. Curve (c) indicates

that criteria are exceeded over large portions of space and time.
provided for in EPA national guidance for assessing criteria attainment (U.S. EPA

1997). Ten percent of the samples collected at a point are allowed to reflect nonattainment of water quality criteria without indicating nonattainment of designated

uses. These criteria exceedances are considered ‘allowable exceedances’ that had

limited impact on the designated use. The 10-percent rule is not directly applicable

in the context of the CFD methodology for defining criteria attainment because it

was designed for samples collected at one location and, therefore, is only reflective

of time. 

A more appropriate approach for defining ‘allowable exceedances’ in the CFD

context is to develop a reference curve (described below) that identifies the amount of

spatial and temporal criteria exceedance that can occur without causing significant

ecological degradation. Such curves can be based on biological indicators of ecological health that are separate from the criteria measures themselves. Biological

indicators can be used to identify areas of the Chesapeake Bay and its tidal tributaries

that have healthy ecological conditions and supportive water quality conditions. CFDs

can be developed for those areas as well. Since healthy ecological conditions exist in

the selected areas, CFDs developed for the area would reflect an extent and pattern of

criteria exceedance that did not have significant ecological impact. Thus, the reference curve approach takes the development of criteria levels beyond those developed

in a laboratory setting and provides actual environmental context. Small incidents of

spatial and temporal criteria exceedance that do not have ecological impacts are identified and allowed in the assessment of criteria attainment. A description of the

application of the reference curve is provided in this section, with more details on

reference curves in the section titled “Defining the Reference Curve.”

Figure VI-9 illustrates the use of the reference curve and the interpretation of criteria

attainment using the CFD. The light blue line illustrates a possible reference curve,

below which a certain amount of spatial or temporal exceedance is allowed. An

actual reference curve could be asymmetrical, indicating that the system could withstand either short-term excursions in time or chronic exceedances in small portions

of space, but not both.

Development of the reference curve is intended to identify such specifics to more

accurately reflect what the ecological system needs to thrive. It also is intended to be

developed as a benchmark that is not changed on a regular basis, recognizing the

potential for updates as new information is gathered. By contrast, the attainment

curve is developed over every assessment period during which monitoring data are

collected. 

The attainment curve is the assessment of the condition in the segment during the

assessment period and is compared to the reference curve. The area above the reference curve and below the attainment curve reflects criteria attainment and is referred

to as “non-allowable exceedances.” It is recommended that separate attainment

curves be developed for each criteria component, for subsequent application in every

spatial assessment unit (Chesapeake Bay Program segment/designated use) and for

at least one full assessment period of three years.

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In cases where the amount of ‘non-allowable exceedances’ is large (e.g., Figure VI8, line c; Figure VI-9), decisions regarding the attainment of designated uses will be

unequivocal. However, situations could arise where small amounts of non-allowable

exceedance could render the decisions less clear. Figure VI-10 illustrates a situation

in which a decision on nonattainment might be clear (a) and one in which the decision might be less clear (b). In the latter case, questions could arise about the

certainty of the analysis and whether the data were adequate to unequivocally decide

that the portion of the Chesapeake Bay was not attaining its designated use. In some

cases, many data points could have contributed to the development of the CFD,

whereas in other cases there may have been only a few. It is possible to define the

decision rule that any non-allowable exceedance would indicate nonattainment of

the established designated use. However, a decision rule based on a statistical test

could help to address some of the uncertainty involved by accounting for differences

in the number of observations on which the analysis is based.

Work is currently under way to devise a statistical test for the application of CFDs

to assess water quality criteria attainment in the Chesapeake Bay. The test currently

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Area of ‘Non-Allowable’

Criteria Exceedance 

Area of ‘Allowable’

Criteria Exceedance 

Reference Curve

Percentage of Time a Specified Percentage or

More of Area/Volume Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure VI-9. Light area reflects amount of ‘allowable’ criteria exceedance defined as

the area under the reference curve (light line). Dark area reflects the amount of ‘nonallowable’ criteria exceedance defined as the area between the attainment 

curve (black line) and the reference curve. 
being evaluated and refined is the

Kolmogorov-Smirnov (KS) test, which

was originally developed to test for significant differences between cumulative

density functions (Haan 1977). The KS

test is nonparametric and is based on the

maximum difference between curves

(Figure VI-11). The maximum difference

is somewhat different than the area

between the curves, which is the preferred

indicator for assessing attainment.

However, it can be shown that the

maximum difference and the area

between the curves are closely correlated

and, therefore, evaluation of one will

reflect an evaluation of the other. 

The KS test is well-documented and

accepted in the statistical literature. Some

refinements that may be necessary are

currently being evaluated. Overall,

however, the KS test has a strong potential

for evaluating water quality criteria attainment in the Chesapeake Bay.

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Maximum

Difference

Between

Curves

Percentage of Time a Specified Percentage

or More of Area/Volume Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure VI-11. Illustration of the basis of the KolmogorovSmirnov statistical test for identifying statistically significant

differences between cumulative density functions. In this case,

the test is applied to identify statistically significant differences

between the reference and attainment curves. 

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Area of ‘Non-Allowable’

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Exceedance 

Area of ‘Non-Allowable’

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a. b. Percentage of Time a Specified Percentage or More of Area/Volume Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria Percentage of Area/Volume Exceeding the Criteria

Percentage of Time a Specified Percentage

or More of Area/Volume Exceeds the Criteria 

Figure VI-10. Light area reflects amount of ‘allowable’ criteria exceedance defined as the area under the

reference curve (light line). Dark area reflects the amount of ‘non-allowable’ criteria exceedance defined as

the area between the attainment curve (black line) and the reference curve. 
DIAGNOSING THE MAGNITUDE OF 

CRITERIA EXCEEDANCE

The CFD is a useful tool for evaluating water quality criteria attainment, but it is

based on pass/fail principles and provides no information on the magnitude of

criteria exceedance, which would interest managers, because it indicates how much

effort is needed to correct any impairment. To fill this need and provide supporting

information for the CFD, it is recommended that interpolator plots be generated for

each monitoring event conducted during an assessment period. Viewed either

individually or as a movie, interpolator plots will shed light on the magnitude of

exceedance during the assessment period.

Two types of interpolator plots are useful for this purpose. The first is the basic interpolator plot of the criteria parameter (i.e., concentration for dissolved oxygen and

chlorophyll a, and percent light-through-water for water clarity; Figure VI-12). Such

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Figure VI-12. Example plot of chlorophyll a concentration (μg liter-1) estimates

generated through spatial interpolation for purposes of evaluating the magnitude 

of criteria exceedance.
plots show problem areas and indicate their distance from criteria attainment.

However, they are limited in evaluating the overall picture of magnitude of criteria

exceedance for the entire Chesapeake Bay. Criteria values vary spatially and thus the

magnitude of exceedance will depend on both actual interpolator values and the

criteria values themselves. To address this need, a second set of interpolator plots

illustrating the magnitude of exceedance as a percentage of the criteria values themselves should be generated (Figure VI-13). Any estimated values below the criteria

level will be less than one and bounded at zero, whereas estimated values above the

criteria level will be in percentage of criteria level.

Other information is available to evaluate the significance of the criteria attainment

assessment results and to place them in context. This includes the size of the designated use (as surface area or volume) and the percentage of the total habitat that is

represented by the designated use. This particular data is especially useful for

dissolved oxygen criteria attainment assessment. The information is used to understand the relative percentage of the total habitat that is accounted for by the

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Figure VI-13. Example plot of chlorophyll a concentration (μg liter-1) estimates

generated through spatial interpolation, expressed as a percentage of a possible spring

season criteria value, for purposes of evaluating the magnitude of criteria exceedance. 
open-water, deep-water or deep-channel designated use habitats in the entire water

column. For example, if the deep-water use was found in nonattainment at a rate of

50 percent but only accounted for 10 percent of the total habitat of the water column,

the management actions taken in response would differ from those taken if the deepwater use accounted for 75 percent of the total habitat. This may prove to be a useful,

additional source of data when difficult decisions must be made. 

DEFINING THE REFERENCE CURVE

The recommended criteria attainment assessment approach is designed to protect the

living resources as defined by the designated uses. The criteria levels themselves

were largely based on scientific studies performed in laboratory settings or under

controlled field conditions. The criteria establish the level of a given habitat condition that living resources need for survival. They do not account for many other

environmental factors that could affect survival. 

Reference curves were developed to provide a scientific-based, direct measure of the

‘allowable’ criteria exceedances. These exceedances are defined to be those that last

a short enough time or cover a small enough area to have no adverse affects on the

designated use. It is assumed that the designated uses can be attained even with some

limited level of criteria exceedances and thus, the reference curves define those

criteria exceedances deemed to be allowable—chronic in time but over small areas,

or infrequent occurrences over large areas. Exceedances that occur over large areas

of space and time would be expected to have significant detrimental effects on

biological communities, which would imply nonattainment of designated uses. 

STRENGTHS AND LIMITATIONS

Although the Chesapeake Bay and its tidal tributaries are listed as impaired water

bodies, there are some places that have met or usually meet the Chesapeake Bay

criteria and support healthy aquatic living resource communities. Reference curves

derived from monitoring these areas reveal patterns of criteria attainment or

exceedances that support the healthy community. That is, they show whether areas

that support a relatively healthy target community: 1) never exceed the applicable

criteria, 2) exceed the criteria frequently, but over a small area or volume, 3) exceed

the criteria infrequently over a large area or volume or 4) exhibit some other pattern.

The EPA recognizes that there are currently a limited number of reference sites, given

the Chesapeake Bay’s nutrient-enriched status. In addition, there are limited data available—both for criteria parameters as well as measures of the biological health of target

communities—with adequate spatial and temporal coverage from which to develop a

full array of biological-based reference curves. However, where sufficient data exist,

the reference curves appear to be stable. The reference curve for the deep-water designated use dissolved oxygen criteria is the most solidly grounded in data.

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This biological reference curve (see below for details) is based on dissolved oxygen

concentration distributions at sites associated with bottom sediment-dwelling

benthic communities scoring 3 or higher on the Chesapeake Bay benthic index of

biotic integrity (benthic-IBI). If several of the reference segments were randomly

removed, the regenerated reference curves do not change much, suggesting that

within designated uses, the attainment curves for reference segments appear to be

very similar. Although less firmly grounded, the reference curves for other designated uses and other criteria also seem to be relatively stable.

APPROACHES TO DEFINING REFERENCE CURVES

At least three options exist for defining a reference curve (Figure VI-14). Fixed

percentages could be selected based on a policy decision or other basis similar to the

10 percent level of acceptable exceedances allowed in 305(b) EPA national guidance

(Figure VI-14a; U.S. EPA 1997). Alternatively, laboratory or empirical field data

from areas known to be unimpaired by the stressor can be used to derive a biologically-based reference curve (Figure VI-14b). Even this second approach, however,

requires technical or policy decisions regarding the acceptable level of biological

effect. Finally, a reference curve could be established to reflect uncertainty based on

the assumption of a normal distribution, and using observed or estimated error variance for both time and space (Figure VI-14c).

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(b) Biologically-Based

(c) Accounting for

Sampling/Measurement Error

(a) 10% Time/Space

(Build from Current EPA Guidance)

Percentage of Area/Volume Exceeding the Criteria

p

Percentage of Time a Specified Percentage

or More of Area/Volume Exceeds the Criteria

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Figure VI-14. Three possible options for setting reference curves for application 

to the cumulative frequency distribution approach for defining criteria attainment: 

(a) fixed percentages based on policy decisions; (b) biological effects-based empirical

field or laboratory data and; (c) observed or estimated uncertainty data.
The reference curves described below for the dissolved oxygen and water clarity

criteria are based on empirical, biologically-based field data where possible. Where

no corroborating field data exist, a normal distribution curve representing approximately 10 percent exceedance is used (see Figure VI-18). Appendix H contains

supporting analyses and detailed descriptions of the methodologies used for defining

these reference curves, as well as the list of reference locations.

REFERENCE CURVES FOR DISSOLVED OXYGEN CRITERIA 

Reference curves for dissolved oxygen are intended to represent the spatial and

temporal distribution of dissolved oxygen concentrations in areas supporting healthy

species and communities the criteria were established to protect. The deep-water

designated use, for example, contained the necessary water quality and biological

source data collected over similar temporal and spatial scales. When such data were

not available at the scales necessary to establish quantitative relationships between

the criteria parameter and measured living resource community health, surrogate

measures of biological and habitat conditions were explored. Ideally, each set of

designated use-based dissolved oxygen criteria should have a separate, individually

derived reference curve. However, satisfactory synoptic water quality and biological

indices data or surrogate measures of habitat condition were found only for the openwater fish and shellfish and deep-water designated uses and were tested only against

the 30-day mean criteria for those uses. 

Migratory Fish Spawning and Nursery Dissolved Oxygen 

Criteria Reference Curve

Current Chesapeake Bay water quality monitoring in migratory fish spawning and

nursery habitats is limited to midchannel stations. There also are insufficient

spawning success fisheries-independent data available to identify biologically-based

reference sites for these criteria. In addition, the criteria duration components for this

designated use are an instantaneous minimum and 7-day mean, and methodologies

to translate less frequently monitored dissolved oxygen measurements into these

time steps have not been finalized.

An attainment curve for exploratory purposes was created for the February-May

spawning period, using a 30-day criterion of 6 mg liter-1 and reference sites identified

using nitrogen, phosphorus, chlorophyll a and total suspended solids as parameters

(Figure VI-15). Attainment was very close to 100 percent. Until more data are

collected to assess the attainment of the 7-day mean and instantaneous minimum

criteria in the migratory fish spawning and nursery designated use, however, the openwater dissolved oxygen criteria reference curve should be applied (Figure VI-16).

Open-Water Dissolved Oxygen Criteria Reference Curve

In the absence of a Chesapeake Bay open-water fish community index of biotic

integrity, reference Chesapeake Bay Program segments with ‘good’ water quality

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were identified based on assessments of surface and above-pycnocline concentrations of four parameters: total nitrogen, total phosphorus, chlorophyll a and total

suspended solids (see Appendix F for details). Cumulative frequency distribution

reference curves for migratory spawning and nursery designated use habitats from

February through May (Figure VI-15) and for open-water designated use habitats in

summer (Figure VI-16) were derived using dissolved oxygen concentration data

from these segments.

The Chesapeake Bay Program’s Tidal Monitoring and Analysis Workgroup developed a procedure to assess relative status for cases in which an absolute point of

reference for a water quality parameter is not available (Alden and Perry 1997). That

procedure uses the logistic distribution of a parameter in a ‘benchmark’ data set as a

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Figure VI-15. Initial attempt at developing a dissolved oxygen criteria reference curve for

migratory, spawning and nursery habitat designated use areas using the 6 mg liter-1 7-day

mean criterion assessed as a 30-day mean.

Figure VI-16. Dissolved oxygen criteria reference curve for defining criteria attainment in

open-water designated use habitats.
standard against which individual data points are assessed. The individual data are

thus scored between 1 and 100. The assessments are conducted separately in salinity

classification and in depth layers corresponding to the designated uses. The median

score of the individual data scores is then calculated. The benchmark distribution is

divided roughly into thirds, which are defined as ‘good’, ‘fair’ and ‘poor’ (these

terms relate only to each other, not necessarily to actual water quality requirements

of living resources). Status of the parameter is assigned depending on where the

median score falls among these divisions.

Using this procedure, open-water concentrations of the four parameters were

assessed for each Chesapeake Bay Program segment, yielding for each parameter an

assessment of ‘good,’ ‘fair’ or ‘poor’ for each segment, year and season (spring and

summer). To qualify as a reference location, at least three out of four parameters had

to be ‘good’ and only one parameter could be ‘fair’. Once the times and locations

were selected, the corresponding monthly average dissolved oxygen concentration

data were evaluated against the migratory fish spawning and nursery dissolved

oxygen criterion value of 6 mg liter-1 (evaluated as a 30-day mean, not as a 7-day

mean) and the open-water dissolved oxygen 30-day mean criterion of 5 mg liter-1 for

spring and summer, respectively. The percent volume failing the criterion was calculated for each month of the season/year. The resulting cumulative frequency

distribution curves are shown in figures VI-15 and VI-16, respectively. Figure VI-16

currently serves as the recommended reference curve for both the migratory fish

spawning and nursery and open-water fish and shellfish designated uses for purposes

of assessing dissolved oxygen criteria attainment. 

Deep-Water Dissolved Oxygen Criteria Reference Curve

Reference areas were identified using a measure of benthic community health, the

Chesapeake Bay Benthic Index of Biological Integrity (benthic-IBI; Weisberg et al.

1997). Sessile benthic communities are good indicators of water quality conditions of

overlying waters. Although relatively tolerant of lower oxygen concentrations, a

dissolved oxygen concentration of 2 mg liter-1 is considered the lower threshold below

which benthic infaunal communities become severely stressed (see Chapter III). A

healthy benthic community, therefore, could indicate that dissolved oxygen conditions

meeting deep-water dissolved oxygen criteria were met. Benthic infaunal community

samples are collected as part of a long-term Chesapeake Bay Benthic Monitoring

Program. Samples are collected at fixed and random locations in the summer season,

usually in August/September. If the benthic-IBI of that sample is ‘good’, in this case 3

or greater on a scale of 1 to 5, then it is likely that dissolved oxygen conditions have

been adequate for the previous one to two months of the summer. 

The benthic-IBI data from 1985 through 1994 were assessed and a list of deep-water

reference locations identified by year and segment was compiled. Then, the summer

(June through September) dissolved oxygen data that were collected as part of the

Chesapeake Bay Water Quality Monitoring Program at the times and places on the

list were evaluated relative to the deep-water criteria. Figure VI-17 shows the

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resulting cumulative frequency distribution curve, which serves as the recommended

reference curve for the deep-water seasonal fish and shellfish designated use for

assessing dissolved oxygen criteria attainment (see Appendix H for documentation

of the validation curves used to confirm the reference curve). 

Deep-Channel Dissolved Oxygen Criteria Reference Curve

The deep-channel seasonal refuge designated use contains dissolved oxygen concentrations that are inadequate to support most Chesapeake Bay species, and the

criterion is set to protect the survival of benthic organisms. Unfortunately, a biologically-based reference curve could not be developed for the deep-channel use at this

time. This area is assumed to be severely degraded and is not now sampled as part

of the Chesapeake Bay Program long-term benthic monitoring program. No other

appropriate biological data were available with which to identify reference sites.

While a biologically-based reference curve is recommended for the future, a default

reference curve such as the normal distribution curve representing approximately 10

percent exceedance is appropriate in this case to account for anticipated natural

criteria exceedances (Figure VI-18). States and other users must recognize that the

deep-channel dissolved oxygen criterion is stated as an instantaneous minimum, thus

any exceedance is assumed to have direct consequences to the survival of the

bottom-dwelling community.

REFERENCE CURVES FOR WATER CLARITY CRITERIA

Reference areas for development of the water clarity criteria reference curve were

identified as Chesapeake Bay Program segments or parts of segments where underwater bay grasses were abundant historically and thriving or increasing in coverage

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Figure VI-17. Dissolved oxygen criteria reference curve for defining criteria attainment in

deep-water designated use habitats.
in recent years. Separate reference curves were developed for low salinity—tidalfresh and oligohaline–and higher salinity–mesohaline and polyhaline–zones. The

supporting analyses for deriving the water clarity criteria reference curves are

provided in Appendix H.

Once the reference Chesapeake Bay Program segments were identified, the water

clarity data (as measured by Secchi depth) for those segments were extracted from

the Chesapeake Bay water quality monitoring program data base. Percent lightthrough-water (PLW) is the operational parameter used for assessing attainment of

the water clarity criteria. PLW = 100exp(-KdZ), where Z is the target restoration

depth and Kd, the coefficient of extinction, is estimated as Kd= 1.45/Secchi depth

(see Chapter III for details). Kd values calculated from the Secchi depth data were

averaged by month for each station. The monthly data were then spatially interpolated baywide for each month in the underwater bay grass growing season from 1985

through 1994 to match the Chesapeake Bay water quality model hydrologic simulation period. PLW was calculated for each interpolation cell using the interpolated

Kd value and the defined segment-specific restoration depth. The PLW values were

compared to the criterion value appropriate to the Chesapeake Bay Program

segment’s salinity zone, and the percent of the shallow-water area (< 2 meters)

failing the criterion in each segment was calculated for each month. The monthly

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Area of Allowable

Criteria

Exceedance

Percentage of Area/Volume Exceeding the Criteria

Percentage of Time a Specified Percentage

or More of Area/Volume Exceeds the Criteria 

Figure VI-18. Cumulative frequency distribution curve in the shape of a hyperbolic curve

that represents approximately 10 percent allowable exceedances equally distributed

between time and space.
attainment percentages for each reference Chesapeake Bay Program segment were

pooled in their respective low and higher salinity groups and plotted as cumulative

frequency distribution curves (figures VI-19 and VI-20). Appendix H contains the

reference curves generated using the more recent 1995-2000 data. All these water

clarity criteria reference curves were derived using data spanning decadal scales,

capturing the full range of wet, dry and average hydrologic conditions.

The derived water clarity criteria reference curves reflect findings published in the

scientific literature for Chesapeake Bay species that indicate that underwater plants

can survive reduced light conditions for periods of days to weeks. Field and laboratory experiments indicated that lower salinity species were more tolerant of longer

periods of reduced light conditions (Rybicki et al. 2002) compared with species

inhabiting higher salinity waters (Goldsborough and Kemp 1988). These salinity

regime differences also are reflected in the different shapes of the derived reference

curves. The lower salinity reference curve allows for more exceedances over time

and space than are allowed for by the higher salinity reference curve (figures VI-19

and VI-20, respectively).

It should be noted that the water clarity criteria were derived, in part, on the basis of

underwater bay grass growing season medians (Batiuk et al. 1992, 2000), but

attainment is measured on a monthly basis over the growing season (see “Developing the Cumulative Frequency Distribution,” p. 152, for details). Appendix H also

shows water clarity reference curves based strictly on growing season median

assessments. 

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chapter vi • Recommended Implementation Procedures

Figure VI-19. Water clarity criteria reference curve for defining criteria attainment in

tidal-fresh/oligohaline shallow-water bay grass designated use habitats.
REFERENCE CURVES FOR CHLOROPHYLL A CRITERIA

As states derive numerical regional and local specific chlorophyll a criteria, they

should either derive biologically-based reference curves that reflect the ‘allowable’

exceedances of local impairments or apply the normal distribution curve representing

approximately 10 percent ‘allowable’exceedance in time and space (see Figure VI-18). 

The cumulative frequency distributions derived from the subset of Chesapeake Bay

water quality monitoring program chlorophyll a data associated with the ‘Better’ and

‘Best,’ and sometimes ‘Mixed_Better Light’ water quality categories closely

matched the normal distribution curve in both spring and summer (figures VI-21 and

VI-22). These categories formed the basis for characterizing the Chesapeake Bay

phytoplankton reference community (see Chapter V and Appendix F for details). The

cumulative frequency distributions were derived from applying the 95th percentiles

of chlorophyll a values occurring in these categories (see Table V-6). In figures 

VI-21 and V-22, respectively, the cumulative frequency distributions of spring

(March–May) and summer (July–September) chlorophyll a concentration exceeding

the 95th percentile phytoplankton reference community values (a) are overlaid with

the normal distribution curve (b). The normal distribution curve matches well with

both seasonal biological-based cumulative frequency distributions, providing further

justification for applying the normal distribution curve as a chlorophyll a criteria

reference curve in the absence of a directly derived biological reference curve.

REFERENCE CURVE IMPLEMENTATION

As the states adopt the Chesapeake Bay criteria and concomitant procedures into

their water quality standards, they may decide to: 1) allow for no criteria exceedance,

2) select the normal distribution curve representing approximately 10 percent

allowable criteria exceedance or 3) apply a biological reference curve. The first two

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Figure VI-20. Water clarity criteria reference curve for defining criteria attainment in

mesohaline/polyhaline shallow-water bay grass designated use habitats.
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Percentage of Area/Volume Exceeding the Criteria

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(a)

(b)

Figure VI-21. Cumulative frequency distribution of spring (March-May) chlorophyll a

concentration exceeding the 95th percentile phytoplankton reference community 

values (a) compared with the normal distribution curve (b).

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Figure VI-22. Cumulative frequency distribution of summer (July-September) chlorophyll

a concentration exceeding the 95th percentile phytoplankton reference community values

(a) compared with the normal distribution curve (b).
options are likely to be more restrictive than the biological reference curve approach.

If states choose to apply the biological reference curve, then there should be a strong

incentive to collect relevant data to strengthen the scientific basis of those reference

curves in the future.

MONITORING TO SUPPORT THE ASSESSMENT 

OF CRITERIA ATTAINMENT

To support the development of cumulative frequency distributions for criteria attainment assessment purposes, additional monitoring will be required. The current

fixed-station Chesapeake Bay Water Quality Monitoring Program will support many

aspects of the effort to assess criteria attainment. However, some aspects will require

new monitoring in areas of Chesapeake Bay tidal waters from which data have not yet

been collected. Other aspects will require new types of monitoring based on new technologies that will better address the technical requirements of the criteria as they are

currently defined. The Chesapeake Bay Program has developed a tidal monitoring

network design that identifies the needs and proposes options for addressing those

needs. Many of those options can feasibly be implemented, but additional monitoring

will be expensive, and it is expected that available funds will limit what can be done.

The following describes options for conducting monitoring to support the assessment of criteria attainment. Given that funding may be limited, the monitoring

options are divided into three categories based on funding level. The first category,

‘recommended’, assumes that funding will be available to conduct monitoring to

fully support the assessment of criteria attainment. The ‘recommended’ level of

monitoring is based on technological needs to provide a set of data that can be

defended legally and scientifically in making decisions regarding the attainment of

designated uses. The second category, ‘adequate’, assumes that funding will be

somewhat limited, but will be sufficient to collect enough data to support the development of cumulative frequency distributions for most criteria components in most

Chesapeake Bay Program segments and tidal-water designated uses. The third category, ‘marginal’, assumes that monitoring will be significantly limited by available

funding and that it will not be possible to assess all criteria components in all

segments of the Chesapeake Bay and its tidal tributaries. 

Efforts are underway to develop the tools necessary to generate verifiable and quantitative estimates of error and the levels of monitoring required for given levels of

accuracy acceptable to management agencies. The three general categories defined

above were developed to give the reader some perspective on the range of options

available and the adequacy of the options.

SHALLOW-WATER MONITORING

Resource managers rely upon habitat and water quality monitoring data to characterize problem areas in a watershed, such as areas of low dissolved oxygen, and to

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detect changes related to management strategies to reduce nutrients and sediments

on a tributary to baywide level. Traditional monitoring programs have collected periodic data at a small number of fixed sampling locations, often in the deeper

midchannel areas. These measurements provide a good baseline for watershed

assessment and long-term trends, but may miss small-scale gradients in tidal water

quality and neglect critical shallow-water habitats.

In the past, intensive water quality monitoring of these shallow-water habitats has

been time-intensive and cost-prohibitive. The advent of a new suite of technologies

known as the DATAFLOW water quality monitoring system, however, has brought

intensive monitoring of shallow-water habitats into reach (http://mddnr.

chesapeakebay.net/sim/index.html). DATAFLOW is a system of shipboard water

quality probes that measure spatial position, water depth, water temperature, salinity,

dissolved oxygen, turbidity (a measure of clarity of the water) and chlorophyll a

from a flow-through stream of water collected near the water body’s surface. The

system allows data to be collected rapidly (approximately every four seconds) and

while the boat is traveling at speeds up to 25 knots. Because the DATAFLOW system

is compact, it can be housed on a small boat, enabling sampling in shallow water and

the ability to map an entire small tributary in less than a day. Typical DATAFLOW

research cruise sampling paths traverse shallow and channel areas to obtain a full

characterization of a tributary’s water quality.

The discussion below focuses on migratory spawning and nursery, open-water, deepwater and deep-channel designated uses. The DATAFLOW system is the only viable

option for monitoring water quality conditions in the shallow-water designated use.

The high temporal and spatial variability expected in shallow-water areas implies

that intensive data collection would be required for any assessment to have credibility. A probability-based approach was considered as a less expensive approach for

shallow-water monitoring, but the cost savings were not sufficient to justify the

reduced amount of information that this approach would provide. The only option

for reduced costs in shallow-water monitoring is to limit the amount that is

conducted during any one year.

DISSOLVED OXYGEN CRITERIA ASSESSMENT 

‘Recommended’ Level of Monitoring

Monitoring for dissolved oxygen criteria attainment should address all four frequencies of dissolved oxygen criteria: 30-day mean, 7-day mean, 1-day mean and

instantaneous minimum. The current fixed-station monitoring program is designed

to provide a long-term record of dissolved oxygen concentrations that reflect

seasonal and interannual variation. For that reason, even though instantaneous measurements are collected, the current monitoring is best suited for assessing the 30-day

mean dissolved oxygen criteria component and poorly suited for assessing the 7-day,

1-day mean and instantaneous minimum criteria components. To address the need

for data that will address the 7-day, 1-day mean and instantaneous minimum criteria

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components, continuous monitors mounted to buoys or piers will be required. At

least one continuous monitor should be located at each assessment location. The

continuous record will then be combined with fixed-station data, used to calibrate

the spectral-analysis model (described below), and all criteria components could be

quantified using that model. Individual criteria component estimates would be

assessed at all fixed locations and interpolated for incorporation in a cumulative

frequency distribution.

‘Adequate’ Level of Monitoring

Assuming that funding will not be available for the ‘recommended’ monitoring

approach, an alternative would be to place a limited number of continuous monitors

at representative locations in the Chesapeake Bay and tidal tributaries. The number

of continuous monitors would be relatively small, but the number would be established to characterize different types of settings in the Chesapeake Bay. Those

representative temporal records would then be combined with fixed-station data in

similar settings, and spectral models would be developed for each fixed-station location. Dissolved oxygen criteria components would be assessed based on the spectral

models, interpolated and used to develop the cumulative frequency distributions.

This approach would entail much greater uncertainty in the assessments. The

absolute variation would be characterized well by regular monthly measurements at

the fixed-stations. However, the higher frequency assessments would be based on

data collected at only a few locations, which would then be extrapolated over large

distances. 

‘Marginal’ Level of Monitoring

Assuming that funding will not be available for even the ‘adequate’ level of monitoring, assessments would need to rely on the fixed-station data only. As stated

above, this type of monitoring was designed for long-term assessments and would

only be truly appropriate for the 30-day mean criteria component. If the ‘marginal’

level of monitoring was selected, it is likely that higher frequency criteria components would not be assessed in most designated use areas. 

Assessing Dissolved Oxygen Criteria Attainment

Addressing Duration Issues. The dissolved oxygen criteria have several

different durations: 30-day mean, 7-day mean, 1-day mean (deep-water only) and

instantaneous minimum. A state’s ability to assess these criteria and to have certainty

in the results depends on the time scale of available data and on the capacity of

models to estimate conditions at those time scales. At present, long-term, fixedstation, midchannel water quality monitoring in the Chesapeake Bay and its tidal

tributaries provides dissolved oxygen measurements twice monthly at most or

approximately every 15 days between April and August. Proposed enhancements to

the tidal water quality monitoring program include shallow-water monitoring, as

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well as high-resolution spatial and temporal monitoring in selected locations.

However, these new components are only in the planning and early implementation

stages at this point, and because of financial constraints or limitations to current

technology, direct monitoring at the scales of the criteria may not be possible in the

foreseeable future. Therefore, the direct assessment of attainment for some

geographic regions and for some short-term criteria elements (e.g., instantaneous

minimum, 1-day mean and 7-day mean) must be waived for the time being or based

on statistical methods that estimate probable attainment. Several approaches to

addressing the duration issue are described below.

Thirty-Day Mean Attainment Procedure. This duration appears to be within the

temporal scale of the current Chesapeake Bay water quality monitoring programs.

The simplest assessment approach is to use the one value or average of two values

collected within a month as the best estimate of the true 30-day mean. At present,

this is the approach recommended for assessing attainment of criteria with this duration. However, it is debatable how well one or two samples per month represent what

is intended as protective by the 30-day mean.

These procedures assume the existence of a baywide tidal-water monitoring program

with a fixed-station sampling design and sampling frequency at least once per month

during the seasons defined by the criteria. The procedures assume that horizontal and

vertical measurements of dissolved oxygen will be sufficiently dense that the interpolator can create an accurate three-dimensional representation of dissolved oxygen

in the defined designated uses. It also assumes that data are sufficient to define the

boundaries of the designated uses where boundaries are variable, depending on

pycnocline depth.

To simplify computations, if there is more than one observation per month, then the

monthly average is calculated prior to input to the volumetric interpolator. Prior to

averaging for the month, each station’s dissolved oxygen profile is interpolated vertically to obtain a value at each half-meter interval from surface to bottom. The

monthly average concentrations at each fixed station at each half-meter are then

interpolated horizontally by the Chesapeake Bay interpolator to yield a basinwide

grid of concentrations for each month. A comparable reference grid or a table of grid

coordinates and depths can be used to relate the monthly cell concentrations to be

evaluated with the correct designated use and corresponding criteria concentrations.

The cell is scored as meeting or not meeting the criterion level and cell volume is

accumulated in the pool of passing or failing totals for each designated use in each

Chesapeake Bay Program segment. From this, the spatial extent of nonattainment,

i.e., the percentage of the total volume exceeding the criterion in each designated use

in each Chesapeake Bay Program segment is tallied for each month in the assessment period (most recent three years). 

Dissolved oxygen criteria attainment is reported seasonally (see Table VI-1). To

assess, for example, attainment of the summer season 30-day mean criterion for the

deep-water seasonal fish and shellfish designated use, the percent exceedance data

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for the months of June through September for a three-year period for all Chesapeake

Bay Program segments with deep-water designated use habitats would be extracted

and evaluated individually using the cumulative frequency distribution approach.

The cumulative frequency distribution attainment curve would be calculated (and

plotted, if desired) and compared to the appropriate reference curve for the designated use and season using the statistical test described earlier. If the two curves are

significantly different, then the segment/designated use is considered out of attainment, and failing by the amount defined by the area between the two curves.

Seven-Day Mean Attainment Procedure. The 7-day time frame is much shorter

than the temporal scale of the current baywide water quality monitoring programs,

and statistical forecasting models are necessary to assess criteria of this duration.

The proposed approach, referred to as the spectral analysis approach in this chapter

and discussed in more detail below, uses long-term, low-frequency data from the

monitoring program and shorter-term, high-frequency data from in situ semi-continuous monitors to synthesize a data set that incorporates both long- and short-term

patterns of variability. The synthetic data set is created at user-specified time intervals, e.g., weekly, daily and hourly. The minimum interval will depend on the

interval length of the continuous data. The synthetic data set is then analyzed at the

appropriate temporal scale, which in this case is seven days. At present there are

insufficient high-frequency data and insufficient validation of the approach to

recommend its implementation. For now, attainment of 7-day mean criteria should

not be assessed unless data are available for a specific location/segment at a temporal

scale consistent with the 7-day duration.

One-day Mean Attainment Procedure. The 1-day attainment procedure is the

same as the 7-day mean procedure described above. For now, attainment of the 

1-day mean criteria should not be assessed unless data are available for a specific

location/segment at a temporal scale consistent with the 1-day duration.

Instantaneous Minimum Attainment Procedure. Again, the instantaneous

minimum time frame is much shorter than is currently sampled. The spectral

analysis approach presented above is one way to estimate attainment of these

dissolved oxygen criteria. Another approach, referred to as the logistic regression

approach in this chapter and described in more detail below, applies by restating the

criterion in slightly different temporal terms. An instantaneous minimum implies

that the criterion is not met if dissolved oxygen concentrations are below the criterion value at any time. The logistic regression approach estimates the relative

frequency or percent of time that a region falls below a specified concentration based

on the empirical relationship between seasonal or monthly mean values and the

percent of dissolved oxygen concentrations above or below the specified level as

observed in the historical data record (of the Chesapeake Bay water quality monitoring program). This method has been applied experimentally with reasonable

results (Jordan et al. 1992) and can approximate criteria exceedance/attainment

frequency. However, at this time the method has not been adequately validated to

recommend implementation for formally assessing criteria attainment. Attainment of

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instantaneous minimum criteria should not be assessed unless data are available for

a specific location/segment at a temporal scale consistent with the instantaneous

minimum duration. 

Spectral Analysis Approach. The foundation for this method was developed by

Neerchal et al. (1992) in the context of implementing the Chesapeake Bay dissolved

oxygen restoration goal (Jordan et al. 1992) and has been modified for criteria application. The method uses spectral analysis to extract the cyclical components of the

long- and short-term time-series records and combines them to create a synthesized

time-series data set with data synthesized at user-specified time steps. At present, the

synthetic data are hourly, with cyclic components limited to two cycles per day. The

synthetic data have the annual and seasonal cyclic and trend characteristics of the

long-term record as well as the tidal, diurnal and any other periodic characteristics

of the short-term, high-frequency record. The long-term record comes from fixedstation monitoring data collected at regular once or twice monthly intervals in the

seasons of interest. The short-term data come from in-situ semicontinuous oxygen

monitors deployed on buoys or other fixed structures at designated locations around

the Chesapeake Bay and its tidal tributaries. These semicontinuous oxygen monitors

are put in place for various lengths of time at many different locations and depths.

Sites are chosen in order to best characterize the dissolved oxygen conditions in each

designated use. The sampling interval of the semicontinuous monitors are commonly

5, 10 or 20 minutes. To be most useful, the interval should be no longer than one

hour. More details are provided in Appendix I.

Application of the Spectral Analysis Approach. The spectral analysis application

shown in Figure VI-23 uses long-term data from station CB4.2C, a monitoring

station in the midregion of the Chesapeake Bay, and a two-month series of continuous dissolved oxygen measurements at a buoy deployment near that station at

approximately 9 meters below the surface. Figure VI-23 shows the observed monthly

dissolved oxygen concentrations (asterisks) at station CB4.2C (8- to 10-meter depth)

and the long-term forecast (line) from the spectral equation. 

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Figure VI-23. Observed monthly dissolved oxygen concentrations (*) at Chesapeake Bay

Monitoring Program station CB4.2C (at the 8 to 10 meter depth) from January 1985 to

January 2000 and the long-term ‘forecast’ (—) from application of the spectral equation.
The synthetic data record is obtained by combining the long- and short-term equations. A sample two-month period, August through September 1987 (indicated by

the two, close-together vertical reference lines in Figure VI-23), is illustrated in

Figure VI-24. This synthetic record can then be analyzed relative to the applicable

criteria elements. In the example shown, the 9-meter depth at station CB4.2C is near

or below the pycnocline and is, therefore, subject to criteria for the deep-water designated use. Summer dissolved oxygen criteria for the deep-water designated use is a

3 mg liter1 30-day mean, 2.3 mg liter1 1-day mean and 1.7 mg liter1 instantaneous

minimum. For demonstration purposes, let a 7-day mean of 2.5 mg liter-1 also apply.

Based on monitoring data alone (two observations each month), the August and

September mean monthly values are 3.4 mg liter1 and 4.2 mg liter1, respectively.

Basing assessment on the synthetic data record, attainment can be measured either

in sequential or rolling time windows, as described below. In some cases the results

vary depending on which option is used (Table VI-5). For the 30-day duration, the

sequential option results in two 30-day periods within the 61 days, between August

1 and September 30, 1987; the rolling time window option yields 31 periods. If there

was a 7-day criterion for deep-water designated use, there would be 8 sequential

versus 55 rolling-window periods in those 61 days. For the 1-day minimum duration,

the question of sequential and rolling-window is moot.

Verifying the Spectral Analysis Approach. The number and distribution of high

frequency semicontinuous dissolved oxygen data sets is small compared to the

variety of habitats, times of year and layers of the water column that need to be characterized. There are gaps in critical seasons, geographic coverage and designated

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Figure VI-24. Expanded view from Figure VI-23 of the two-month period August–

September 1987 synthetic data record obtained by combining the long- and short-term

spectral equations.
uses. Nevertheless, the number of such data sets on hand is substantial and growing,

relative to the number and location of fixed monitoring stations. 

Developing and verifying the method will be an ongoing process. Short-term forecasts based on synthetic data are created and compared to actual semicontinuous

records not used in the original forecasting process. There are some, but not many,

instances in which semicontinuous data are available at the same site in different

years. Also, in some instances, multiple semicontinuous records are available for the

same region. In these cases, one record is used in the spectral analysis and equation

development and the other is used to verify the results. With data recorders deployed

for the specific purpose of validating and refining the forecasting models, better verification will be available in the future. 

Even with these issues resolved, there are still questions concerning how synthetic

time-series data sets should be adapted to enable an assessment of spatial extent and

frequency of attainment in a manner consistent with criteria assessed by other analytical methods.

Logistic Regression Approach. This method modifies and significantly updates

a method developed originally to measure attainment of the 1992 Chesapeake Bay

dissolved oxygen restoration goal (Jordan et al. 1992). The early work demonstrated

predictable relationships, on a segment-by-segment basis, between seasonal mean

dissolved oxygen concentrations and the percent of observations above a target

concentration. The relationships proved to be strong and applicable in areas where

dissolved oxygen concentrations ranged above and below the goal target concentrations. Given the tidal water quality monitoring data record that spans more than 17

years with the measurements from multiple depths (the vertical dissolved oxygen

profile is collected at 1- to 2-meter intervals), the regression models are now monthand depth-specific in many segments. Based on the monthly mean dissolved oxygen

concentration measured at a specified depth, the models predict the percent of time

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Table VI-5. Sample attainment results when assessing with varying time windows

Dissolved Oxygen Criterion

Time Windows 

Meeting Criterion

 Percent of

Observations

 at or above Criterion

 Sequential

 Rolling window

 2 of 2

 31 of 31

100%

100%

 Sequential

 Rolling window

 7 of 8

 46 of 55

87.5%

83.6%

 Pool of hourly measurements 1,250 of 1,484

Instantaneous Minimum (1.7 mg liter -1)

7-day Mean (2.5 mg liter -1):

30-day Mean (3 mg liter -1):

84.2%
that the dissolved oxygen concentration at that depth in a segment is at or above a

user-specified concentration, e.g., an instantaneous minimum of 1.7 mg liter-1 (see

Appendix I for more details).

Application of the Logistic Regression Approach. The method can be applied

using the three-dimensional baywide interpolations of monthly average dissolved

oxygen, as described for the determination of 30-day duration criteria. The monthly

average concentrations at each fixed station at each half-meter are interpolated horizontally by the Chesapeake Bay interpolator to yield a basinwide grid of

concentrations for each month. A comparable reference grid or a table of grid coordinates and depths relate the monthly cell concentrations to be evaluated with the

correct designated use and corresponding criteria concentration (e.g, instantaneous

minimum of 1.7 mg liter-1). In the data processing step, a segment- and criterion

level-specific prediction model uses the cell’s monthly average concentration, depth

and month as factors in predicting the percent of the time that particular cell is at or

above the criterion. The cell is scored as passing or failing the criterion level

depending on the model results. The cell volume is accumulated in the pool of

passing or failing totals for each designated use in each segment. Like the method

for assessing the 30-day mean, the spatial extent of nonattainment, i.e., the

percentage of the total volume exceeding the criterion in each designated use in each

segment, is tallied for each month in the assessment period (most recent three years).

The cumulative frequency distribution attainment and reference curves can then be

derived, and the same statistical test for determining attainment as described for the

direct assessment method can be applied. 

Strengths and Current Limitations. The logistic models are based on conditions

represented by the fixed stations in the current monitoring program, which in most

tributaries are sited in the main channel. Until more data are collected, the similarity

of shallow areas to the midchannel in the same segment is not known. This approach

would assume, in the absence of other data, that the main channel data are representative of similar depths in the shallows. If salinity or other physical data from the

shallows indicate that all or part of the shallow water column is more similar to a

different depth in the midchannel (as is sometimes the case for various reasons), then

the more representative depth would be used to estimate percent attainment. For

example, the pycnocline typically is deeper in the portion of the Chesapeake Bay

than on the flanks, and the depth of the pycnocline on one flank is typically deeper

than the other. Thus a 4-meter-deep, above-pycnocline water parcel on one flank

may be most similar to the 4-meter-above-pycnocline depth in the midchannel

profile, while the 4-meter-deep, subpycnocline parcel on the opposite flank is more

similar to the 5-meter depth in the midchannel profile. 

To date, dissolved oxygen concentrations have shown little significant trend in most

areas of the Chesapeake Bay and its tidal tributaries and, therefore, history-based

estimation models are reasonable. However, where significant trends are detected, it

would be important to review the models and their basis in light of new, emerging

empirical relationships at those locations. This approach provides an estimate of the

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amount of time a water parcel is above or below a particular concentration, but does

not address the length of individual exposure, rate of re-exposure, or a specific eventduration such as daily or 7-day mean. 

WATER CLARITY CRITERIA ASSESSMENT 

‘Recommended’ Level of Monitoring

Because the DATAFLOW technology is the only viable approach for assessing water

quality conditions in shallow-water designated use areas, there is only a ‘recommended’ level of monitoring for assessing the water clarity criteria. Significant

spatial and temporal variability are expected in the shallow-water designated use

area. The DATAFLOW is best suited to address the high level of variability and

provide data for credible assessments of criteria attainment. The only option for

reduced costs in shallow-water monitoring is to limit either the total number of tidal

systems assessed and/or the frequency of monitoring events for each system that are

conducted during a single year.

Assessing Attainment of the Shallow-Water Bay Grass 

Designated Use

Restoring underwater bay grasses to areas supporting “the propagation and growth

of balanced, indigenous populations of ecologically, recreationally and commercially important fish and shellfish inhabiting vegetated shallow-water habitats” is

ultimately the best measure of attainment of the shallow-water bay grass designated

use. To determine the return of water clarity conditions necessary to support restoration of underwater grasses and, therefore, attainment of the shallow-water designated

use, states may: 1) evaluate the number of acres of underwater bay grasses present

in each respective Chesapeake Bay Program segment, comparing that acreage with

the segment’s bay grass restoration goal acreage; and/or 2) determine the attainment

of the water clarity criteria within the area designated for shallow-water bay grass

use. The shallow-water bay grass use designated use area may be defined by either:

1) applying the appropriate water clarity criteria application depth (i.e., 0.5, 1 or 2

meters) along the entire length of the segment’s shoreline (with exception of those

shoreline areas determined to be underwater bay grass no-grow zones; see U.S. EPA

2003 for details); or 2) determining the necessary total acreage of shallow-water

habitat within which the water clarity criteria must be met using a salinity regime

specific ratio of underwater bay grass acres to be restored within a segment to acres

of shallow-water habitat that must meet the water clarity criteria within the same

segment (regardless of specifically where and at what exact depth those shallow

water habitat acreages reside within the segment). These approaches to assessing

attainment of the shallow-water bay grass designated use are described below in

more detail.

Assessing Underwater Bay Grasses Restoration. In response to a commitment in the Chesapeake 2000 agreement, the Chesapeake Bay watershed partners

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chapter vi • Recommended Implementation Procedures
adopted a baywide underwater bay grasses restoration goal of 185,000 acres. This

baywide restoration goal was established “to reflect historic abundance, measured as

acreage and density from the 1930s to present” (Chesapeake 2000, Chesapeake

Executive Council 2000).

The single best year of underwater bay grasses growth observed in each Chesapeake

Bay Program segment from the entire available record of aerial photographs (1938-

2000) was determined by state and federal agency resource managers and

Chesapeake Bay scientists as the best available data on underwater bay grasses

occurrence over the long-term. The underwater bay grasses goal acreage was set

using the single best year acreage out to a Chesapeake Bay Program segmentspecific application depth determined as summarized in Table VI-6 and described in

detail in the Technical Support Document for the Identification of Chesapeake Bay

Designated Uses and Attainability (U.S. EPA 2003). Based on the implementation

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Table VI-6. Methodology for establishing the 185,000 Chesapeake Bay baywide

underwater grasses restoration goal.

The baywide underwater bay grasses goal acreage was set using the single best year

acreage out to an application depth determined as follows:

1. Bathymetry data and aerial photographs were used to divide the single best year

underwater bay grasses acreage in each Chesapeake Bay Program segment into three

depth zones: 0-0.5 meters, 0.5-1.0 meters and 1-2 meters.

2. The aerial photographs were then used to determine the maximum depth to which the

underwater bay grass beds grew in each segment with either a minimum abundance or

minimum persistence. The underwater bay grass goal for a Chesapeake Bay Program

segment is the portion of the single best year acreage that falls within this determined

depth range. The decision rules for this were as follows:

In all segments, the 0-0.5 meter depth interval was designated for shallow-water

bay grass use. In addition, the shallow-water bay grass use was designated for

greater depths within a segment if either:

A) The single best year of underwater bay grasses distribution covered at least

20 percent of the potential habitat in a deeper zone; or,

B) The single best year of underwater bay grasses distribution covered at least

10 percent of the potential habitat in the segment-depth interval, and at least

three of the four five-year periods of the more recent record (1978-2000)

show at least 10 percent SAV coverage of potential habitat in the segmentdepth interval. 

3. The single best year underwater bay grasses distribution acreage of all Chesapeake

Bay Program segments were clipped at the deeper depth of the segment-depth

interval, determined above. The resulting underwater bay grass acreages for each

segment were added, resulting in the total baywide underwater bay grass restoration

goal of 185,000 acres.

Source: U.S. Environmental Protection Agency 2003
of this methodology, each Chesapeake Bay Program segment (see Figure VI-1 and

Table VI-4) has an underwater bay grass restoration goal acreage, with the exception

of those segments documented as underwater bay grass no-grow zones along their

entire shoreline, with the total acreage summed up from all segments equaling

185,000 acres.

In adopting and implementing their water quality standards for protecting the

shallow-water bay grass designated use, states may: 1) adopt the segment-specific

underwater bay grass restoration goal acreages that make up the baywide 185,000

restoration goal; or 2) adopt a lower initial Chesapeake Bay Program segmentspecific underwater bay grass acreage, below the established goal acreage for a

segment, and use their upcoming triennial reviews of state water quality standards to

continually evaluate and appropriately increase the segment-specific acreages

towards the ultimate underwater bay grass restoration goal acreage. If states choose

to adopt a lower initial segment-specific acreage, at a minimum they must adopt an

underwater bay grass acreage for that Chesapeake Bay Program segment equal to or

greater than the existing use underwater bay grasses acreage defined as either the

single best year of composite acreage of underwater bay grasses mapped through the

baywide underwater bay grasses aerial survey since 1975. The Chesapeake Bay

Program segment-specific acreages that, added together, make up the baywide

185,000 restoration goal are documented in the Technical Support Document for the

Identification of Chesapeake Bay Designated Uses and Attainability along with the

segment-specific existing use underwater bay grasses acreages (U.S. EPA 2003).

Achieving the Chesapeake Bay Program segment-specific underwater bay grass

restoration acreages should be measured as the single best year of acreage as

observed through the most recent three years of data from the Chesapeake Bay

underwater bay grasses aerial survey. All mapped acreages of underwater bay

grasses in a segment should be counted towards achievement of each segmentspecific restoration goal regardless of the depth. Chesapeake Bay segment level

acreages of underwater bay grasses are published annually and can be accessed

through the Chesapeake Bay Program’s web site at http://www.chesapeakebay.

net/data, or directly through the Virginia Institute of Marine Science’s “Bay Grass in

Chesapeake Bay and Delmarva Peninsula Coastal Bays” web site at http://www.

vims.edu/bio/sav/index.html.

Assessing Water Clarity Criteria Attainment at an Established Application Depth. The recommended method for assessing water clarity criteria

attainment is, first, to interpolate monthly values of Kd to obtain a Kd value for each

interpolator cell, then to calculate PLW for each cell using the interpolated value of

Kd and the Chesapeake Bay Program segment-specific shallow-water bay grass

designated use boundary depth (see U.S. EPA 2003 for a full listing of the recommended shallow-water bay grass designated use boundary depths). Note that for

statistical reasons, the interpolations are performed using a log transformation of the

light values (log[Kd]). The resulting interpolated cell values are converted back to

their untransformed status for the PLW calculation. 

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As described previously in this chapter, the interpolator cells can be associated with

the proper Chesapeake Bay Program segment and salinity zone so that each cell’s

PLW value can be compared to the proper salinity regime-based water clarity criterion value. The cell area is then accumulated in the ‘fail’ or ‘pass’ tally for each

Chesapeake Bay Program segment for each month. The cumulative frequency distribution curve resulting from the monthly percent attainment measures over the

respective underwater bay grass growing season (see Table VI-2) and three-year

attainment period is then compared statistically to the reference curve for the appropriate salinity zone to determine the degree of attainment or nonattainment. If the

curves are differ significantly, then the segment/designated use is considered out of

attainment and fails by the amount defined by the area between the two curves.

Assessing Water Clarity Criteria Attainment throughout a Defined

Shallow-Water Habitat Acreage. Restoring underwater bay grasses within a

segment requires that the particular shallow-water habitat meets the Chesapeake Bay

water clarity criteria across acreages much greater than those actually covered by

underwater bay grasses. The ratio of underwater bay grass acreage to the required

shallow-water habitat acreage achieving the necessary level of water clarity to

support return of those underwater bay grasses varies, based upon the different

species of bay grasses inhabiting the Chesapeake Bay’s four salinity regimes. The

average baywide ratio of underwater bay grass acreage to suitable shallow-water

habitat acreage is approximately one acre of underwater bay grasses for every three

acres of shallow-water habitat achieving the Chesapeake Bay water clarity criteria

(U.S. EPA 2003).

The salinity regime and, therefore, bay grass community-specific underwater bay

grass acreage to shallow-water habitat acreage ratios have been derived through an

evaluation of extensive underwater bay grass distribution data within tidal-fresh,

oligohaline, mesohaline and polyhaline salinity regimes (reflecting different levels

of coverage by different underwater bay grass communities). The Technical Support

Document for the Identification of Chesapeake Bay Designated Uses and Attainability documents the methodology followed and the resulting underwater bay

grasses acreage to shallow-water habitat acreage ratios derived for each of the four

salinity regimes (U.S. EPA 2003).

The same procedures as described above in “Assessing Water Clarity Criteria Attainment at an Established Application Depth” are followed for determining attainment

of the water clarity criteria across the total required shallow-water habitat acreage for

a specific Chesapeake Bay Program segment. The only difference is that a segmentspecific water clarity criteria application depth is not applied. Instead, the depth of

attainment of the water clarity criteria is determined for each interpolator cell. The

area in each interpolator cell from the intertidal zone out to the water-column depth

at which the water clarity criteria was attained is combined along with other similar

areas determined for the other interpolator cells comprising the shallow-water areas

in a specific segment.

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chapter vi • Recommended Implementation Procedures
Factoring in the Influence of Tidal Range 

on Water Clarity Attainment

Chesapeake Bay Submerged Aquatic Vegetation Water Quality and Habitat-Based

Requirements and Restoration Targets: A Second Technical Synthesis specifies that

half the diurnal tidal range for that Chesapeake Bay Program segment should be

added to the restoration depth Z before calculating PLW or PLL (Batiuk et al. 2000,

page 102). These half tidal-range values, taken from tidal-range tables and averaged

by Chesapeake Bay Program segment, were listed on page 202 of that document in

Table D-4. However, for the purposes of testing attainment of the water clarity

criteria, the EPA recommends using the water clarity criteria application depths

without adding half the diurnal tidal range to it (see U.S. EPA 2003). This recommendation is based on the biologically-based water clarity criteria reference curves.

The methodology followed in the derivation of those reference curves did not

include adding the half tidal range to the restoration depth, Z (see Appendix H). The

EPA believes it is important to maintain consistency throughout the entire set of

procedures for determining water clarity criteria attainment.

Using Midchannel Data to Estimate Shallow-water Conditions

The majority of baywide, regional and local tidal Bay water quality monitoring

programs in the past have collected data only from fixed midchannel stations. Incorporating a rotational shallow-water monitoring into the tidal monitoring network is

leading to the generation of shallow-water data for evaluating attainment for the

water clarity criteria. However, given the rotational nature of this shallow-water

monitoring network component, fixed midchannel stations are still going to be used

in criteria assessment. It is relevant, in assessing water clarity criteria attainment, to

note the extent to which water quality monitoring data collected from midchannel

stations in the Chesapeake Bay and its tidal tributaries represent conditions at

shallow-water sites where underwater bay grasses potentially occur and the water

clarity criteria apply. 

Evaluation of Midchannel and Nearshore Data Comparability. Several

studies have addressed the shallow-water versus midchannel sampling issue in the

Chesapeake Bay (Stevenson et al. 1991; Batiuk et al. 1992; Ruffin 1995; Bergstrom,

unpublished data; Parham 1996; Karrh 1999; Hunley, unpublished data). While most

studies indicate that midchannel data can be used to describe shallow-water conditions, several suggest the opposite. There is no doubt that demonstrable differences

in water quality can occur between shallow-water and midchannel stations over

varying temporal and spatial scales, especially when bay grasses are present (Ward

et al. 1984; Moore 1996). Other possible causes of variability between shallow-water

and midchannel environments include localized resuspension of sediments, algal

patchiness, point source effluents or sediment chemistry variability (Goldsborough

and Kemp 1988; Moore 1996).

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chapter vi • Recommended Implementation Procedures
Using Shallow-water Water Quality Data where Available. Because of

these sources of variability, the use of midchannel data to evaluate the water-clarity

criteria should be avoided whenever shallow-water data are available. Managers of

tidal-water quality monitoring programs should consider the need for enhanced evaluation of the shallow-water environment in future monitoring efforts and requests for

funding.

Guidance for Using Midchannel Data when Shallow-water Information

Is Absent. When nearshore shallow-water monitoring data are not available, Karrh

(1999) and Batiuk et al. (2000) provide guidance on the use of midchannel information. The findings published by Karrh (1999) and reported by Batiuk et al. (2000)

were based on a comprehensive analysis of shallow-water and midchannel data in

the Chesapeake Bay, which have been collected since 1983 to determine whether

such data can be used to characterize shallow-water environments. Data for the

Karrh (1999) study, obtained from state monitoring efforts, academic researchers

and citizen monitors, were incorporated from the entire Chesapeake Bay and its tidal

tributaries, including the upper Chesapeake Bay region; the Middle, Magothy,

Rhode, Chester, Choptank, Patuxent, Potomac, Rappahannock, Poquoson, York and

James rivers; and Mobjack Bay.

These reports indicated that underwater bay grass habitat quality conditions (relative

to attainment or nonattainment of the 1992 bay grass habitat requirements published

by Batiuk et al. in 1992 and Dennison et al. in 1993) were comparable between

nearshore and adjacent midchannel stations 90 percent of the time, when station

pairs were separated by less than two kilometers.

Midchannel and nearshore areas usually show similar attainment/nonattainment of

the individual water quality parameters—Kd or Secchi depth, dissolved inorganic

nitrogen, dissolved inorganic phosphorus, chlorophyll a and total suspended

solids—published in 1992 as the original set of Chesapeake Bay underwater bay

grass habitat requirements (Batiuk et al. 1992; 2000). These same water quality

parameters are used in calculating percent light-at-the-leaf (PLL) and applying the

supporting diagnostics tools (see Chapter VII).

The Karrh (1999) study results also indicated that individual water quality parameter

concentrations at many of the comparison sites differed significantly between shallowwater and midchannel areas, from a statistical standpoint. These differences suggest

that although the attainment/nonattainment status may have been comparable, the

magnitude of attainment/nonattainment and the diagnosis of the water quality factors

involved between the shallow-water and midchannel areas could be affected.

It should be noted that the comparisons made between shallow-water and

midchannel areas may also have been affected by temporal factors, given that the

pairs were not sampled on the same day. Water quality managers should also be

aware that these reports were developed to support the application of nonregulatory

bay grass habitat requirements and restoration goals, not regulatory aquatic life

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chapter vi • Recommended Implementation Procedures
water quality criteria. Therefore, the report’s recommendations for the allowable use

of midchannel data should be used with appropriate caution only in the absence of

shallow-water quality monitoring data. 

Estimating Areas Characterized by Midchannel Stations. It is possible to

determine a distance from a specific midchannel station for which it is appropriate

to use the midchannel distance to characterize the shallow-water environment.

Results revealed that the underwater bay grass habitat quality conditions are indistinguishable between shallow-water and adjacent midchannel stations 90 percent of

the time, when station pairs were separated by less than two kilometers. This radius

differs on a site-by-site basis (see Batiuk et al. 2000, Chapter IX, Table IX-3 and

figures IX-4a through IX-4o). Decisions to use midchannel data to characterize

shallow-water conditions should be made on a site-by-site, tributary-by-tributary

basis. Karrh (1999) provides detailed illustrations of estimated distances from

midchannel monitoring stations to the farthest point where the shallowwater/midchannel data are comparable.

River Input and Flow Considerations

States responsible for measuring water clarity/shallow-water bay grass designated

use attainment near the fall-lines of where major free flowing rivers enter tidal

waters should recognize the strong influences of intra- and interannual flows on

conditions in the shallow-water habitats. The quality of the waters entering the tidalfresh reaches of these rivers is greatly influenced by flow levels. The decadal scale

record of underwater bay grasses distributions and concurrent water quality monitoring data provides the states and other users with a wealth of information from

which to gather information on the relative influence of flow conditions on observed

attainment. In the case of water clarity attainment and restoration of underwater

grasses, the EPA recommends recognition within states’ water quality standards of

the influence of river flow conditions on water clarity and underwater bay grasses

(through chlorophyll a and suspended solids contributions to reduced light penetration) particularly for the tidal reaches just below the major river fall lines.

Management actions directed toward attaining the water clarity criteria and shallowwater bay grass designated use attainment in these tidal reaches should also reflect

the long-term flow conditions and influences on local shallow-water habitat quality.

CHLOROPHYLL A CRITERIA ASSESSMENT 

‘Recommended’ Level of Monitoring

Monitoring for chlorophyll a criteria assessment requires a significant amount of

spatially and temporally intensive data. Algal blooms tend to occur sporadically and

in patches throughout the Chesapeake Bay. The severe nature of blooms, associated

dissolved oxygen extremes and associated releases of toxins are what cause ecological impacts. 

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chapter vi • Recommended Implementation Procedures
To capture data that reflect those blooms, spatially and temporally intensive data are

required. In the shallow-water designated use areas, the DATAFLOW system can

adequately characterize the spatial variability in chlorophyll a.

A ‘recommended’ monitoring program for the open-water and migratory spawning

and nursery designated use areas would include a combination of fixed-station,

continuous track and remotely sensed data collection. Fixed-station data is usually

considered the most reliable type of data collection because it includes ambient

sample analysis in the laboratory. For that reason, it serves as the baseline for all

other types of chlorophyll a measurement. Continuous-track (‘flow-through’) monitoring should be developed for all vessels used to conduct the fixed-station

monitoring program. Like the DATAFLOW system, the continuous-track monitoring

would provide intensively collected data that would significantly improve our

assessment of the spatial variation in chlorophyll a. One of the limitations of continuous-track monitoring is that it does not cover the entire Chesapeake Bay. Thus, the

third type of recommended monitoring is remote sensing, which can provide estimates of chlorophyll a for most locations in the Bay. It is not clear at this point that

remote sensing is ready for the criteria assessment application, but it does offer great

potential. All three types of monitoring (fixed-station, continuous track, remote

sensing) are recommended because each provides complementary types of information that are useful for evaluating different parts of the Chesapeake Bay. 

‘Adequate’ Level of Monitoring

Assuming that funding will not be available for the recommended monitoring

approach, an alternative would be to collect only fixed-station data enhanced with

continuous track monitoring. This provides spatially intensive data wherever cruises

occur, including most tidal tributaries. Furthermore, it represents a relatively small

cost, particularly when considered in proportion to the amount of information that

could be generated. The improvement of this approach over current monitoring is

that spatially intensive data collection would be collected on a regular basis in most

large tidal tributaries. The limitation would be that data would only be collected

along cruise tracks and not intensively throughout the Chesapeake Bay (i.e., as might

be possible with remote sensing). For that reason, the uncertainty associated with the

‘adequate’ monitoring plan would be higher than the ‘recommended’ plan.

‘Marginal’ Level of Monitoring

If funding is not available for even the adequate level of monitoring, assessments

would need to rely on fixed-station data only. This type of monitoring is limited in

its ability to assess the spatial and temporal variability of chlorophyll a found in most

of the Chesapeake Bay. The uncertainty associated with the assessment of chlorophyll a criteria attainment using only the fixed-station monitoring program would be

expected to be quite high.

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chapter vi • Recommended Implementation Procedures
Assessing Chlorophyll a Criteria Attainment

Phytoplankton are actively growing and consuming nutrients throughout the surface

mixed layer of the water column. The pycnocline region below the this layer, as well

as other depth strata below the pycnocline, rarely contain sufficient light for active

photosynthesis. Therefore, there is little or no autotrophic growth below the surface

mixed layer, although phytoplankton accumulate within and below the pycnocline

due to the physical processes of sinking and estuarine circulation. Given that the

chlorophyll a concentrations throughout the water column will be expressed at the

surface at some point during the natural cycling of phytoplankton and for the

sampling reasons described above, the chlorophyll a criteria are applied to surface

waters only.

Chlorophyll a samples used in determining attainment of numerical chlorophyll a

criteria should be collected at 0.5 to 1 meter below the surface. The majority of

historical and current chlorophyll a data are collected from a discrete surface depth.

The potential for assessing broad areas of the estuary via high-speed vessels and

flow-through technologies or remote sensing can only be tapped if the criteria apply

only to surface chlorophyll a distributions. In general, chlorophyll a concentrations

are highest in the surface layer of the water column.

The formulation and ultimately the assessment of numerical chlorophyll a criteria

should be based upon seasonal dynamics and concentrations of chlorophyll a in the

Chesapeake Bay and its tidal tributaries. Spring and summer were chosen for these

purposes because chlorophyll a concentrations attain annual peaks during these

months in the estuary’s various salinity regimes. Any site-specific numerical

impairment-based chlorophyll a criteria should be applied as salinity regime-based

spring (March through May) and summer (July through September) seasonal mean

concentrations.

In spring, river inputs with high dissolved inorganic nitrogen dominate, dissolved

inorganic nitrogen is abundant, phytoplankton are primarily limited by the availability of phosphorus, and bottom waters are oxygenated. By contrast, under summer

conditions, recycling of nitrogen and phosphorus is the dominant supply, both

dissolved inorganic nitrogen and dissolved inorganic phosphorus are low, phytoplankton are primarily limited by the availability of nitrogen and deep bottom waters

are anoxic. The ecological implications of chlorophyll a concentrations in spring and

summer are vital to physical and chemical processes such as hypoxia and anoxia,

nutrient recycling and light attenuation, and biological processes such as the availability of sufficient and appropriate food for filter and suspension-feeders.

After years of monitoring the Chesapeake Bay and its tidal tributaries, characterizing

phytoplankton dynamics and analyzing these data, Bay scientists have found that

June is indeed a ‘transition’ month from spring to summer. During certain years,

June behaves more like spring in the types and quantity of phytoplankton that are

present, while in other years, the flora reflect the summer patterns of composition

and densities. This means that in attempts to measure ‘spring’ and ‘summer’

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chapter vi • Recommended Implementation Procedures
phytoplankton populations, June is either springlike, summerlike or uniquely

different from either season.

At present, the recommended method for assessing numerical chlorophyll a criteria

attainment is to interpolate monthly chlorophyll a concentrations for each surface

interpolator cell from the available fixed stations. The interpolator cells can be associated with the proper segment and salinity zone, so that each cell’s chlorophyll a

concentration can be compared to the proper chlorophyll a criterion value. The cell

area is then accumulated in the fail or pass tally for each Chesapeake Bay Program

segment for each month. The cumulative frequency distribution curve resulting from

the monthly percent attainment measures over the spring or summer seasons and the

three-year attainment period is then compared statistically to the reference curve to

determine the degree of attainment/nonattainment. If the curves are significantly

different, then the segment/designated use is considered out of attainment, and

failing by the amount defined by the area between the two curves.

River Input and Flow Considerations

States responsible for measuring chlorophyll a criteria attainment near the fall lines

where major free-flowing rivers enter tidal waters should recognize the strong influences of intra- and interannual flows on conditions in the adjacent tidal-fresh

habitats. In addition to their upstream contributions of chlorophyll a, the flow levels

of waters directly entering the tidal-fresh reaches of these rivers greatly influence the

resulting tidal habitat chlorophyll a concentrations. The decadal scale record of

water quality monitoring data provides the states and other users with a wealth of

information from which to understand the relative influence of flow conditions on

observed chlorophyll a criteria attainment. The EPA recommends recognition within

states’ water quality standards of the influence of river flow conditions on chlorophyll a concentrations, particularly in the tidal reaches just below the major fall

lines. Management actions directed toward chlorophyll a criteria attainment in these

tidal reaches should also reflect the long-term flow conditions and influences on

local water quality.

EVALUATION OF CHESAPEAKE BAY 

WATER QUALITY MODEL OUTPUT

The Chesapeake Bay Program has developed what have become standard estuarine

modeling tools, including a watershed model (Donigian et al. 1994; Linker et al.

1996, 2000), airshed model (Shin and Carmichael 1992; Appleton 1995, 1996),

estuarine hydrodynamic model (Wang and Johnson 2000), estuarine water quality

model (Cerco 1993, 1995a, 1995b; Thomann et al. 1994; Cerco and Meyers 2000;

Cerco 2000; Cerco and Moore 2001; Cerco et al. 2002) and estuarine sediment

diagenesis model (Di Toro 2001). Together these linked simulations provide a

system to estimate dissolved oxygen, water clarity and chlorophyll a in 35 major

segments of the Chesapeake Bay and its tidal tributaries. The same criteria

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chapter vi • Recommended Implementation Procedures
attainment assessment process applied to observed data is applied to integrated

modeling/monitoring ‘scenario’ data to determine likely criteria attainment under

management loading scenarios.

The watershed and airshed models are loading models. As such, they provide an estimate of management actions through air controls, agricultural best management

practices, or point source controls which will reduce nutrient or sediment loads to

the Chesapeake Bay tidal waters. The advantage of using loading models is that the

full simulation through different hydrologies of wet, dry and average periods can be

simulated on existing or hypothetical land use patterns. All of the Chesapeake Bay

Program models used in this system simulate the 10-year period from 1985 to 1994

(Linker and Shenk 2000). 

CHESAPEAKE BAY WATERSHED MODEL

The Chesapeake Bay Watershed Model is designed to simulate nutrient and sediment

loads delivered to the Chesapeake Bay under different management scenarios

(Donigian et al. 1994; Linker et al.1996; Linker 1996). The simulation is an overall

mass balance of nitrogen and phosphorus in the basin, so the ultimate fate of the

input nutrients is incorporation into crop or forest plant material, incorporation into

soil, or loss through river runoff. 

The Chesapeake Bay Watershed Model has been in continuous operation in the

Chesapeake Bay Program since 1982 and has had many upgrades and refinements.

The current version of the Watershed Model, Phase 4.3, is a comprehensive package

for the simulation of watershed hydrology, nutrient and sediment export from

pervious and impervious land uses and the transport of these loads in rivers and

reservoirs. The model is based on a modular set of computer codes called Hydrologic Simulation Program—Fortran (HSPF). A slightly modified version of HSPF

release 11.1 (Bicknell et al. 1996) is applied in the watershed simulation. Version 11

is a widely-used public-domain model supported by the U.S. EPA, U.S. Geological

Survey and U.S. Army Corps of Engineers (Shenk et al. 1998).

The Watershed Model allows for the integrated simulation of land and soil contaminant runoff processes with in-stream hydraulic and sediment-chemical interactions.

The model takes into account watershed land uses and the application of fertilizers

and animal manure; loads from point sources, atmospheric deposition and onsite

wastewater management systems; and best management practice reduction factors

and delivery factors. Land uses, including cropland, pasture, urban areas and forests,

are simulated on an hourly time-step. 

Fourteen calendar years (1984–1997) of varying hydrology are simulated by the

Watershed Model, although only 10 of those years (1985–1994) are used in this

study because of the more limited simulation period of the Chesapeake Bay water

quality model. Scenarios are run on a 1-hour time step and results are often aggregated into 10-year-average annual loads for reporting and comparisons among

scenarios. Watershed Model results, in the form of daily flows and nutrient and sediment loads, are used as input to the Chesapeake Bay water quality model.

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chapter vi • Recommended Implementation Procedures
CHESAPEAKE BAY WATER QUALITY MODEL

The complex movement of water within the Chesapeake Bay, particularly the density

driven vertical estuarine stratification, is simulated with a Chesapeake Bay hydrodynamic model of more than 13,000 cells (Wang and Johnson 2000).

Three-dimensional equations of the intertidal physical system, including equations

of continuity, momentum, salt balance and heat balance, are employed to provide the

correct simulation of the movement, or the barriers to movement, of the water quality

constituents of dissolved oxygen, water clarity and chlorophyll a. Correct formulation of vertical mixing, including the simulation of vertical eddy diffusion

coefficients in the hydrodynamic model is particularly important for the dissolved

oxygen criteria as the principal barrier to vertical movement of dissolved oxygen

from surface waters to the deep water is the pycnocline simulated by the hydrodynamic model.

The water quality model is linked to the hydrodynamic model and uses complex

nonlinear equations describing 26 state variables relevant to the simulation of

dissolved oxygen, water clarity and chlorophyll a (Cerco 1993, 1995a, 1995b, 2000;

Thomann et al. 1994; Cerco and Meyers 2000). Dissolved oxygen is simulated as the

mass balance calculation of reaeration at the surface, respiration of algae, benthos

and underwater bay grasses; photosynthesis of algae, benthic algae and underwater

bay grasses; and the diagenesis, or decay of organics, by microbial processes in the

water column and sediment. This mass balance calculation is made for each model

cell and for associated sediment cells at each hourly time step, providing an estimate

of dissolved oxygen from nutrient loads from the watershed and airshed to the waters

of the 35 major segments of the Chesapeake Bay and its tidal tributaries. Chlorophyll

a is estimated based on Monod calculations of algal growth given resource

constraints of light, nitrogen, phosphorous or silica. Water clarity is estimated from

the daily input loads of sediment from the watershed and shoreline acted on by

regionally-calibrated settling rates, as well as estimated advection due to hydrodynamics. Coupled with the water quality model are simulations of settling to the

sediment of organic material and its subsequent decay and flux of inorganic nutrients from the sediment (Di Toro 2001) as well as a coupled simulation of underwater

bay grasses in shallow waters (Cerco and Moore 2001).

INTEGRATION OF MONITORING AND MODELING 

FOR CRITERIA ASSESSMENT 

The load allocation process requires that specific water quality conditions be met

over critical time periods within designated use areas. These areas are given either a

‘pass’ or ‘fail’ status. While the Chesapeake Bay water quality model can estimate

changes in water quality due to changes in input loads with reasonable accuracy, an

exact match of the simulated and observed data is impossible. The following method

was developed to make the best use of the strengths of the Chesapeake Bay water

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chapter vi • Recommended Implementation Procedures
quality model and the Chesapeake Bay Water Quality Monitoring Program in

assessing criteria attainment.

The observed data is used to assess criteria attainment during a ‘base’ period corresponding to the years of calibration for the Chesapeake Bay water quality model,

1985–1994. The Chesapeake Bay water quality model is used in scenario mode to

determine the effect of changes in nutrient and sediment loads on water quality

concentrations. A modified 1985–1994 observed data set is generated for each

scenario using both the model and the observations. The same criteria attainment

assessment process applied to the observed data is then applied to this scenario data

to determine likely criteria attainment under modified loading scenarios.

To generate the modified data set for a particular scenario (e.g., 2010 Clean Air Act),

the EPA compared the output of the scenario to the output of the calibration on a

point-by-point and month-by-month basis. For each point in space and time where

an observation exists during the 1985–1994 period, a mathematical relationship

between the model scenario and the model calibration was established by regressing

the 30 or so daily values for the month when the observation occurred in the water

quality model cell that contains the observation. The regression generates a unique

equation for each point and month that transforms a calibration value to a scenario

value. This relationship is then applied to the monitored observation as an estimate

of what would have been observed had the Chesapeake Bay watershed been under

the scenario management rather than the management that existed during

1985–1994. This procedure is repeated for each monitored observation of dissolved

oxygen, water clarity and chlorophyll a to generate an ‘observed’ data set for the

scenario. For a full discussion of this procedure, see A Comparison of Chesapeake

Bay Estuary Model Calibration With 1985–1994 Observed Data and Method of

Application to Water Quality Criteria (Linker et al. 2002). 

LITERATURE CITED

Alden, R. W. III and E. S. Perry 1997. Presenting Measurements of Status: Report to the

Chesapeake Bay Program Monitoring Subcommittee’s Data Analysis Workgroup. Chesapeake Bay Program, Annapolis, Maryland.

Appleton, E. 1996. Air quality modeling’s brave new world: A new generation of software

systems is set to tackle regional and multipollutant air quality issues. Environmental Science

and Technology 30(5):200A-204A.

Appleton, E. L. 1995. A cross-media approach to saving the Chesapeake Bay. Environmental

Science and Technology 29(12):550-555.

Bahner, L. 2001. The Chesapeake Bay and Tidal Tributary Volumetric Interpolator, VOL3D

Version 4.0. National Oceanic and Atmospheric Administration, Chesapeake Bay Office.

http://www.chesapeakebay.net/cims/interpolator.htm

Batiuk, R. A., P. Bergstrom, M. Kemp, E. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V.

Carter, N. B. Rybicki, J. M. Landwehr, C. Gallegos, L. Karrh, M. Naylor, D. Wilcox, K. A.

Moore, S. Ailstock and M. Teichberg. 2000. Chesapeake Bay Submerged Aquatic Vegetation

▼197

chapter vi • Recommended Implementation Procedures
Water Quality and Habitat-Based Requirements and Restoration Targets: A Second Technical Synthesis. CBP/TRS 245/00 EPA 903-R-00-014. U.S. EPA Chesapeake Bay Program,

Annapolis, Maryland.

Batiuk, R. A., R. Orth, K. Moore, J. C. Stevenson, W. Dennison, L. Staver, V. Carter, N.

Rybicki, R. Hickman, S. Kollar and S. Bieber. 1992. Chesapeake Bay Submerged Aquatic

Vegetation Habitat Requirements and Restoration Targets: A Technical Synthesis. CBP/TRS

83/92. U.S. EPA Chesapeake Bay Program, Annapolis, Maryland.

Bicknell, B., J. Imhoff, J. Kittle, A. Donigian Jr., R. Johanson and T. Barnwell, 1996. Hydrologic Simulation Program–Fortran User’s Manual for Release 11. U.S. EPA Environmental

Research Laboratory, Athens, Georgia.

Cerco, C. F., L. Linker, J. Sweeney, G. Shenk and A. J. Butt. 2002. Nutrient and solids

controls in Virginia’s Chesapeake Bay tributaries. Journal of Water Resources Planning and

Management May/June:179-189.

Cerco, C. F. and K. Moore. 2001. System-wide submerged aquatic vegetation model for

Chesapeake Bay. Estuaries 24(4):522-534.

Cerco, C. and M. Meyers. 2000. Tributary Refinements to Chesapeake Bay Model. Journal

of Environmental Engineering 126(2):164-174.

Cerco, C. F. 2000. Phytoplankton kinetics in the Chesapeake Bay Eutrophication Model.

Journal of Water Quality and Ecosystem Modeling 1(1-4):5-49.

Cerco, C. F. 1995. Response of Chesapeake Bay to nutrient load reductions. Journal of Environmental Engineering 121:8 549-556.

Cerco, C. F. 1995. Simulation of Long-Term Trends in Chesapeake Bay Eutrophication.

Journal of Environmental Engineering 121(4):298-310.

Cerco, C. F. 1993. Three-Dimensional Eutrophication Model of Chesapeake Bay. Journal of

Environmental Engineering 119(6): 1006-1025.

Chesapeake Bay Program (CBP). 1999. Analytical Segmentation for the 1997 Reevaluation

and Beyond. Report from the Chesapeake Bay Program Monitoring Subcommittee’s Data

Analysis Workgroup. Annapolis, Maryland.

Chesapeake Executive Council. 2000. Chesapeake 2000. Chesapeake Bay Agreement,

Annapolis, MD

Dennison, W. C., R. J. Orth, K. A. Moore, J. C. Stevenson, V. Carter, S. Kollar, P. W.

Bergstrom and R. A. Batiuk. 1993. Assessing water quality with submersed aquatic vegetation habitat requirements as barometers of Chesapeake Bay health. Bioscience 43:86-94.

Di Toro, D. M. 2001. Sediment Flux Modeling. John Wiley and Sons, Inc. New York, New

York. 624 pp.

Donigian, J., S. Anthony, B. R. Bicknell, A. S. Patwardhan, L. C. Linker, C. H. Chang and

R. Reynolds. 1994. Watershed Model Application to Calculate Bay Nutrient Loadings: Final

Findings and Recommendations. U.S. EPA Chesapeake Bay Program, Annapolis, Maryland.

Goldsborough, W. J. and W. M. Kemp. 1988. Light responses of a submersed aquatic macrophyte: Implications for survival in turbid waters. Ecology 69:1775-1786.

Haan, C.T. 1977. Statistical Methods in Hydrology. Iowa State University Press. Ames, Iowa.

378 pp. 

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chapter vi • Recommended Implementation Procedures
Helsel, D. R. and R. M. Hirsch. 1992. Statistical Methods in Water Resources. Elsevier

Science Publishing Company, Inc. New York. 522 pp.

Jordan, S. J., C. Stenger, M. Olson, R. A. Batiuk and K. Mountford. 1992. Chesapeake Bay

Dissolved Oxygen Goal for Restoration of Living Resource Habitats: A Synthesis of Living

Resource Requirements with Guidelines for their Use in Evaluating Model Results and Monitoring Information. CBP/TRS 88/93. Chesapeake Bay Program, Annapolis, Maryland.

Karrh, L. 1999. Comparison of Nearshore and Midchannel Water Quality Conditions.

Chesapeake Bay Program, Annapolis, Maryland. 200 pp.

Linker, L.C., 1996. Models of the Chesapeake Bay. Sea Technology 37(9):49-55. 

Linker, L.C., G. W. Shenk, P. Wang, C. F. Cerco, A. J. Butt, P. J. Tango and R. W. Savidge.

2002 A Comparison of Chesapeake Bay Estuary Model Calibration With 1985-1994

Observed Data and Method of Application to Water Quality Criteria. Modeling Subcommittee, Chesapeake Bay Program Office, Annapolis, Maryland.

Linker, L. C., G. W. Shenk, D. L. Dennis and J. S. Sweeney. 2000. Cross-Media Models of

the Chesapeake Bay Watershed and Airshed. Water Quality and Ecosystem Modeling 1(1-

4):91-122.

Linker, L. C., C. G. Stigall, C. H. Chang and A. S. Donigian, Jr., 1996. Aquatic accounting: 

Chesapeake Bay Watershed Model quantifies nutrient loads. Water Environment and Technology 8(1):48-52.

Moore, K. A. 1996. Relationships between seagrass growth and survival and environmental

conditions in a lower Chesapeake Bay tributary. Ph.D. dissertation. University of Maryland,

College Park, Maryland. 188pp.

National Research Council. 2001. Assessing the TMDL Approach to Water Quality Management. Committee to Assess the Scientific Basis of the Total Maximum Daily Load Approach

to Water Pollution Reduction, Water Science and Technology Board, Division on Earth and

Life Studies. National Academy Press, Washington, D. C.

Neerchal, N. K., G. Papush and R. Shafer. 1992. Statistical Method of Measuring DO

Restoration Goals by Combining Monitoring Station and Buoy Data. Chesapeake Bay

Program, Annapolis, Maryland.

Parham, T. 1996. Analysis of SAV and Shellfish Habitat in the Patuxent River and Choptank

River Tributaries. Chesapeake Bay Program, Annapolis, Maryland.

Ruffin, K. 1995. The effects of hydraulic clam dredging on nearshore turbidity and light

attenuation in Chesapeake Bay, Maryland. Master’s thesis, University of Maryland, College

Park, Maryland. 97 pp.

Rybicki, N. B. and V. Carter. 2002. Light and temperature effect on the growth of Vallisneria

americana and Hydrilla verticillata (L.f.) Royle. Journal of Aquatic Plant Management

40:92- 99.

Shenk, G. W., L. C. Linker and A. S. Donigian, 1998. The Chesapeake Bay Program Models.

Federal Interagency Hydrologic Modeling Conference, Las Vegas, Nevada.

Shin, W. C. and G. R. Carmichael. 1992. Sensitivity of acid production/deposition to emission reductions. Environmental Science and Technology 26(4):715-725.

Stevenson, J. C., L. W. Staver and P. Hensel. 1991. Evaluation of water quality monitoring

in shallows versus deep water for submersed aquatic vegetation along an estuarine gradient.

Estuaries 16:346-361.

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Thomann, R. V., J. R. Collier, A. Butt, E. Casman and L. C. Linker. 1994. Response of the

Chesapeake Bay Water Quality Model to Loading Scenarios. Chesapeake Bay Program

Office, Annapolis, Maryland.

U.S. Environmental Protection Agency (EPA). 1997. Guidelines for Preparation of the

Comprehensive State Water Quality Assessments (305 (b) Reports) and Electronic Updates.

Assessment and Watershed Protection Division, Office of Wetlands, Oceans and Watersheds,

Office of Water, U.S. EPA, Washington, D. C.

U.S. EPA. 2003. Technical Support Document for the Identification of Chesapeake Bay

Designated Uses and Attainability. EPA 903-R-03-004. Chesapeake Bay Program Office,

Annapolis, Maryland.

Wang, H. V. and B. H. Johnson. 2000. Validation and application of the second generation

three- dimensional hydrodynamic model of Chesapeake Bay. Journal of Water Quality and

Ecosystem Modeling 1(1-4):51-90.

Ward, L. G., W. M. Kemp and W. R. Boynton. 1984. The influence of water depth and

submerged vascular plants on suspended particulates in a shallow estuarine embayment.

Marine Geology 59:85-103.

Weisberg, S. B., J. A. Ranasinghe, D. M. Dauer, L. C. Schaffner, R. J. Diaz and J. B. Frithsen.

1997. An estuarine benthic index of biotic integrity (B-IBI) for Chesapeake Bay. Estuaries

20:149-158. 

Weibull, W. 1939. The Phenomenon of Rupture in Solids: Ingeniors Vetenskaps Akademien

Handlinga 153. Stockholm, Sweden.17 pp.

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ADDRESSING NATURAL EXCEEDANCE 

OF THE CHESAPEAKE BAY CRITERIA

Through the refinement of tidal-water designated uses to better reflect natural habitats defined by season and physical features (e.g., bathymetry, stratification and

hydrodynamic process) and the development of criteria that specifically support

these uses, a full consideration of natural conditions has been directly interwoven

into the two major components of state water quality standards. Within the recommended implementation procedures for defining criteria attainment, occasional

exceedance of criteria, often natural in origin, has been directly accounted for in

deriving and applying biologically based reference curves (see Chapter VI). Finally,

possible errors in sampling and natural spatial and temporal variability have been

accounted for, in part, through applying a statistical test for the significance of the

observed nonattainment. Outside of extreme climatic events, application of the

complete set of integrated Chesapeake Bay criteria, designated uses and attainment

determination procedures will clearly identify nonattainment of desired water

quality conditions due to anthropogenic impacts.

This combination of refined uses, habitat-tailored criteria and comprehensive implementation procedures factors in many circumstances, described below, in which

natural conditions affect criteria attainment. In some situations extreme weather

events or conditions may result in criteria exceedances beyond those accounted for

in the combined criteria-uses-implementation procedures. In such situations, additional steps should be taken to quantify, where possible, exceedances that are due to

natural events or conditions versus anthropogenic, pollutant-based stresses. This

section describes known natural events or conditions that will influence attainment

of the Chesapeake Bay dissolved oxygen, water clarity and chlorophyll a criteria.

Tools that can be used to diagnose and quantify factors contributing to nonattainment

also are described.

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chaptervii

Diagnostic Procedures for

Natural Processes and

Criteria Nonattainment
NATURAL EXCURSIONS OF LOW 

DISSOLVED OXYGEN CONDITIONS

Physical (e.g., temperature, stratification or wind- and tide-driven mixing), chemical

(e.g., salinity) and biological (e.g., respiration and photosynthesis) processes can

independently and interactively affect the concentration of dissolved oxygen faster

than new equilibrium can be reached with the atmosphere. As a result, for relatively

short periods of time, or under sustained conditions of reduced physical mixing (i.e.,

stratification of the water column), dissolved oxygen concentrations can be driven

well below saturation. Dissolved oxygen concentrations can decrease to near zero

(anoxia), especially in deep or stratified bodies of water, or to 20 mg liter1 (supersaturation) during dense algal blooms in surface waters.

The refined tidal-water designated uses were defined largely on the basis of natural

conditions that divide the Bay and its tidal tributaries into different habitat zones. By

devising Bay dissolved oxygen criteria to protect each designated use habitat, natural

conditions that directly influence dissolved oxygen conditions have been largely

accounted for through this process. In addition, by definition, the biologically-based

reference curves derived for the respective designated uses directly incorporate

allowable criteria exceedances due to natural causes in those habitats. The application of the statistical test of significant differences between the curves also addresses

sampling error. Nevertheless, extreme occurrences in the natural processes may

occur and the EPA strongly recommends that managers consider the natural factors

listed below when evaluating criteria attainment.

Temperature and Salinity Effects

The amount of oxygen dissolved in the water changes as a function of temperature,

salinity, atmospheric pressure and biological and chemical processes. The equilibrium (or saturated) concentration of dissolved oxygen in natural waters ranges from

about 6 to 14 mg liter1. Seawater at equilibrium at a given temperature contains

substantially less dissolved oxygen than freshwater. The higher the temperature and

salinity, the lower the equilibrium dissolved oxygen concentration. The saturation

concentration for dissolved oxygen decreases with increasing salinity (about -0.05

mg liter1/psu1) and increasing temperature (about -0.2 mg liter1/

oC).

An analysis of the degree of saturation given existing temperature and salinity conditions within a designated use habitat can indicate whether these natural conditions

will or are preventing criteria attainment. A spreadsheet analysis tool for conducting

such analyses is described below and available on the Chesapeake Bay Program’s

web site at http://www.chesapeakebay.net/tools.

High or Low River Flow Events

Because of its morphology and estuarine circulation, the Chesapeake Bay and some

of its tidal tributaries have a natural tendency to produce reduced dissolved oxygen

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conditions, particularly in deeper waters. The Chesapeake Bay’s highly productive

shallow waters, coupled with its tendency to retain, recycle and regenerate nutrients

delivered from the atmosphere and surrounding watershed, create a nutrient-rich

environment. The mainstem Chesapeake Bay and the major tidal rivers flowing off

of shallower, broad shoal waters, along with the significant influx of freshwater

flows, produce a stratified water column that prevents the water at the bottom from

mixing with more highly oxygenated surface waters. The combination of nutrient

retention and recycling and water-column stratification leads to severe reductions in

dissolved oxygen concentrations, usually from June to September. 

The timing and extent of hypoxic and anoxic water conditions vary from year to year

because of regional weather patterns, the timing and magnitude of freshwater river

flows, the flow of nutrients and sediments into tidal waters and the corresponding

springtime phytoplankton bloom. The actual freshwater flow is the natural condition

that should be considered in determining attainment. It is important to remember that

under the low-flow conditions between 1950 and 1965, there was far less hypoxia in

the mainstream Chesapeake Bay than there has been in the comparable low-flow

years of the late-1980s to the present. Likewise, historical high-flow years produced

less hypoxia and anoxia than current high-flow years (Hagy 2002). The impact from

extremely high or low river flows can be evaluated by accounting for variations in

the stratification of the water column. Basing the determination of the boundaries

between the open-water, deep-water and deep-channel designated uses on sampling

event calculations of the upper and lower pycnocline depths is the most straightforward means of addressing the effects of river flow on dissolved oxygen criteria

attainment.

The data required to calculate sampling event-based pycnocline boundary depths can

be found on the Chesapeake Bay Program’s web site at http://www.chesapeakebay.

net/data. Analysts are urged to use the Chesapeake Bay Water Quality Monitoring

Program’s protocol for calculating the upper and lower boundaries of the pycnocline

(found at http://www.chesapeakebay.net/tools.htm), as this protocol was used to set

the designated use boundaries. (Also see Appendix J in U.S. EPA 2003.) Extensive

data on river flow can be found on the U.S. Geological Survey’s Chesapeake Bay

web site at http://chesapeake.usgs.gov.

Upwelling of Hypoxic Water

Nearshore, shallow waters in the Chesapeake Bay periodically experience episodes

of low- to no-dissolved-oxygen conditions that result in part from intrusions of

bottom water forced onto the shallows by sustained winds. Such seiching events are

natural, but a large percentage of the low dissolved oxygen that intrudes into these

shallow habitats is not due to natural causes. Therefore, attaining the deep-water and

deep-channel dissolved oxygen criteria will greatly reduce or even prevent the influx

of oxygen-depleted bottom waters into the shallows.

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These pycnocline seiche events often take place over time scales that are missed by

the monitoring program’s sampling frequency. When they have occurred during a

sampling cruise, the seiching events result in a clear tilting of the pycnocline. Such

events often are triggered by sustained winds in a single direction over a period of

several days. To verify that observed tilting of the pycnocline and the resulting

excursion of less than 5 mg liter1 waters into shallow- and open-water designated use

habitats were due to natural seiching events, it is recommended that offshore salinity

with depth profiles and the wind direction and speed data be analyzed.

Extensive salinity with depth profile data are available on the Chesapeake Bay

Program’s web site at http://www.chesapeakebay.net/data. For the Chesapeake Bay’s

tidal waters, the best sources of information on continuous wind direction and speed

are the Patuxent Naval Air Station, Baltimore-Washington International Airport and

Norfolk International Airport1. Data from these wind monitoring stations can be

accessed through the NOAA National Climatic Data Center at http://ww.ncdc.

noaa.gov.

Natural Diel Fluctuations

Diel cycles of low dissolved oxygen conditions often occur in nonstratified shallow

waters where nightly water-column respiration temporarily depletes dissolved

oxygen levels. The lowest dissolved oxygen readings, generally observed in the early

morning hours from 0.5 to 2 hours after sunrise, are frequently missed by typical

daytime shipboard water quality monitoring, where sampling usually starts in the

morning and continues into the late afternoon. These diel fluctuations are the result

of natural processes such as daily temperature cycles and photoperiod cycles, but

anthropogenic stresses further exaggerate the fluctuations.

The Chesapeake Bay dissolved oxygen criteria were derived to protect aquatic

animals in the defined designated uses during the applicable time frames, regardless

of time of day. It should be noted that daytime measurements of dissolved oxygen

may not fully reflect actual attainment of the criteria over the 24-hour cycle.

To achieve the most protective degree of criteria attainment, the oxygen dynamics of

a particular water body should be characterized using oxygen meters that monitor

semicontinuously. If diel fluctuations in oxygen conditions are found to exist, two

further steps should be taken. The level of oxygen saturation should be analyzed to

confirm that the criteria meet the given natural temperature and salinity conditions.

Users also should build in a determination of diurnal minimum concentrations

through translation or correction of fixed stations using semicontinuous buoy data.

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1 A time-series of hour/wind direction and velocity for 1985-1994 for each of these three stations was

developed for use in the Chesapeake Bay water quality model. Wind data was adjusted to account for

over-water conditions by multiplying the east-west component by a factor of 1.0, 1.43 and 1.25 for

BWI, Patuxent and Norfolk, respectively. Likewise, the north-south component was multiplied by

factors of 1.50, 2.05 and 1.25, respectively. 
The Maryland Department of Natural Resources (MD DNR) is developing a method

to temporally standardize dissolved oxygen measurements to a diurnal minimum.

Averaged spring and summer data from MD DNR’s continuous monitors indicate

that dissolved oxygen minima are reached at approximately 6:30 a.m., while

dissolved oxygen maxima are achieved at 3:30 p.m. These diurnal fluctuations in

dissolved oxygen produce increasing values during water quality mapping cruises,

where thousands of point samples are collected throughout a tributary over the

course of several hours. In order to produce realistic interpolated surfaces of the

spatially intensive monitoring data, the ‘time of day’ artifact must be removed from

the dissolved oxygen data. MD DNR has chosen to standardize data to the dissolved

oxygen minimum time of 6:30 a.m. to represent the worst conditions that living

resources might face in the tributary, even though this methodology could just as

easily be applied to other times of the day.

The first step in temporal standardization is to obtain a 15-minute interval average of

continuous monitoring data during a two-week period that encompasses a water

quality mapping cruise. The two-week average is somewhat arbitrary, but helps to

filter out small-scale noise in the dissolved oxygen signal. In MD DNR’s case, the

two-week period will be reevaluated in the coming months with additional, concurrent continuous and spatial data collected in 2002. A third-order polynomial is fit to

the two-week dissolved oxygen average from 5:30 a.m. (one hour before dissolved

oxygen minimum) to one hour after the completion of the water quality mapping

cruise of interest. The third-order polynomial model is used to back-calculate each

water quality mapping sample to its theoretical 6:30 a.m. value. The standardized

data is then put into geostatistical interpolation models to produce a dissolved

oxygen minimum map.

Methods to incorporate multiple monitors into the standardization process should be

developed. Also, the effect of chlorophyll a concentrations on dissolved oxygen

concentrations should be studied and possibly included in the correction.

Release of Organic Materials from Tidal Wetlands

Tidal wetlands are a valuable component of estuarine systems. They have been

shown as net sinks for sediments (Neubauer et al. 2001) and in most cases also serve

to remove nutrients from overlying water (Anderson et al. 1997). High rates of

organic production, accompanied by high rates of respiration (Neubauer et al. 2000),

can significantly reduce dissolved oxygen and enhance dissolved inorganic carbon

levels both in sediment pore water and overlying water in wetland systems. Another

process that can deplete dissolved oxygen in wetland sediments is nitrification,

which converts ammonium to nitrite and nitrate (Tobias et al. 2001). 

Studies of South Carolina estuaries demonstrate that small tidal salt marsh creeks

have significantly lower dissolved oxygen levels than large tidal creeks (Van Dolah

et al., in press). Cai et al. (1999, 2000) determined that a significant export of high

dissolved inorganic carbon from marshes was responsible for the low dissolved

oxygen concentrations observed in five estuaries in South Carolina and Georgia. In

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a series of studies of the York River estuary, Raymond et al. (2000) showed that the

system is supersaturated with respect to carbon dioxide pressure (pCO2); conservative mixing diagrams demonstrated a mid-estuary source of dissolved organic

carbon, which caused respiration to exceed production in the system. Further studies

by Neubauer and Anderson (2003) showed that the export of dissolved inorganic

carbon from tidal freshwater and saltwater marshes could account for approximately

47 percent of the excess dissolved inorganic carbon observed by Raymond et al.

(2000) in the York River estuary.

These effects need to be considered in cases where there is a large wetland-to-water

ratio or high residence times of water in extensive nearby wetlands. The Mattaponi

and Pamunkey rivers, two large tidal tributaries to the York River in Virginia, are the

two best examples of such systems in the Chesapeake Bay region. Computer

simulation modeling may be used to help quantify the impact on dissolved oxygen

criteria attainment.

NATURAL REDUCTIONS IN WATER CLARITY LEVELS

The shallow-water bay grasses designated use excludes those habitats where natural

physical factors (e.g., wave action) will prevent underwater bay grasses from ever

growing. Other natural conditions found in potential and current underwater bay

grass habitats (e.g., resuspension) are addressed using a comparison of ambient data

with a biologically-based reference curve. This reference curve defines the water

clarity criteria exceedances through time and space that can occur without impairing

the underwater bay grass community.

High Flow Events

High river flows resulting from major storms will carry elevated loads of suspended

solids from the upper watersheds and lead to reduced water clarity levels in the

midchannel and shallow-water habitats. According to recent U.S. Geological Survey

studies, most of the sediment that has been delivered to free-flowing stream corridors occurred during land clearance in the 1800s. Much of the sediment mobilized

from stream banks and adjacent flood plains and delivered to the tidal rivers and

mainstem Chesapeake Bay may be these ‘legacy’ sediments. The U.S. Geological

Survey is conducting research to determine the amount of sediment that is caused by

recent erosion from land sources versus the sediment that is eroded from within the

stream corridors themselves. The latest findings and extensive data on river flows

can be found on the U.S. Geological Survey’s Chesapeake Bay web site at

http://chesapeake.usgs.gov.

The influence of high flow events is largely accounted for through the derivation

(and application) of the biologically-based water clarity criteria reference curves.

These reference curves were developed based on almost two decades’ worth of

underwater bay grass distributions and water quality data. The mid-1980s to early

2000s data record contains the full array of long-term drought to extreme storm

events (e.g., hurricanes) to sustained, very wet hydrological conditions.

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Wind-Driven Events

Sustained high winds can cause shallow-water sediments to become resuspended

and thus lead to reduced water clarity levels. The U.S. Geological Survey is identifying areas where poor water-clarity conditions are likely to exist due to wind-driven

events. The latest research findings for management application can be found on the

U.S. Geological Survey’s Chesapeake Bay web site at http://chesapeake.usgs.gov.

The biologically-based reference curves should account for allowable criteria

exceedances due to such short-term wind-driven events.

Estuarine Turbidity Maximum Zones

The area in the Bay’s larger tidal tributaries and the upper Bay mainstem where the

warmer, lighter freshwater flows first mix with saltier, denser water flowing

upstream (originally from the coastal Atlantic Ocean) is called the zone of maximum

turbidity, or estuarine turbidity maximum zone (Lin and Kuo 2001; Sanford et al.

2001). The intersection of these two water masses causes nutrients and sediment to

be naturally mixed and continually resuspended. The general locations of these

zones are illustrated in Figure VII-1, which was mapped using long-term salinity and

total suspended solids records over the past 20 years. The actual location varies from

year to year, depending on the timing and volume of freshwater flows.

The natural effect of the estuarine turbidity maximum zone on water clarity in shallow

habitats has been directly factored into the selection of the Chesapeake Bay water

clarity criteria application depths (see U.S. EPA 2003 for more details). The historical

(1930s to early 1970s) and more recent (1978–2001) record of bay grasses distributions included the effects of the estuarine turbidity maximum zones located in the

tidal tributaries and the mainstem Chesapeake Bay. The shallow-water bay grass

designated use depth boundaries for Chesapeake Bay Program segments, within

which the estuarine turbidity maximum zones are located, generally have lower water

clarity application depths, reflecting the fact that total suspended solids concentrations would be naturally elevated leading to less water clarity (U.S. EPA 2003). 

Natural Water Color

Several tidal tributaries throughout the Chesapeake Bay drain extensive tidal,

wetland-dominated watersheds. The organic materials from those areas tend to color

or stain the water naturally, which reduces water clarity. A background level of water

color was factored into the scientific basis for the Chesapeake Bay water-clarity

criteria and the supporting diagnostic tools (see Batiuk et al. 2000 and Gallegos 2001

for details). However, in tidal-fresh habitats along the lower Eastern Shore where

water color plays a significant role in reducing water clarity, the habitats were

considered underwater bay grass no-growth zones. Since no shallow-water bay grass

designated use applies in these habitats, the water clarity criteria do not apply (see

U.S. EPA 2003 for details).

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Turbidity

< Median (50%)

> Median (50%)

> 90%,salinity < 7 ppt

 

_ _

Figure VII-1. The estuarine turbidity maximum zone is generally found at the interface

of fresh and salt water. It is illustrated here as the region within each river basin where

mean concentration of total suspended solids is at or above the 90th percentile of concentrations measured within that basin in the last decade, i.e., between 1991–2000. 

The regions of lesser turbidity are divided into two categories: those with mean concentrations less than the median (50th percentile) or greater than the median, but less

than the 90th percentile. ‘Hot spots’ of relatively high turbidity in downstream mesoand polyhaline areas are not shown. ‘Major’ basins are the mainstem Bay (including 

Mobjack Bay) and the Chester, Choptank, Nanticoke, Pocomoke, Patuxent, Potomac,

Rappahannock, York and James rivers. In some of these river basins, the turbidity

maximum is too far upriver to be clearly displayed on this map.
NATURAL ELEVATED CHLOROPHYLL A CONCENTRATIONS

Many of the factors influencing chlorophyll a concentrations are related to physical

processes affecting the residence time of a water mass in a tidal river, creek or

embayment, and light penetration due to channel morphology or physical mixing. In

regions or specific tidal-water habitats where these listed physical processes lead to

chlorophyll a-related impairments, states should derive local scale numerical chlorophyll a criteria directly addressing these natural conditions.

High Residence Time and Reduced Flushing Rates

In many small tidal rivers, the reduced flushing of more confined open-water habitats

often leads to elevated chlorophyll a concentrations, given that phytoplankton populations are exposed to nutrient-enriched conditions for longer periods. Nutrient loadings

that would not otherwise lead to increased chlorophyll a concentrations in well-flushed

tidal open-water habitats generate bloom conditions in these smaller systems.

There has been relatively little analysis of the appropriateness and attainability of

specific chlorophyll a values in poorly flushed tidal systems. For example, most of the

analyses performed in support of generating chlorophyll a target concentrations have

focused on well-flushed open-water systems (see Chapter V). Natural elevations of

chlorophyll a should be considered when setting designated use boundaries and when

setting specific numeric targets and criteria for addressing regional and local algalrelated impairments.

Through the development and application of biologically-based reference curves, the

numerical chlorophyll a criteria attainment methodology can factor in the spatial

extent of criteria attainment or nonattainment. This allows for limited spatial extent

with elevated chlorophyll a concentrations and larger spatial areas with lower, yet

nonattaining, chlorophyll a concentrations. If a Chesapeake Bay Program segment

contains a very high portion of tidal habitats with high residence times, more

detailed analyses of the relative contribution of naturally reduced flushing rates

versus excessive anthropogenic nutrient loadings should be undertaken.

Channel Morphology

Tidal rivers and creeks with shallow and wide channels (versus narrower and deep

channels) will tend to have higher chlorophyll a concentrations, given the greater

volume of the photic zone relative to the total channel volume. In addition, the

shallow and wide channels tend to be less well-flushed, allowing greater accumulation of phytoplankton and chlorophyll a. 

Natural Algal Blooms Independent of Nutrient Conditions

Although anthropogenic nutrient loading is a principal factor in the overall primary

productivity of the Chesapeake Bay system, its relationship to blooms of specific

taxa is not well understood. Such blooms have been observed to occur in the absence

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of elevated nutrient conditions as a result of a complex set of physical, chemical and

biological stimuli. Species composition data from the Chesapeake Bay Phytoplankton Monitoring Program should be consulted to determine if the observed algal

bloom conditions are due principally to species that fall within this category. These

phytoplankton monitoring data can be accessed through the Chesapeake Bay

Program website at http://www.chesapeakebay.net/data.

DIAGNOSING CAUSES OF CRITERIA NONATTAINMENT

DISSOLVED OXYGEN CRITERIA

Percent Saturation

An analysis of the degree of saturation given existing temperature and salinity conditions within a designated use habitat can be performed by applying the following

equation. For temperature in degrees Celsius and salinity in mg liter-1:

dissolved oxygen saturation = 14.6244 – 0.367134(TempoC) + 0.0044972 

(TempoC)2 – 0.0966(salinity) + 0.00205 (salinity) (TempoC) + 0.0002739

(salinity)2. 

A spreadsheet version of this diagnostic analysis tool is available on the Chesapeake

Bay Program’s web site at http://www.chesapeakebay.net/tools.htm.

Chesapeake Bay Water Quality Model

As explained in Chapter VI, the Chesapeake Bay water quality model is linked to the

Chesapeake Bay hydrodynamic model and uses complex nonlinear equations

describing 26 state variables relevant to the simulation of dissolved oxygen, chlorophyll a and water clarity. Dissolved oxygen is simulated as the mass balance

calculation of reaeration at the surface; respiration of algae, benthos and underwater

bay grasses; photosynthesis of algae, benthic algae and underwater bay grasses; and

the diagenesis, or decay of organics, by microbial processes in the water column and

bottom sediments. This mass balance calculation is made for each model cell and for

associated bottom sediment cells at each hourly time step. Estimates of dissolved

oxygen from nutrient loads from the watershed and airshed are simulated in the tidal

waters of the 35 major segments of the Chesapeake Bay and its tidal tributaries. This

state-of-the-science modeling tool is available to management agencies and others to

help diagnose the reasons behind nonattainment of the Chesapeake Bay dissolved

oxygen criteria.

For the dissolved oxygen criteria, the daily output of dissolved oxygen concentration

for 10 years (1985–1994) for the 13,000 cells provides a detailed estimate of the

transport and transformation of nutrients and organic matter that ultimately consume

oxygen in the waters of the Chesapeake Bay and its tidal tributaries. Influential

aspects, such as the limiting nutrient, seasonal changes in dissolved oxygen, changes

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in the nutrient flux of bottom sediments that change with bottom-water oxygen

levels, and other temporal and spatial aspects of dissolved oxygen concentrations

and dynamics, can be diagnosed by evaluating water quality model output to gain

insights into the reasons behind nonattainment of the dissolved oxygen criteria.

WATER CLARITY CRITERIA

In Chesapeake Bay Submerged Aquatic Vegetation Water Quality and Habitat-Based

Requirements and Restoration Targets: A Second Technical Synthesis, a set of diagnostic tools were developed not only to better interpret the relative degree of

achievement of the Bay water clarity criteria, but also to understand the relative

contributions of different water quality parameters to overall light attenuation

(Batiuk et al. 2000). Two management-oriented diagnostic tools have been developed. The water-column diagnostic tool quantifies the relative contributions to total

light attenuation in the water column that is attributable to light absorption and scattering by total suspended solids and chlorophyll a. The leaf surface attenuation

diagnostic tool further quantifies the light attenuation at the leaf surface attributable

to epiphytes and total suspended solids settled out on the leaf surface. Both diagnostic tools are available as spreadsheet-based application tools and can be accessed

through the Chesapeake Bay Program’s web site at http://www.chesapeakebay.net

/tools.htm.

Water-Column Light Attenuation Diagnostic Tool

Water-column attenuation of light measured by the light attenuation coefficient, Kd,

can be divided into contributions from four sources: water, dissolved organic matter,

chlorophyll a and total suspended solids. The basic relationships can be expressed in

a series of simple equations, which were combined to produce the equation for the

water-column diagnostic tool (Gallegos 2001). The resulting equation calculates

linear combinations of chlorophyll a and total suspended solids concentrations that

just meet the percent light-through-water (PLW) criteria value for a particular depth

at any site or season in the Chesapeake Bay and its tidal tributaries. This diagnostic

tool can also be used to consider various management options for improving water

quality conditions when the water clarity criteria are not currently met.

Generation of Management Options. The water-column diagnostic tool

spreadsheet program calculates median water quality concentrations and evaluates

them in relation to PLW criteria for growth to 0.5-, 1- and 2-meter restoration depths.

Provisions are included for specifying a value for PLW criteria appropriate for

mesohaline and polyhaline regions (22 percent) or for tidal-fresh and oligohaline

areas (13 percent). When the observed median chlorophyll a and total suspended

solids concentrations do not meet the PLW criteria, up to four target chlorophyll a

and total suspended solids concentrations that do meet the PLW criteria are calculated based on four different management options (Figure VII-2). Under some

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Figure VII-2. Illustration of management options for determining target concentrations

of chlorophyll a and total suspended solids. It illustrates the use of the diagnostic tool to

calculate target growing-season median concentrations of total suspended solids (TSS)

and chlorophyll a for restoration of underwater bay grasses to a given depth. Target

concentrations are calculated as the intersection of the percent light-through-water

criteria line, with a line describing the reduction of median chlorophyll a and TSS concentrations calculated by one of four strategies: (A) projection to the origin (i.e., chlorophyll

a=0, TSS=0); (B) normal projection, i.e., perpendicular to the percent light-through-water

requirement; (C) reduction in total suspended solids only; and (D) reduction in chlorophyll

a only. A strategy is not available (N/A) whenever the projection would result in a ‘negative concentration.’ In (D), reduction in chlorophyll a also reduces TSS due to the dry

weight of chlorophyll a, and therefore moves the median parallel to the line (long dashes)

for ChlVS, which describes the minimum contribution of chlorophyll a to TSS.
conditions, some of the management options are not available because a ‘negative’

chlorophyll a or total suspended solids concentration would be calculated.

Option 1 is based on projections from existing median conditions to the origin

(Figure VII-2a). This option calculates target chlorophyll a and total suspended

solids concentrations as the intersection of the PLW criteria line with the line

connecting the existing median concentration and the origin, i.e., chlorophyll a = 0,

TSS = 0. Option 1 always results in positive concentrations of both chlorophyll a and

total suspended solids.

Option 2 is based on normal projections (Figure VII-2b). It calculates target chlorophyll a and total suspended solids concentrations as the projection from existing

median conditions perpendicular to the PLW criteria. Geometrically, Option 2

requires the least overall reductions in chlorophyll a and total suspended solids

concentrations. In practice, target chlorophyll a and total suspended solids concentrations for the normal projection, when permissible (i.e., no negative concentrations

are calculated), are frequently very similar to those calculated in Option 1 using

projection to the origin.

Option 3 is based on a total suspended solids reduction only (Figure VII-2c). This

option calculates target chlorophyll a and total suspended solids concentrations,

assuming the target can be met only by reducing the concentration of total suspended

solids. Option 3 is not available whenever the median chlorophyll a exceeds the total

suspended solids = 0 intercept. When a system is nutrient-saturated and light-limited,

a reduction of total suspended solids alone poses the risk of relieving light limitation

and promoting further phytoplankton growth. Such a tendency is indicated on the

diagnostic tool plot whenever data points tend to align parallel to the PLW criteria

lines (Figure VII-2c).

Option 4 is based on a chlorophyll a reduction only. This option calculates target

chlorophyll a and total suspended solids concentrations, assuming that the target can

be met only by reducing the concentration of chlorophyll a (Figure VII-2d). Due to

the suspended solids removed by reduction of phytoplankton and associated carbon,

i.e., ChlV, the target total suspended solids concentration reported for Option 4 is

actually lower than the existing median. Option 4 is not available whenever the

median total suspended solids concentration exceeds the chlorophyll a = 0 intercept

of the PLW criteria line.

The precision of the calculations implies a degree of control over water quality

conditions that clearly is not always attainable. Nevertheless, reporting of four

potential targets provides managers with an overall view of the magnitude of the

necessary reductions and some of the available tradeoffs. Furthermore, the spreadsheet reports the frequency with which the PLW criteria for each restoration depth

are not achieved by the individual measurements.

Evaluating Management Options. Option 1 will likely be the most useful for

generating target concentrations because it always results in the calculation of positive concentrations. Also, most efforts to control loadings involve a reduction of total

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chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment
runoff, which reduces both suspended solids and nutrients. Under certain conditions

managers may choose to apply Option 3, when data plots indicate that attenuation is

dominated by flood-borne or resuspended sediments (Figure VII-3a). Similarly,

Option 4 may be useful when diagnostic plots indicate that light attenuation is dominated by algal blooms (Figure VII-3b). For details on how best to evaluate the four

possible management options, refer to Chesapeake Bay Submerged Aquatic Vegetation Water Quality and Habitat-Based Requirements and Restoration Targets: A

Second Technical Synthesis (Batiuk et al. 2000, pp. 47-49).

Leaf Surface Light Attenuation Diagnostic Tool

Building from the diagnosis and quantification of water-column contributions to

attenuation of light, a second diagnostic tool focuses on how changes in water

quality variables alter the light available to underwater plant leaves and considers

effects of light attenuation resulting from substances both in the overlying water

column (phytoplankton, suspended particles and dissolved organics) and attached to

underwater bay grass leaves (epiphytic algae, organic detritus and inorganic particles). A simple model was developed to calculate photosynthetically available

radiation (PAR) at the leaf surface for plants growing at a given restoration depth (Z)

under specific water quality conditions. The computed value for PAR at the plant

leaves is compared to the applicable Bay water clarity criteria.

The overall objective is to apply this model using water quality monitoring data to

estimate growing season mean light levels at bay grass leaves for a particular site or

geographic region. The calculated light levels at bay grass leaves are then compared

to the applicable light-at-the-leaf water clarity requirement to assess whether water

quality conditions are suitable to support the survival and growth of underwater bay

grasses. The relative contributions of water-column versus epiphytic substances in

attenuating incident light to underwater bay grass leaves also are computed. 

The scientific basis of this model is described in detail in Batiuk et al. (2000) and

Kemp et al. (in review).

Generating Diagnostics. To compute median PAR at the bay grass leaf surface,

the diagnostic spreadsheet model requires underwater bay grass growing season

medians for four water quality variables: 1) dissolved inorganic nitrogen (nitrate +

nitrite + ammonia), or DIN; 2) dissolved inorganic phosphorus (primarily phosphate),

or DIP; 3) total suspended solids (TSS); and 4) diffuse downwelling PAR attenuation

coefficient (Kd). Values for Kd are either obtained from direct measurements of

decrease in PAR with water depth using a cosine-corrected sensor, or calculated from

observations on the depth at which a Secchi disk disappears (see Chapter III in Batiuk

et al. 2000 for the details on the recommended Secchi depth/Kd conversion of Kd =

1.45/Secchi depth). The restoration depth is defined by the Chesapeake Bay Program

segment-specific shallow-water designated use outer depth boundary (U.S. EPA

2003). Figure VII-4 and Table VII-1 lays out the steps for running the spreadsheet

model, the data required, and the scientific basis for the calculation.

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chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment

Figure VII-3. Application of the water column light attenuation diagnostic tool to two mainstem Chesapeake

Bay stations and one tidal tributary station, which demonstrates three primary modes of variation in the data:

(A) variation in diffuse attenuation coefficients is dominated by (flow-related) changes in concentrations of

total suspended solids (TSS) (upper Chesapeake Bay station, CB2.2); (B) variations in attenuation coefficients is

dominated by changes in chlorophyll a concentration (Baltimore Harbor, MWT 5.1); and (C) maximum

chlorophyll a concentration varies inversely with TSS, indicating light-limited phytoplankton (lower middle

Chesapeake Bay, CB5.2). Plots show individual measurements (points) and growing season median (asterisk) in

relation to the percent light-through-water (PLW) criteria for restoration to depths of 0.5m (short dashes), 1m

(solid line) and 2m (dotted line); and PLW calculated by equations IV-1 and J-1 (see Chapter IV and Appendix J).

Note the change in scale. Approximate minimum contribution of chlorophyll a to TSS (ChIVS) is calculated by

Equation IV-11 (long dashes) in Batiuk et al. 2000. The data is from the Chesapeake Bay Water Quality

Monitoring Program, April through October, 1986-1996. 

Sources: Batiuk et al. 2000; Gallegos 2001.
Evaluating Diagnostic Outputs. To examine the components of light attenuation, as determined by the spreadsheet percent light-at-the-leaf (PLL) calculator,

several fields in addition to PLL are shown. This permits insight into the contribution to light total attenuation from the water column, leaf surface epiphytes and leaf

surface total suspended solids (TSS). The additional fields are:

• PLW—percent-light-through-water. Comparing PLL to PLW gives an indication of the contribution of leaf surface light attenuation to the total

attenuation.

• PLLnoTSS—PLL calculated without TSS light attenuation. Indicates the

relative importance of epiphytes and TSS.

• %EpiAtten. Refers to the percentage of the light attenuation on the leaf

surface that is due to the growth of epiphytes.

• %LeafTSSAtten. Refers to the percentage of the light attenuation on the leaf

surface that is due to deposited TSS.

• Requirement. Indicates whether the calculated PLL meets or fails the PLL

diagnostic minimum light requirement. Assessment takes into account the

salinity regime of the station.

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chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment

 

Percent Light-through-Water (PLW) Percent Light-at the-Leaf (PLL)

 Inputs

Kd measured directly

 or

Kd calculated from 

 Secchi depth

 Inputs

Kd

Total suspended solids

Dissolved inorganic

 Dissolved inorganic phosphorus

nitrogen

Evaluation

*Ke = Epiphyte attenuation

*Be = Epiphyt e biomass

Evaluation

Water

 Color

Total Suspended

Solids

Algae

PLW

Epiphyte

Attenuation

Leaf Surface

PLL

100% Ambient Light of Water Surface

PLW=100exp(-K d Z)

 Calculation Calculation

PLL=100[exp(-Kd e Z)][exp(-K B )] e

Calculated PLW vs. PLW criteria PLL vs. PLL Diagnostic Requirement 

Figure VII-4. Illustration of percent light-at-the-leaf (PLL) and percent light-through-water (PLW) calculation

comparisons for underwater bay grasses in the Chesapeake Bay.

Source: Batiuk et al. 2000
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chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment

Table VII-1. Summary of the approach to estimate photosynthetically available radiation 

at the leaf surface of underwater bay grasses using water quality data routinely monitored

in the Chesapeake Bay.

Step in Model Calculation Input Source of Model

Functional Relation Data Relationship Units

1) Decide limiting nutrient DIN, DIP Fisher et al. 1992 μM

DIN/DIP > 16 , use DIP

DIN/DIP ≤ 16 , use DIN

2) Derive general equation DIN, DIP Numerical model Be, gCgC-1

to calculate epiphyte biomass (Madden and Kemp 1996) DIN, μM

Be = (Be)m [1 + 208 (DIN-KN(OD))]-1 KN(OD), none

• (Be)m = maximum Be value

• KN(OD) = characteristic coeff.

3) Calculate PAR effect on KN(OD) Kd, Z Numerical model Kd, m-1

and (Be)m (Madden and Kemp 1996) Z, m

(Be)m = 2.2 - [0.251 (OD1.23)]

• OD = Optical Depth = (Kd)(Z)

KN(OD) = 2.32 (1 - 0.031 OD1.42)

4) Calculate epiphyte dry weight TSS Regression from TSS, mg l-1

Bde = 0.107 TSS + 0.832 Be Be experimental data Be, mg chl gdw-1

(e.g., Staver 1984) Bde, gdw gdw-1

5) Calculate epiphyte biomass- Be, Regression from Be, μg chl cm-2

specific PAR attenuation coeff. Bde experimental and Bde , mg dw cm-2

Ke = 0.07 + 0.32 (Be/Bde)-0.88 field data Ke, cm2 μg chl-1

6) Calculate PAR at SAV leaves (Ize) DIN, DIP, Combining steps 1–5 DIN, μM

Ize/Io = [exp(-KdZ)][exp(-KeBe)] Kd, TSS, Z (from above) DIP, μM

TSS, mg l-1

Kd, m-1

7) Compare SAV leaf PAR with Ize/Io See Chapter VII %

Light-at-the-Leaf Requirement in Batiuk et al. 2000

Note that units used for specific variables change at different steps in calculation, but are consistent with

conventions of data and model sources.

Source: Batiuk et al. 2000.
Chesapeake Bay Water Quality Model

Outputs from the Chesapeake Bay water quality model include quantification of the

various components of light attenuation from sediment, algae or color. Further evaluation of the relative contributions of these various components of light attenuation can

provide insights into the reasons behind nonattainment of the water clarity criteria.

CHLOROPHYLL A CRITERIA

Chesapeake Bay Water Quality Model

The Chesapeake Bay community also has access to water-quality models that represent excellent tools for diagnosing the causes for nonattainment of the chlorophyll a

criteria. Time and space aspects of the criteria and the understanding of the fundamental behavior and significant influences on chlorophyll a in the Chesapeake Bay

designated use habitats is based primarily on resource limitation of algae. Resource

limitation on the growth of algae include nitrogen and phosphorus limitation, light

limitation and, for diatoms, limitation of silica. Interactions of the chlorophyll a and

water clarity criteria include algal self-shading and light attenuation due to sediment

or the color imparted to natural waters due to dissolved organic material. Through

the Chesapeake Bay water quality model, the total fate and transformation of algae

based on the Monod structure of temperature corrected algal growth operating on a

hourly time step can be evaluated. Diagnostics of chlorophyll a criteria nonattainment that can be examined through model outputs include nitrogen and phosphorus

limitation, light limitation and, for diatoms, limitation of silica. See the Water Clarity

section above for diagnostics related to factors limiting light.

LITERATURE CITED

Anderson, I. C., C. R. Tobias, B. B. Neikirk and R. L. Wetzel. 1997. Development of a

process-based mass balance model for a Virginia Spartina alterniflora salt marsh: Implications for net DIN flux. Marine Ecology Progress Series 159:13-27.

Batiuk, R. A., P. Bergstrom, M. Kemp, E. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V.

Carter, N. B. Rybicki, J. M. Landwehr, C. Gallegos, L. Karrh, M. Naylor, D. Wilcox, K. A.

Moore, S. Ailstock and M. Teichberg. 2000. Chesapeake Bay Submerged Aquatic Vegetation

Water Quality and Habitat-based Requirements and Restoration Targets: A Second Technical

Synthesis. CBP/TRS 245/00 EPA 903-R-00-014. U. S. EPA Chesapeake Bay Program,

Annapolis, Maryland.

Cai, W. J., W. J. Weibe, Y. Wang and J. E. Sheldon. 2000. Intertidal marsh as a source of

dissolved inorganic carbon and a sink of nitrate in the Satilla River-estuarine complex in the

southeastern U. S. Limnology and Oceanography 45:1743-1752. 

Cai, W. J., L. R. Pomeroy, M. A. Moran and Y. Wang. 1999. Oxygen and carbon dioxide mass

balance for the estuarine-intertidal marsh complex of five rivers in the southeastern U. S.

Limnology and Oceanography 44:639-649. 

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chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment
Fisher, T. R., E. R. Peele, J. W. Ammerman and L. W. Harding, Jr. 1992. Nutrient limitation

of phytoplankton in Chesapeake Bay. Marine Ecology Progress Series 82: 51-63.

Gallegos, C. L. 2001. Calculating optical water quality targets to restore and protect

submersed aquatic vegetation: Overcoming problems in partitioning the diffuse attenuation

coefficient for photosynthetically active radiation. Estuaries 24:381-397.

Hagy, J. D. 2002. Eutrophication, hypoxia and trophic transfer efficiency in Chesapeake Bay.

Ph.D. dissertation, University of Maryland, College Park, Maryland.

Kemp, W. M., R. Batiuk, R. Bartleson, P. Bergstrom, V. Carter, C. L. Gallegos, W. Hunley, L.

Karrh, E. W. Koch, J. M. Landwehr, K. A. Moore, L. Murray, M. Naylor, N. B. Rybicki, C.

Stevenson and D. J. Wilcox. In review. Habitat requirements for submerged aquatic vegetation

in Chesapeake Bay: Water quality, light regime and physical-chemical factors. Estuaries.

Lin, J. and A. Y. Kuo. 2001. Secondary turbidity maximum in a partially mixed microtidal

estuary. Estuaries 24:707-720.

Madden, C. J. and W. M. Kemp. 1996. Ecosystem model of an estuarine submersed plant

community: Calibration and simulation of eutrophication responses. Estuaries 19(2B): 457-

474.

Neubauer, S. C. and Anderson, I. C. 2003. Transport of dissolved inorganic carbon from a

tidal freshwater marsh to the York and Pamunkey river estuary. Limnology and Oceanography 48:299-307.

Neubauer, S. C., I. C. Anderson, J. A. Constantine and S. A. Kuehl. 2001. Sediment deposition and accretion in a mid-Atlantic (U.S.A.) tidal freshwater marsh. Estuarine Coastal and

Shelf Science.

Neubauer, S. C., W. D. Miller and I. C. Anderson, I. C. 2000. Carbon cycling in a tidal freshwater marsh ecosystem: A gas flux study. Marine Ecology Progress Series 199:13-30.

Raymond, P.A., J. E. Bauer and J. J. Cole. 2000. Atmospheric CO2 evasion, dissolved inorganic carbon production and net heterotrophy in the York River estuary. Limnology and

Oceanography 45:1701-1717.

Sanford, L. P., S. E. Suttles and J. P. Halka. 2001. Reconsidering the physics of the Chesapeake Bay estuarine turbidity maximum. Estuaries 24:655-669.

Staver, K. 1984. Responses of epiphytic algae to nitrogen and phosphorus enrichment and

effects on productivity of the host plant, Potamogeton perfoliatus L., in estuarine waters.

Master’s thesis, University of Maryland, College Park, Maryland.

Tobias, C. R., I. C. Anderson, E. A. Canuel and S. A. Macko. 2001. Nitrogen cycling through

a fringing marsh-aquifer ecotone. Marine Ecology Progress Series 210:25-39.

U.S. Environmental Protection Agency. 2003. Technical Support Document for the Identification of Chesapeake Bay Designated Use and Attainability. EPA 903-R-03004. Chesapeake

Bay Program Office, Annapolis, Maryland.

Van Dolah, R. F., D. E. Chestnut, J. D. Jones, P. C. Jutte, G. Reikirk, M. Levinson and W.

McDermott. In press. The Importance of Considering Spatial Attributes in Evaluating Estuarine Habitat Conditions: The South Carolina Experience.

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chapter vii • Diagnostic Procedures for Natural Processes and Criteria Nonattainment
BACKGROUND

Federal water quality standards regulations establish that states must specify appropriate water uses to be achieved and protected. Current designated uses applied to

the waters of the Chesapeake Bay and its tidal tributaries do not fully reflect natural

conditions and are too broad in their definition of ‘use’ to support the adoption of

more habitat-specific aquatic life criteria. Furthermore, they change across jurisdictional borders in the same body of water.

Under the federal water quality standards regulation, states may adopt subcategories

of uses, seasonal uses and may remove uses under certain conditions (including

natural, physical and socio-economic conditions). If a state wishes to remove or

establish a subcategory of a designated use that requires a less stringent water quality

criteria, the state must conduct a use-attainability study. States must also demonstrate that all water uses present on or after November 28, 1975, will always be

protected.

The Chesapeake 2000 agreement and the subsequent six state, District of Columbia

and EPA memoranda of understanding challenged the Bay watershed jurisdictions

to, “by 2010, correct the nutrient- and sediment-related problems in the Chesapeake

Bay and its tidal tributaries sufficiently to remove the Bay and the tidal portions of

its tributaries from the list of impaired waters under the Clean Water Act” (Chesapeake Executive Council 2000; Chesapeake Bay Watershed Partners 2001).

These agreements included commitments to “define the water quality conditions

necessary to protect aquatic living resources” by 2001 and to have the jurisdictions

with tidal waters “use their best efforts to adopt new or revised water quality standards consistent with the defined water quality conditions” by 2003. Against this

backdrop of a renewed commitment to restore Bay water quality (in part through the

adoption of a consistent set of Chesapeake Bay water quality criteria as state standards), it was recommended that the underlying tidal-water designated uses must be

refined to better reflect desired Bay water quality conditions.

▼A-1

appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries

appendixA 

Refined Designated Uses for the

Chesapeake Bay and Tidal Tributaries
In considering the refinement of the tidal-water designated uses, the six Bay watershed states and the District of Columbia should take into account five principal

considerations:

• Habitats used in common by sets of species and during particular life stages

should be delineated as separate designated uses;

• Natural variations in water quality should be accounted for by the designated uses;

• Seasonal uses of different habitats should be factored into the designated uses;

• The Chesapeake Bay criteria for dissolved oxygen, water clarity and chlorophyll a should be tailored to support each designated use; and

• The refined designated uses applied to the Chesapeake Bay and its tidal tributaries will support the federal Clean Water Act and state goals for uses existing

in these water since 1975 and for potential uses not currently met.

The five proposed designated uses were derived to reflect the habitats of an array of

recreationally, commercially and ecologically important species. The supporting prey

communities were given full consideration along with the ‘target species’ in defining

the designated uses. The Chesapeake Bay criteria were based on effects data from a

wide array of species and biological communities to capture the range of sensitivities

of the thousands of aquatic species inhabiting the Chesapeake Bay and tidal tributary

estuarine habitats. As the U.S. Environmental Protection Agency (2003a) documents

extensively, the only species formally listed as threatened or endangered that would

be affected by the Chesapeake Bay criteria was the shortnose sturgeon. Low dissolved

oxygen effects data for shortnose sturgeon were part of the larger scientific data base

used to derive the Chesapeake Bay dissolved oxygen criteria.

This appendix broadly describes the five designated uses and the general boundaries

between the migratory fish spawning and nursery; shallow-water bay grass; openwater fish and shellfish; deep-water seasonal fish and shellfish; and deep-channel

seasonal refuge designated use habitats (Table A-1). Figure 1 in the Executive

Summary illustrates the conceptual framework of the refined tidal-water designated

uses. More detailed descriptions of and documentation on the five designated uses

are published in the Technical Support Document for the Identification of Chesapeake Bay Designated Uses and Attainability (U.S. EPA 2003b).

CHESAPEAKE BAY TIDAL-WATER DESIGNATED USES

The following descriptions of designated uses provide the context for deriving

dissolved oxygen, water clarity and chlorophyll a water quality criteria for the

Chesapeake Bay provided in this Regional Criteria Guidance. Correct application of

water quality criteria depends on the accurate delineation of these designated uses.

For example, each of these designated uses have distinct dissolved oxygen criteria

derived to match the respective level of protection required. 

▼A-2

appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
The watershed states with tidally influenced Chesapeake Bay waters—Maryland,

Virginia, Delaware and the District of Columbia—have the ultimate responsibility

for defining and adopting the designated uses into their state water quality standards.

These uses will be adopted as subcategories of current state tidal-water designated

uses, which are designed to protect aquatic life. The formal process for refining

designated uses will meet the requirements of the Clean Water Act. The adopted

designated uses will protect existing aquatic and human uses of the tidal waters that

have been present since 1975, as well as potential uses. The specific use definitions

and the spatial application of the final designated uses will undergo public review

and the four jurisdictions’ respective regulatory adoption processes prior to final

approval by EPA.

MIGRATORY FISH SPAWNING AND NURSERY DESIGNATED USE

Waters with this designated use support the survival, growth and propagation of

balanced indigenous populations of ecologically, recreationally and commercially

▼A-3

appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries

Table A-1. General descriptions of the five proposed Chesapeake Bay tidal-water designated uses.

Migratory Fish Spawning and Nursery Designated Use: Aims to protect migratory finfish

during the late winter/spring spawning and nursery season in tidal freshwater to low-salinity

habitats. This habitat zone is primarily found in the upper reaches of many Bay tidal rivers and

creeks and the upper mainstem Chesapeake Bay and will benefit several species including

striped bass, perch, shad, herring and sturgeon.

Shallow-Water Designated Use: Designed to protect underwater bay grasses and the many

fish and crab species that depend on the shallow-water habitat provided by grass beds.

Open-Water Fish and Shellfish Designated Use: Designed to protect water quality in the

surface water habitats within tidal creeks, rivers, embayments and the mainstem Chesapeake

Bay year-round. This use aims to protect diverse populations of sportfish, including striped

bass, bluefish, mackerel and seatrout, bait fish such as menhaden and silversides, as well as the

listed shortnose sturgeon.

Deep-Water Seasonal Fish and Shellfish Designated Use: Aims to protect living resources

inhabiting the deeper transitional water column and bottom habitats between the well-mixed

surface waters and the very deep channels during the summer months. This use protects many

bottom-feeding fish, crabs and oysters, as well as other important species, including the bay

anchovy.

Deep-Channel Seasonal Refuge Designated Use: Designed to protect bottom sedimentdwelling worms and small clams that act as food for bottom-feeding fish and crabs in the very

deep channel in summer. The deep-channel designated use recognizes that low dissolved

oxygen conditions prevail in the deepest portions of this habitat zone and will naturally have

very low to no oxygen during the summer.
important anadromous, semi-anadromous and tidal-fresh resident fish species inhabiting spawning and nursery grounds from February 1 through May 31.

Chesapeake Bay tidal waters support spawning areas and juvenile nurseries for a

host of anadromous and semi-anadromous fish, important not only to Chesapeake

Bay fishery populations, but also to those of the entire East Coast, such as striped

bass. The eggs, larvae and early juveniles of anadromous and semi-anadromous

species often have more sensitive habitat quality requirements than other species and

life stages (Funderburk et al. 1991; Jordan et al. 1992). Thus, the combined migratory spawning and nursery habitats were delineated as a refined tidal-water

designated use for the Chesapeake Bay and its tidal tributaries.

Designated Use Boundary Delineation

The boundaries of the migratory fish spawning and nursery designated use are

broadly delineated from the upriver extent of tidally influenced waters to the downriver and lower Bay spawning and nursery habitats that have been determined

through a composite of all targeted anadromous and semi-anadromous fish species’

spawning and nursery habitats. Free-flowing streams and rivers, where several of the

target species (such as shad and river herring) migrate for spawning, are protected

through other existing state water quality standards.

From February 1 through May 31, the migratory fish spawning and nursery designated use coincides with and, therefore, encompasses portions of the shallow-water

bay grass and open- water fish and shellfish designated use habitats. Therefore, the

horizontal and vertical delineations for the migratory fish spawning and nursery

designated use are the same as those of the open-water fish and shellfish designated

uses. For those areas designated for migratory spawning and nursery uses, the designated use extends horizontally from the intertidal zone (mean low water) across the

body of water to the adjacent intertidal zone, and down through the water column to

the bottom sediment-water interface.

SHALLOW-WATER BAY GRASS DESIGNATED USE

Waters with this designated use support the survival, growth and propagation of

rooted, underwater bay grasses necessary for the propagation and growth of

balanced, indigenous populations of ecologically, recreationally and commercially

important fish and shellfish inhabiting vegetated shallow-water habitats.

Designated Use Rationale

The shallow-water bay grass designated use protects a wide variety of species, such

as largemouth bass and pickerel, which inhabit vegetated tidal-fresh and low-salinity

habitats; juvenile speckled sea trout in vegetated higher salinity areas; and blue crabs

that inhabit vegetated shallow-water habitats covering the full range of salinities

encountered in the Chesapeake Bay and its tidal tributaries. Underwater bay grasses,

▼A-4

appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
the critical community that the designated use protects, provide the shelter and food

that make shallow-water habitats so unique and integral to the productivity of the

Chesapeake Bay ecosystem. Many Chesapeake Bay species depend on vegetated

shallow-water habitats at some point during their life cycle (Funderburk et al. 1991).

Given the unique nature of this habitat and its critical importance to the Chesapeake

Bay ecosystem, shallow waters were delineated as a refined tidal-water designated

use for the Chesapeake Bay and its tidal tributaries.

The shallow-water bay grass designated use is intended specifically to delineate the

habitats where the water clarity criteria would apply. The open-water fish and shellfish

designated use and the accompanying dissolved oxygen criteria will fully protect the

biological communities inhabiting shallow-water habitats. The open-water fish and

shellfish designated use extends into the intertidal zone and protects shallow-water

organisms that do not depend on bay grasses. The seasonal shallow-water bay grass

designated use, similar to the migratory fish spawning and nursery use, actually coincides with the year-round open-water designated use and provides specific protection

for underwater bay grasses through the application of water clarity criteria.

Designated Use Boundary Delineation

The shallow-water bay grass designated use covers tidally influenced waters from

the intertidal zone to a Chesapeake Bay Program segment-specific depth contour

from 0.5 to 2 meters. The shallow-water designated use applies during the bay grass

growing season: April 1 through October 31 for tidal-fresh, oligohaline and mesohaline segments, and March 1 through May 31 and September 1 through November 30

for polyhaline segments.

OPEN-WATER FISH AND SHELLFISH DESIGNATED USE

Waters with this designated use support the survival, growth and propagation of

balanced, indigenous populations of ecologically, recreationally and commercially

important fish and shellfish species inhabiting open-water habitats.

Designated Use Rationale

The natural temperature and salinity stratification of open waters influence dissolved

oxygen concentrations, and thus the distribution of Chesapeake Bay species. Waters

located above the pycnocline with higher oxygen levels support a different community of species than deeper waters from late spring to early fall. Several well-known

species that inhabit these open waters are menhaden, striped bass and bluefish. Their

habitat requirements and prey needs differ from those of species and communities

inhabiting deeper water habitats during the summer months.

Designated Use Boundary Delineation

From June 1 through September 30, the open-water designated use includes tidally

influenced waters extending horizontally from the shoreline measured at mean low

▼A-5

appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
water, to the adjacent shoreline, and extending through the water column to the

bottom. If the presence of a pycnocline prevents oxygen replenishment, the openwater fish and shellfish designated use extends only as far as the upper boundary of

the pycnocline. If a pycnocline exists but other physical circulation patterns (such as

the inflow of oxygen-rich oceanic bottom waters) provide oxygen replenishment to

the deep waters, the open-water fish and shellfish designated use extends to the

bottom water-sediment interface. 

From October 1 through May 31, the boundaries of the open-water designated use

include all tidally influenced waters extending horizontally from the shoreline,

measured at mean low water, to the adjacent shoreline, and down into the water

column to the bottom water-sediment interface.

DEEP-WATER SEASONAL FISH AND SHELLFISH DESIGNATED USE

Waters with this designated use protect the survival, growth and propagation of

balanced, indigenous populations of important fish and shellfish species inhabiting

deep-water habitats.

Designated Use Boundary Delineation

This designated use refers to tidally influenced waters located between the measured

depths of the upper and lower boundaries of the pycnocline, where a measured pycnocline is present and presents a barrier to oxygen replenishment from June 1 through

September 30. In some areas, the deep-water designated use extends from the upper

boundary of the pycnocline down to the sediment/water interface at the bottom,

where a lower boundary of the pycnocline is not calculated due to the depth of the

water column.

DEEP-CHANNEL SEASONAL REFUGE DESIGNATED USE

Waters within this designated use must protect the survival of balanced, indigenous

populations of ecologically important benthic infaunal and epifaunal worms and

clams, which provide food for bottom-feeding fish and crabs. 

Designated Use Boundary Delineation

Deep-channel seasonal refuge designated use waters are defined as tidally influenced

waters at depths greater than the measured lower boundary of the pycnocline in

isolated deep channels. The deep-channel designated use is defined laterally by

bathymetry of the trough and vertically by the lower boundary of the pycnocline

above, and below, at the sediment-water interface on the bottom.

▼A-6

appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
LITERATURE CITED

Chesapeake Bay Watershed Partners. 2001. Memorandum of Understanding among the State

of Delaware, the District of Columbia, the State of Maryland, the State of New York, the

Commonwealth of Virginia, the State of West Virginia and the United States Environmental

Protection Agency Regarding Cooperative Efforts for the Protection of the Chesapeake Bay

and its Rivers. Chesapeake Bay Program, Annapolis, Maryland

Chesapeake Executive Council. 2000. Chesapeake 2000 agreement. Chesapeake Bay

Program, Annapolis, Maryland.

Funderburk, S. L., J. A. Mihursky, S. J. Jordan and D. Riley (eds.). 1991. Habitat Requirements for Chesapeake Bay Living Resources: Second Edition. Chesapeake Research

Consortium, Solomons, Maryland.

Jordan, S. J., C. Stenger, M. Olson, R. Batiuk and K. Mountford. 1992. Chesapeake Bay

dissolved oxygen goal for restoration of living resource habitats: a synthesis of living

resource requirements with guidelines for their use in evaluating model results and monitoring information. CBP/TRS 88/93. Chesapeake Bay Program, Annapolis, Maryland.

U.S. Environmental Protection Agency. 2003a. Biological Evaluation for the Issuance of

Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for

the Chesapeake Bay and its Tributaries. Chesapeake Bay Program Office, Annapolis, Maryland.

U.S. Environmental Protection Agency. 2003b. Technical Support Document for the Identification of Chesapeake Bay Designated Uses and Attainability. Chesapeake Bay Program

Office, Annapolis, Maryland.

▼A-7

appendix A • Refined Designated Uses for the Chesapeake Bay and Tidal Tributaries
This appendix provides the following lines of evidence to support the conclusion that

the data used in the calculation of the Virginian Province saltwater dissolved oxygen

criteria (U.S. EPA 2000) are appropriate for the Chesapeake Bay dissolved oxygen

criteria development. 

For the juvenile criterion (Final Acute Value), most test temperatures ranged from

19°C to 30°C. Three species were tested at temperatures less than 19°C: Homarus

americanus (15°C), Carcinus maenus (10°C) and Rithropanopeus harrisii (10°C).

Fourteen genera were tested at 19°C to 21.5°C and eight genera were tested at 23°C

to 30°C. Figure B-1 shows the cumulative rank plot for genus mean acute values

(GMAV) using data from Appendix B of the Virginian Province saltwater dissolved

oxygen criteria document (U.S. EPA 2000). The data were segregated into ‘20°C’

and ‘26°C’groups, representing the 14 and 8 genera groups mentioned above,

respectively. Plots of the two sets of data overlap, showing that both groups give a

similar estimate of the range for the juvenile community’s sensitivity to hypoxia. The

criteria minimum concentrations (CMC) calculated for the two sets of data are likewise very similar, 2.36 mg/L for the ‘20°C’ group, and 2.26 mg/L for the ‘26°C’

group.

The same type of analysis was conducted using the 24-hour larval LC50 data (lethal

concentration at which 50 percent mortality of the test organisms was observed)

from Appendix D of the Virginian Province document (Figure B-2; U.S. EPA 2000).

The temperature ranges were also similar, 18°C to 22°C for the ‘20°C’ group, and

23°C to 30°C for the ‘26°C’ group. There were 14 genera in the former and 9 genera in the latter. The conclusion is the same for larvae as for juveniles, a similar distribution of community sensitivity to hypoxia for both sets of temperatures.

In addition to the data from the Virginian Province document, the EPA has conducted tests comparing the sensitivity to hypoxia for northern and southern populations

of two invertebrates (the mud crab, Dyspanopeus sayi and the grass shrimp,

Palaemonetes vulgarus, larvae for both species) and one fish (the inland silverside,

Menidia beryllina, juveniles and larvae; Thursby, personal communication). All of

the northern populations were from Rhode Island. The southern populations of

▼B-1

appendix B • Sensitivity to Low Dissolved Oxygen Concentrations

appendixB 

Sensitivity to Low Dissolved Oxygen

Concentrations for Northern and 

Southern Atlantic Coast Populations 

of Selected Test Species
invertebrates were from Georgia, and the fish were from Florida. The exposure

response data are shown in figures B-3 through B-6. The northern and southern populations of each species responded similarly to low dissolved oxygen conditions,

even though they were conducted at different temperatures.

▼B-2

appendix B • Sensitivity to Low Dissolved Oxygen Concentrations Dissolved Oxygen GMAV (mg liter-1)

Percent Rank

GMAV ‘26 ̊C’

GMAV ‘20 ̊C’

1.80

1.60

1.40

1.20

1.00

0.80

0.60

0.40

0.20

0.00

0% 20% 40% 60% 80% 100%

Figure B-1. Plot of juvenile genus mean acute values (GMAVs ) against percent rank.

Data are from Appendix B in the Virginian Province saltwater dissolved oxygen criteria

document (U.S. EPA 2000). Dissolved Oxygen GMAV (mg liter-1)

Percent Rank

GMAV ‘26 ̊C’

GMAV ‘20 ̊C’

3.00

2.50

2.00

1.50

1.00

0.50

0.00

0% 20% 40% 60% 80% 100%

Figure B-2. Plot of larval genus mean acute values (GMAVs ) against percent rank. Data

are from Appendix D in the Virginian Province saltwater dissolved oxygen criteria document (U.S. EPA 2000).
▼B-3

appendix B • Sensitivity to Low Dissolved Oxygen Concentrations

Figure B-3. Ninety-six hour dose-response for larvae of the marsh grass shrimp

Palaemonetes vulgaris exposed to various levels of low dissolved oxygen. Open symbols are

for tests conducted with populations from Rhode Island (RI) (three at 20°C and four at

25°C). The closed circles are data for a population from Georgia (GA) conducted at 28°C. All

of the RI data are from tests included in the Virginian Province saltwater dissolved oxygen

criteria document and are listed in Poucher and Coiro (1997). Tests were initiated with larval

less than 24 hours old.

Source: U.S. EPA 2000.

Figure B-4. Ninety-six hour dose-response for larvae of the Say mud crab Dyspanopeus

sayi exposed to various levels of low dissolved oxygen. Open symbols are for tests conducted with populations from Rhode Island (RI) (one at 20°C, seven at 25°C, and one at

30°C). The closed circles are data for a population from Georgia (GA) conducted at 28°C. 

All of the Rhode Island data are from tests included in the Virginian Province saltwater

dissolved oxygen criteria document and are listed in Poucher and Coiro (1997). Tests were

initiated with larval animals ranging from stage 1 to stage 3 in development.

Source: U.S. EPA 2000.

RI–20°C

RI–25°C

GA–28°C

RI–20°C

RI–25°C

RI–30°C

GA–28°C

Dissolved Oxygen (mg liter -1)

Dissolved Oxygen (mg liter -1)
▼B-4

appendix B • Sensitivity to Low Dissolved Oxygen Concentrations

Figure B-5. Ninety-six hour dose-response for larvae of the inland silverside Menidia

beryllina exposed to various levels of low dissolved oxygen at two temperatures. Open circles are for a test conducted with a population from Rhode Island (RI) (25°C) and closed

circles are for two tests with a population from Georgia (GA) conducted at 28°C. The

Rhode Island data are from a test listed in Poucher and Coiro (1997). Tests were initiated

with 7-day-old larvae. 

Source: U.S. EPA 2000.

Figure B-6. Seventy-two hour dose-response for juveniles of the inland silverside 

Menidia beryllina exposed to various levels of low dissolved oxygen at two temperatures.

Open circles are for a test conducted with a population from Rhode Island (RI) (25°C) and

closed circles are for a test with a population from Georgia (GA) conducted at 28°C.

Source: U.S. EPA 2000.

Dissolved Oxygen (mg liter -1)

Dissolved Oxygen (mg liter -1)

RI–25°C

GA–28°C

RI–25°C

GA–28°C
LITERATURE CITED

Poucher, S. and L. Coiro. 1997. Test Reports: Effects of Low Dissolved Oxygen on Saltwater

Animals. Memorandum to D. C. Miller. U. S. EPA, Atlantic Ecology Division, Narragansett,

Rhode Island.

U.S. EPA 2000. Ambient Aquatic Life Water Quality Criteria for Dissolved Oxygen (Saltwater): Cape Cod to Cape Hatteras. EPA-822-R-00-012. Office of Water, Office of Science

and Technology, Washington, D.C. and Office of Research and Development, National

Health and Environmental Effects Research Laboratory, Atlantic Ecology Division,

Narragansett, Rhode Island.

▼B-5

appendix B • Sensitivity to Low Dissolved Oxygen Concentrations
▼C-1

appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species

appendixC 

Summary of Literature on the Tolerance

of Chesapeake Bay Macrobenthic Species 

to Low Dissolved Oxygen Conditions

Species Life

Stage

Dissolved

Oxygen

(mg liter-1)

Temp

(°C)

Observed Response Reference

Mollusca

Abra alba Adult 0 10 LD50 in 200 hrs Dries and Theede 1974

Adult 0 10 50% mortality in 7 days Thamdrup 1935

referenced in O’Connor

(unpublished manuscript)

Cardium

edule

Adult 0.15 10 50% mortality in 102 hrs

(4.3 days) without sulfide,

96 hrs (4 days) with sulfide

(50 mg liter-1 Na2

S.9H2O

Theede et al. 1969; Theede

1973

Carium

lamarki

Adult 0 10 LD50 in - 220 hrs (9.2 days) Dries and Theede 1974

Littorina

littoria

Adult 0.15 10 LD50 in 365 hrs (15.2 days)

without sulfide, 180 hrs

(7.5 days) with sulfide;

50 mg liter-1

Theede et al. 1969; Theede

1973

Littorina

saxatilus

Adult 0.15 10 LD50 in 365 hrs (15.2 days)

without sulfide, 72 hrs

(3 days) with sulfide;

50 mg liter-1

Theede et al. 1969; Theede

1973

Adult 0 10 4% mortality in 7 days Thamdrup 1935; referenced

in O’Conner (unpublished

manuscript)

Macoma

balthica

Adult 0 10 LD50 in 500 hrs (20.8 days) Dries and Theede 1974

Larvae 0.9-2.4 25 Reduced growth Morrison 1971

0.2 25 100% mortality in 14 days Morrison 1971

Mercenaria

mercenaria

NR 0.9 25 0% mortality in 14 days Morrison 1971
▼C-2

appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species

Species Life

Stage

Dissolved

Oxygen

(mg liter-1)

Temp

(°C)

Observed Response Reference

Juvenile/

Adult

(31-38

mm)

5.7 19-24 Maximum burrowing rate Savage 1976

NR 0.9-1.8 17-24 Reduced burrowing rate Savage 1976

NR 0.9 19 No mortality in 21 days

and 30 days (two trials)

Savage 1976

Juvenile

(5 mm)

0 10 LT50 in 10.5 days without

sulfide, 4.3 days with

sulfide; 644 mg liter-1

Na2S.9H2O

Shumway and Scott 1983;

referenced in O’Conner

(unpublished manuscript)

Mulina

lateralis

NR 0 20 LT50 in 7.5 days Shumway and Scott 1983;

referenced in O’Conner

(unpublished manuscript)

NR 0 30 LT50 in 2 days Shumway and Scott 1983;

referenced in O’Conner

(unpublished manuscript)

Adult

(10 mm)

0 10 LT50 in 10 days without

sulfide, 3.8 days with

sulfide; 644 mg liter-1

Na2S.9H2O

Shumway and Scott 1983;

referenced in O’Conner

(unpublished manuscript)

NR 0 20 LT50 in 2.5 days Shumway and Scott 1983;

referenced in O’Conner

(unpublished manuscript)

Mulina

lateralis

NR 0 30 LT50 in 1.8 days Shumway and Scott 1983;

referenced in O’conner

(unpublished manuscript)

NR 0 ‘very

low’

Survived for ‘weeks’ Collip 1921; referenced in

O’Conner (unpublished

manuscript)

NR 0 14 Survived 8 days Collip 1921; referenced in

O’Conner (unpublished

manuscript)

Mya arenaria

NR 0 31 Survived 1 day Collip 1921; referenced in

O’Conner (unpublished

manuscript)
▼C-3

appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species

Species Life

Stage

Dissolved

Oxygen

(mg liter-1)

Temp

(°C)

Observed Response Reference

Mya arenaria Adult 0.2 10 LC50 in 21 days without

sulfide, 17 days with

sulfide.

Theede et al. 1969; Theede

1973; referenced in

O’Conner (unpublished

manuscript)

Adult 0.2 10 LC50 in 35 days without

sulfide, 25 days with

sulfide

Theede et al. 1969; Theede

1973; referenced in

O’Conner (unpublished

manuscript)

Mytilus

edulis

Adult 0 10 20% mortality in 7 days Thamdrup 1935; referenced

in O’Conner (unpublished

manuscript)

Adult

(49-64

mm)

5.3-6.0 11-22 Maximum burrowing rate Savage 1976

NR 0.8-1.6 11-22 Reduced burrowing rate,

mortality

Savage 1976

NR 1.6 21.7 1 of 9 dead in 5 days Savage 1976

NR 0.9 21.0 3 of 9 dead in 5 days Savage 1976

Juvenile/

Adult

(31-

28mm)

5.7 19-24 Maximum burrowing rate Savage 1976

Spisula

solidissima

NR 0.9-1.8 17-24 Reduced burrowing rate Savage 1976

Spisula

solidissima

NR 0.9 19 No mortality in 21 days

and 30 days (two trials)

Savage 1976

Juvenile

(5 mm)

0 10 LT50 in 10.5 days without

sulfide, 4.3 with sulfide;

644 mg liter-1 Na2S.9H2O

Shumway and Scott 1983;

referenced in O’Conner

(unpublished manuscript)

NR 0 20 LT50 in 7.5 days Shumway and Scott 1983;

referenced in O’Conner

(unpublished manuscript)

Mulinia

lateralis

NR 0 30 LT50 in 2 days Shumway and Scott 1983;

referenced in O’Conner

(unpublished manuscript)
▼C-4

appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species

Species Life

Stage

Dissolved

Oxygen

(mg liter-1)

Temp

(°C)

Observed Response Reference

Adult

(10 mm)

0 10 LT50 in 10 days without

sulfide, 3.8 days with

sulfide; 644 mg liter-1

Na2S.9H2O

Shumway and Scott 1983;

referenced in O’Conner

(unpublished manuscript)

NR 0 20 LT50 in 2.5 days Shumway and Scott 1983;

referenced in O’Conner

(unpublished manuscript)

NR 0 30 LT50 in 1.8 days Shumway and Scott 1983;

referenced in O’Conner

(unpublished manuscript)

Adult

(100

mm)

1.0 10 LC50 in 15 days; initial

mortality in 8 days; total

mortality in 30 days

Thurberg and Goodlett 1979

NR 3.0 10 No mortality in 2 months Thurberg and Goodlett 1979

Juvenile/

Adult

(3.7-5

cm)

1.0 10 LC50 in 7 days Thurberg and Goodlett 1979

Mulinia

lateralis

Juvenile/

Adult

(3.8-4.6

cm)

2.0 10 LC50 in 21 days Thurberg and Goodlett 1979

Polychaeta

Capitella

capitata

Adult 0 12 Mortality in 8 days Jacubowa and Malm 1931;

referenced in O’Conner

(unpublished manuscript)

Capitomastus

minimus

Adult 0 12 Mortality in 8 days Jacubowa and Malm 1931;

referenced in O’Conner

(unpublished manuscript)

Etoeone picta Adult 0 12 Mortality in 6 days Jacubowa and Malm 1931;

referenced in O’Conner

(unpublished manuscript)

Glycera

convoluta

Adult 0 12 Mortality in 10 days Jacubowa and Malm 1931;

referenced in O’Conner

(unpublished manuscript)
▼C-5

appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species

Species Life

Stage

Dissolved

Oxygen

(mg liter-1)

Temp

(°C)

Observed Response Reference

Harmothae

incerta

Adult 0 12 Mortality in 5 days Jacubowa and Malm 1931;

referenced in O’Conner

(unpublished manuscript)

Nephtys

ciliata

Adult 0 10 LD50 in 140 hr (5.8 days) Dries and Theede 1974

Adult 0.2 10 LC50 in 5 days without

sulfide, 4 days with

sulfide; referenced in

O’Conner (unpublished

manuscript)

Theede et al. 1969; Theede

1973

Adult 0 10 LD50 in 120 hrs (5 days) Dries and Theede 1974

Nerevis

diversicolor

Adult 0 6-8 72 hrs with no mortality;

ATP conc. 59% of initial

value (after 72 hrs)

Schottler 1979

Nereis

pelagica

Adult 0 6-8 40% mortality after 36 hrs;

ATP conc. 51% of initial

value (after 72 hrs)

Schottler 1979

Nereis virens Adult 0 6-8 72 hrs with no mortality;

ATP conc. 57% of initial

value (after 72 hrs)

Schottler 1979

Pectinaria

neapolitana

Adult 0 12 Mortality in 8 days Jacubowa and Malm 1931;

referenced in O’Conner

(unpublished manuscript)

Terebellides

stroemi

Adult 0 10 LD50 in 72 hrs (3 days) Dries and Theede 1974

Source: Holland et al. 1989.

NR = not reported.

LC50 = lethal concentration at which 50 percent mortality of the test organisms was observed.

LD50 = lethal dose (same as LC50). LT50 = lethal threshold (same as LC50).
LITERATURE CITED

Dries, R. R. and H. Theed. 1974. Saverstoffmangelresistenz Mariner Bodenvertebraten aus

der Westlichen Ostsee. Marine Biology 25:327-333.

Holland, A. F., A. T. Shaughnessy, L. C. Scott, V. A. Dickens, J. Gerritsen and J. A. Ranasinghe. 1989. Long-Term Benthic Monitoring and Assessment Program for the Maryland

Portion of Chesapeake Bay: Interpretative Report. CBRM-LTB/EST-2. Maryland Department of Natural Resources, Annapolis, Maryland.

Morrison, G. 1971. Dissolved oxygen requirements for embryonic and larval development of

the hardshell clam Mercenaria mercenaria. Journal of Fisheries Research Board Canada

28:379-381.

Savage, N. B. 1976. Burrowing activity in Mercenaria mercenaria (L.) and Spisula solidissima (Dillwyn) as a function of temperature and dissolved oxygen. Marine Behavior and

Physiology 3:221-234.

Schottler, U. 1979. On the anaerobic metabolism of the three species of Nereis (Annelida).

Marine Ecology Progress Series 1:249-254.

Theede, H. 1973. Comparative studies on the influence of oxygen deficiency and hydrogen

sulphide on marine bottom invertebrates. Netherlands Journal of Sea Research 7:244-252.

Theede, H., A. Ponat, K. Hiroki and C. Schlieper. 1969. Studies on the resistance of marine

bottom invertebrates to oxygen-deficiency and hydrogen sulphide. Marine Biology 2:325-

337.

Thurberg, E. P. and R. B. Goodlett. 1979. Impact on clams and scallops. Part II. Low

dissolved oxygen concentrations and surf clams, a laboratory study. In: Oxygen Depletion

and Associated Benthic Mortalities in New York Bight, R. B. Swanson and C. J. Sindermann,

(eds.). 1976. NOAA Professional Paper 11, U.S. Government Printing Office, Washington,

D. C. Pp. 227-280.

▼C-6

appendix C • Summary of Literature on the Tolerance of Chesapeake Bay Macrobenthic Species
▼D-1

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

appendixD 

Narrative, Numerical and Method-based 

Chlorophyll a Criteria Adopted as Water Quality

Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

Alabama Narrative criteria for specific lakes and

reservoirs

- Walter F. George

- West Point

- Weiss

Numeric chlorophyll a criteria

-16 μg liter-1

-27 μg liter-1

-20 μg liter-1

Chlorophyll a levels set

for samples collected

between April-October.

Samples collected

monthly at deepest

points.

Alaska (1) Fresh water

(A) water supply

 (i) drinking, culinary and food processing;

 (ii) agriculture, including irrigation and

 stock watering;

 (iii) aquaculture;

 (iv) industrial;

(B) water recreation

 (i) contact recreation;

 (ii) secondary recreation;

(C) growth and propagation of fish, shellfish,

other aquatic life and wildlife; and

(2) Marine water

(A) water supply

 (i) aquaculture;

 (ii) seafood processing;

 (iii) industrial;

(B) water recreation

 (i) contact recreation;

 (ii) secondary recreation;

(C) growth and propagation of fish, shellfish,

other aquatic life and wildlife; and

(D) harvesting for consumption of raw

mollusks or other raw aquatic life.

Narrative criteria

Aesthetic Qualities

CRITERIA

All waters free from substances attributable to

wastewater or other discharges that:

(5) Produce undesirable or nuisance aquatic life.

Arizona Designated uses of a surface water may

include full body contact, partial body contact,

domestic water source, fish consumption,

aquatic and wildlife (warm-water fishery),

aquatic and wildlife (ephemeral), aquatic and

wildlife (effluent dependent water) agricultural

irrigation, and agricultural livestock watering.

The designated uses for specific waters are

listed in Appendix B of the article.

R 18-11-108 Narrative Water Quality Standards

A. A surface water shall be free from pollutants in

amounts or combinations that: 6) cause the growth of

algae or aquatic plants that inhibit or prohibit the

habitation, growth or propagation of other aquatic life or

that impair recreational uses.
▼D-2

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

California

Water Quality

Control Board

Region 5

Central Valley Sacramento and San Joaquin

River Basins

Wineries with stills produce substantial quantities of

stillage waste which is high in concentrations of BOD

and nitrogen. The stillage is normally discharged directly

to land without any prior treatment. There is a potential

for the waste to affect water quality and to create

nuisance conditions. A study has been conducted to

develop recommendations for minimizing water quality

effects and nuisance conditions resulting from land

application of stillage waste. There is a need to

implement guidelines for land disposal of stillage waste

that can be used by the industry as a general indication of

minimum disposal practices when accompanied with

suitable soil, weather, groundwater and other conditions

affecting the discharge.

California

Water Quality

Control Board

San Francisco Bay/

Sacramento-San Joaquin

Delta Estuary

Water Quality Compliance and Baseline Monitoring

Monthly monitoring for chlorophyll a at several stations.

Water Quality Objective: To prevent nuisance.

California

Water Quality

Control Board

Region 2

San Francisco Bay region One criterion to protect the aesthetic value of water used

for recreation from excessive algal growth is based on

chlorophyll a.

Biostimulatory substances can cause high chlorophyll a

level rise.

Colorado Numeric water quality criteria by designated

use and for indicated rivers and streams.

Bear Creek Reservoir ...Traditionally, the average

concentration of chlorophyll a has been selected by the

commission as the indicator of lake condition. For Bear

Creek Reservoir, however, peak algal biomass

(chlorophyll a) was selected as the most important of

these indicators upon which to assess trophic response

because algal blooms are most often associated with

impaired uses. To achieve the goal of change in trophic

status, a 16 percent reduction in the frequency of

nuisance algal blooms during the growing season would

need to be achieved, as well as a reduction in frequency

and magnitude of the peak chlorophyll a concentrations.

Lake: Oligotrophic

May be class A, AA or class

B water.

0-2 μg liter-1 midsummer

Lake: Mesotrophic

May be Class AA, Class A,

or Class B.

2-15 μg liter-1 midsummer

Lake: Eutrophic

May be Class AA, Class A,

or Class B water.

15-30 μg liter-1 midsummer

Connecticut Inland Surface waters

-Class AA

Existing or proposed drinking water supply;

fish and wildlife habitat; recreational use;

agricultural, industrial supply and other

purposes (recreational uses may be restricted).

-Class A

Potential drinking water supply; fish and

wildlife habitat; recreational use; agricultural,

industrial supply and other legitimate uses,

including navigation.

-Class B

Recreational use; fish and wildlife habitat;

agricultural and industrial supply and other

legitimate uses, including navigation.

-Class C and Class D (goal to be class A or B)

Lake: Highly Eutrophic

 May be Class AA, Class A,

or Class B water.

30+ μg liter-1 midsummer
▼D-3

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

Georgia Chlorophyll a criteria for only Lakes and

Major Lake tributaries

West Point Lake: shall not exceed 27 μg liter-1 (AprilOctober)

Walter F. George Lake: shall not exceed 18 μg liter-1

(April-October)

Lake Jackson: shall not exceed 20 μg liter-1 (April –

October)

Lake Alatoona

 Upstream from the Dam – 10 μg liter-1

 Allatoona Creek upstream from I-75 10 μg liter-1

 Mid-Lake downstream from Kellogg Creek – 10μg liter-1

 Little River upstream from Highway 205 – 15 μg liter-1

 Etowah River upstream from Sweetwater Creek– 12 μg

liter-1

Lake Sidney Lanier

 Upstream from the Flowery Branch confluence – 5 μg

liter-1

 At Browns Bridge Road (State Road 369) – 5 μg liter-1

 At Bolling Bridge (State Road 53) on Chestatee River –

10 μg liter-1

 At Lanier Bridge (State Road 53) on Chattahoochee

River – 10 μg liter-1

Hawaii Criteria for all Estuaries except Pearl Harbor

Criteria for Pearl Harbor Estuary

Open coastal waters (note that criteria for open

coastal waters differ, based on fresh water

discharge.)

Oceanic waters

Chlorophyll a (μg liter-1) – geometric mean not to

exceed the given value of 2.00 μg liter-1. Not to exceed

the given value more than 10 percent of the time 5.00 μg

liter-1. Not to exceed the given value more than 2 percent

of the times of 10.00 μg liter-1.

Chlorophyll a (μg liter-1) – geometric mean not to exceed

the given value of 3.50 μg liter-1. Not to exceed the given

value more than 10 percent of the time – 10.00 μg liter-1.

Not to exceed the given value more than 2

percent of the time – 20.00 μg liter-1.

Chlorophyll a (μg liter-1) – geometric mean not to

exceed the given value of 0.30 μg liter-1*, 0.15 μg liter1

**

Not to exceed the given value more than 10 percent of the

time – 0.90 μg liter-1*, 0.50 μg liter-1**

Not to exceed the given value more than 2 percent of the

time – 1.75 μg liter-1*, 1.00 μg liter-1**

*“Wet” criteria apply when the coastal waters receive

more than three million gallons per day of fresh water

discharge per shoreline mile.

** “Dry” criteria apply when the open coastal waters

receive less than three million gallons per day of fresh

water discharge per shoreline mile.

Chlorophyll a

0.06 μg liter-1 – geometric mean not to exceed the given

value

0.12 μg liter-1 – not to exceed the given value more than

10 percent of the time

0.20 μg liter-1 – not to exceed the given value more than 2

percent of the time
▼D-4

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

Idaho All waters Excess Nutrients. Surface waters of the state shall be free

from excess nutrients that can cause

visible slime growths or other nuisance aquatic growths

impairing designated beneficial uses.

Iowa By designated uses

General use segments.

Designated use segments:

Primary contact recreation (Class "A").

Cold water aquatic life (Class "B (CW)").

High quality water (Class "HQ").

High quality resource water (Class "HQR").

Significant resource warm water (Class

"B(WW)").

Limited resource warm water (Class"B(LR)").

Lakes and wetlands (Class "B(LW)").

Drinking water supply (Class "C").

General Water Quality criteria

b. Such waters shall be free from floating debris, oil,

grease, scum and other floating materials attributable to

wastewater discharges or agricultural practices in

amounts sufficient to create a nuisance.

c. Such waters shall be free from materials attributable to

wastewater discharges or agricultural practices producing

objectionable color, odor or other aesthetically

objectionable conditions.

Kansas Aquatic Life support use

Recreation use

Nutrients. The introduction of plant nutrients into

streams, lakes or wetlands from artificial sources shall be

controlled to prevent the accelerated succession or

replacement of aquatic biota or the production of

undesirable quantities or kinds of aquatic life.

The introduction of plant nutrients into surface waters

designated for primary or secondary contact recreational

use shall be controlled to prevent the development of

objectionable concentrations of algae or algal by-products

or nuisance growths of submersed, floating or emergent

aquatic vegetation.

Louisiana Narrative criteria for all waters Nutrients. The naturally occurring range of nitrogenphosphorous ratios shall be maintained. This range shall

not apply to designated intermittent streams. To establish

the appropriate range of ratios and compensate for natural

seasonal fluctuations, the administrative authority will

use site-specific studies to establish limits for nutrients.

Nutrient concentrations that produce aquatic growth to

the extent that it creates a public nuisance or interferes

with designated water uses shall not be added to any

surface waters.
▼D-5

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

Maine All Lakes

1. Class GPA waters. Class GPA shall be the

sole classification of great ponds and

natural ponds and lakes less than 10 acres in

size.

B. Class GPA waters shall be described by

their trophic state based on measures of the

chlorophyll a content, Secchi disk

transparency, total phosphorus content and

other appropriate criteria. Class GPA waters

shall have a stable or decreasing trophic state,

subject only to natural fluctuations and shall be

free of culturally induced algal blooms which

impair their use and enjoyment.

Trophic state - Maine

Trophic State Index (TSI)

Trophic state is the ability of

a body of water to produce

algae and other aquatic

plants. The trophic state of a

body of water is a function of

its nutrient content and may

be estimated using the Maine

Trophic State Index (TSI) as

follows....

In addition, a scale of 0 to

100 is established in order to

measure the trophic state or

degree of enrichment of lakes

due to nutrient input.

TSI = 70 log (mean

chlorophyll a + 0.7)

Massachusetts All waterbody - Narrative criteria Control of Eutrophication. ...there shall be no new or

increased point source discharge of nutrients primarily

phosphorus and nitrogen that would encourage cultural

eutrophication or the growth of weeds or algae in these

Lakes or ponds. Any existing point source discharge

containing nutrients in concentration which encourage

eutrophication or growth of weeds or algae in these lakes

or ponds shall be provided with all reasonable control for

non-point source.

Michigan All waters narrative criteria R 323.1060 Plant nutrient

Nutrients shall be limited to the extent necessary to

prevent stimulation of growths of aquatic rooted,

attached, suspended, and floating plants, fungi or bacteria

which are or may become injurious to the designated uses

of the waters of the state.

Minnesota Narrative criteria for all waters Nuisance conditions prohibited.

-Excessive growths of aquatic plants, or other offensive

or harmful effects.

Missouri General criteria for all waters Waters shall be free from substances or conditions in

sufficient amounts to cause unsightly color or turbidity,

offensive odor or prevent full maintenance of beneficial

uses.

Montana Eight classifications by designated use (h) No increases of carcinogenic, bioconcentrating, toxic

or harmful parameters, pesticides and organic and

inorganic materials, including heavy metals, above

naturally occurring concentrations, are allowed.

Nebraska Agricultural use This use applies to all surface waters of the state. To be

aesthetically acceptable, waters shall be free from humaninduced pollution which causes: 1) noxious odors; 2)

floating, suspended, colloidal, or settleable materials that

produce objectionable films, colors, turbidity, or deposits;

and 3) the occurrence of undesirable or nuisance aquatic

life (e.g., algal blooms). Surface waters shall also be free

of junk, refuse, and discarded dead animals.
▼D-6

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

Nevada Lake Mead - 445A.194 Requirements to

maintain existing higher quality for area of

Lake Mead; standards for beneficial uses for

area not covered by NAC 445A. 196. (NRS

445A.425,

445A.520)

Designated use

Recreation involving contact with water,

propagation of aquatic life, including, without

limitation, a warm water fishery, recreation not

involving contact with water and municipal or

domestic supply, or both.

Chlorophyll a –μg liter-1 requirement to maintain existing

Higher Quality

b. The requirements for chlorophyll a are:

(1) Not more than one monthly mean in a calendar year at

Station 3 may exceed 45 μg liter-1.

(2) The mean for chlorophyll a in summer (July 1 -

September 30) must not exceed 40 μg liter-1 at Station 3,

and the mean for 4 consecutive summer years must not

exceed 30 μg liter-1. The sample must be collected from

the center of the channel and must be representative of

the top 5 meters of the channel. "Station 3" means the

center of the channel at which the depth is from 16 to 18

meters.

(3) The mean for chlorophyll a in the growing season

(April l-September 30) must not exceed 16 μg liter-1 at

LM4 and 9 μg liter-1 at LMS. LM4 is located just outside

of the Las Vegas Bay launch ramp and marina, next to

buoy RW "1." LM5 is located next to buoy RW "A" with

the southshore landmark of Cresent Island.

(4) The mean for chlorophyll a in the growing season

(April I - September 30) must not exceed 5 μg liter-1 in

the open water of Boulder Basin, Virgin Basin, Gregg

Basin and Pierce Basin. The single value must not exceed

10 μg liter-1 for more than 5 percent of the samples.

(5) Not less than 2 samples must be collected between the

months of March and October. During months when

only one sample is available, that value must be used in

place of the monthly mean.

New

Hampshire

Narrative criteria related to all waters e) There shall be no new or increased discharge(s)

containing phosphorus or nitrogen to tributaries of lakes

or ponds that would contribute to cultural eutrophication

or growth of weeds or algae in such lakes and ponds.

New Jersey Narrative criteria for all waters 2. Except as due to natural conditions, nutrients shall not

be allowed in concentrations that cause objectionable

algal densities, nuisance aquatic vegetation, or otherwise

render the waters unsuitable for the designated uses.

3. Activities resulting in the non-point discharge of

nutrients shall implement the best management practices

determined by the Department to be necessary to protect

the existing or designated uses.

New Mexico Narrative criteria for all waters E. Plant Nutrients: Plant nutrients from other than natural

causes shall not be present in concentrations which will

produce undesirable aquatic life or result in a dominance

of nuisance species in surface waters of the State.

New York Narrative criteria for all State waters Waters shall contain no phosphorus and nitrogen in

amounts that will result in growths of algae, weeds and

slimes that will impair the waters for their best usage.
▼D-7

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

North Carolina Freshwater – Class C waters and tidal salt

water

For lakes and reservoirs and other waters

subject to growths of macroscopic and

microscopic vegetation not designated as trout

waters

Lakes, reservoirs and other waters subject to

growths of macroscopic or microscopic

vegetation designated as trout waters ( not

applicable to lakes and reservoirs less that 10

acres in surface area)

Not to exceed 40 μg liter-1

Not to exceed 15 μg liter-1

North Dakota 1) Municipal and domestic water.

2) Recreation Fishing and Wildlife

3) Agricultural uses

4) Industrial water

1) Free from substances attributable to municipal,

industrial, or other discharges or agricultural practices

that will cause the formation of putrescent or otherwise

objectionable sludge deposits.

(2) Free from floating debris, oil, scum, and other floating

materials attributable to municipal, industrial, or other

discharges or agricultural practices in sufficient amounts

to be unsightly or deleterious.

(3) Free from materials attributable to municipal,

industrial, or other discharges or agricultural practices

producing color, odor, or other conditions to such a

degree as to create a nuisance or render any undesirable

taste to fish flesh or, in any way, make fish inedible.

Ohio Narrative criteria for all waters 3745-1-04 Criteria applicable to all waters.

(E) Free from nutrients entering the waters as a result of

human activity in concentrations that create nuisance

growths of aquatic weeds and algae.

Oklahoma Narrative criteria for all waters To determine excess nutrient by using Carlson’s Trophic

State Index. Using chlorophyll a, a value of 62 or greater,

is otherwise listed as “NLW” in Appendix A of chapter.

Water are to be designated as “Nutrient-limited

watershed” which means a watershed of a waterbody

with a designated beneficial use which is adversely

affected by excess nutrients as determined by Carlson’s

Trophic State Index.

A) Narrative criterion applicable to all waters of the state.

Nutrients from point source discharges or other sources

shall not cause excessive growth of periphyton,

phytoplankton, or aquatic macrophyte communities

which impairs any existing or designated beneficial use.
▼D-8

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

Oregon Water use designation by basin. 340-041-0150

Nuisance Phytoplankton Growth

The following values and implementation program shall

be applied to lakes, reservoirs, estuaries and streams,

except for ponds and reservoirs less than ten acres in

surface area, marshes and saline lakes:

(1) The following average chlorophyll a values shall be

used to identify water bodies where phytoplankton may

impair the recognized beneficial uses:

(a) Natural lakes which thermally stratify: 0.01 mg liter-1;

(b) Natural lakes which do not thermally stratify,

reservoirs, rivers and estuaries: 0.015 mg liter-1;

(c) Average chlorophyll a values shall be based on the

following methodology (or other methods approved by

the Department): A minimum of three samples collected

over any three consecutive months at a minimum of one

representative location (e.g., above the deepest point of a

lake or reservoir or at a point mid-flow of a river) from

samples integrated from the surface to a depth equal to

twice the Secchi depth or the bottom (the lesser of the

two depths); analytical and quality assurance methods

shall be in accordance with the most recent edition of

Standard Methods for the examination of Water and

Wastewater.

Rhode Island Narrative criteria related for all waters Freshwater

10 b. None in such

concentration that would

impair any usages

specifically assigned to said

Class or cause undesirable or

nuisance aquatic species

associated with cultural

eutrophication, nor cause

exceedance of the criterion of

10(a) above in a downstream

lake, pond, or reservoir.

Seawater

Where waters have low

tidal flushing rates,

applicable treatment to

prevent or minimize

accelerated or cultural

eutrophication may be

required for regulated

nonpoint source

activities.

South Dakota (1) Domestic water supply waters;

(2) Coldwater permanent fish life propagation

waters;

(3) Coldwater marginal fish life propagation

waters;

(4) Warmwater permanent fish life

propagation waters;

(5) Warmwater semipermanent fish life

propagation waters;

(6) Warmwater marginal fish life propagation

waters;

(7) Immersion recreation waters;

(8) Limited contact recreation waters;

(9) Fish and wildlife propagation, recreation,

and stock watering waters;

(10) Irrigation waters; and

(11) Commerce and industry waters.

74:51:01:09. Nuisance aquatic life.

Materials which produce nuisance aquatic life may not be

discharged or mused to be discharged into surface waters

of the state in concentrations that impair a beneficial use

or create a human health problem.
▼D-9

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

Texas Narrative criteria for all waters §307.4. General Criteria.

(e) Nutrient parameters. Nutrients from permitted

discharges or other controllable sources shall not cause

excessive growth of aquatic vegetation which impairs an

existing, attainable, or designated use. Site-specific

nutrient criteria, nutrient permit limitations, and/or

separate rules to control nutrients in individual

watersheds will be established where appropriate after

notice and opportunity for public participation and proper

hearing.

Utah High Quality Waters – Category 1, 2, 3

6.1 Class 1 − Protected for use as a raw water

source for domestic water systems.

a. Class 1A − Reserved.

b. Class 1B − Reserved.

c. Class 1C − Protected for domestic purposes

with prior treatment by treatment processes as

required by the Utah Division of Drinking

Water

6.2 Class 2 − Protected for recreational use and

aesthetics.

a. Class 2A − Protected for primary contact

recreation such as swimming.

b. Class 2B − Protected for secondary contact

recreation such as boating, wading, or similar

uses.

6.3 Class 3 − Protected for use by aquatic

wildlife.

a. Class 3A − Protected for cold water species

of game fish and other cold water aquatic life,

including the necessary aquatic organisms in

their food chain.

b. Class 3B − Protected for warm water

species of game fish and other warm water

aquatic life, including the necessary aquatic

organisms in their food chain.

c. Class 3C − Protected for non-game fish and

other aquatic life, including the necessary

aquatic organisms in their food chain.

d. Class 3D − Protected for waterfowl, shore

birds and other water-oriented wildlife not

included in Classes 3A, 3B, or 3C, including

the necessary aquatic organisms in their food

chain.

e. Class 3E − Severely habitat-limited waters.

Narrative standards will be applied to protect

these waters for aquatic wildlife.

6.4 Class 4 − Protected for agricultural uses

including irrigation of crops and stock

watering.

6.5 Class 5 − The Great Salt Lake. Protected

for primary and secondary contact recreation,

aquatic wildlife, and mineral extraction.

7.2 Narrative Standards

It shall be unlawful, and a violation of these regulations,

for any person to discharge or place any waste or other

substance in such a way as will be or may become

offensive such as unnatural deposits, floating debris, oil,

scum or other nuisances such as color, odor or taste; or

cause conditions which produce undesirable aquatic life

or which produce objectionable tastes in edible aquatic

organisms; or result in concentrations or combinations of

substances which produce undesirable physiological

responses in desirable resident fish, or other desirable

aquatic life, or undesirable human health effects, as

determined by bioassay or other tests performed in

accordance with standard procedures.
▼D-10

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

Virginia Narrative criteria/plan of action for all state

waters.

The Board recognizes that nutrients are contributing to

undesirable growths of aquatic plant life in surface waters

of the Commonwealth. This standard establishes a

designation of "nutrient enriched waters". Designations

of surface waters of the Commonwealth as “nutrient

enriched waters" are determined by the Board based upon

an evaluation of the historical water quality data for one

or more of the following indicators of nutrient

enrichment: chlorophyll a concentrations, dissolved

oxygen fluctuations, and concentrations of total

phosphorus.
▼D-11

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

Washington - Class AA (extraordinary)

Water quality of this class shall markedly and

uniformly exceed the requirements for all or

substantially all uses.

Characteristic uses:

i) water supply (domestic, agricultural,

industrial)

ii) stock watering

iii) fish and shellfish *

iv) wildlife habitats

v) Recreation

vi) Commerce and navigation

vii) Aesthetic Values **

- Class A (excellent)

Same as class AA except for fecal coliform

levels are lower in the AA category in

freshwater.

- Class B (good)

Water quality for this class shall meet or

exceed the requirements for most uses.

Characteristic uses:

i) water supply (industrial and agricultural)

(all other uses stay the same as above classes;

different numeric criteria for DO and fecal

coliform.)

- Class C (fair)

Water quality of this class shall meet or exceed

the requirements of selected essential uses.

i) water supply (industrial) (different criteria

for DO and fecal coliform in this class).

Lake class

-Establishing Lake Nutrient criteria

Narrative or Numeric Chlorophyll a Standards

Lakes in the Willamette, East Cascade Foothills, or Blue

Mountain ecoregions do not have recommended values and

need to have lake-specific studies in order to receive criteria

as described in (c)(i) of this subsection.

(b) The following actions are recommended if ambient

monitoring of a lake shows the epilimnetic total phosphorus

concentration, as shown in Table 1 of this section, is below

the action value for an ecoregion:

(i) Determine trophic status from existing or newly gathered

data. The recommended minimum sampling to determine

trophic status is calculated as the mean of four or more

samples collected from the epilimnion between June through

September in one or more consecutive years. Sampling must

be spread throughout the season.

(ii) Propose criteria at or below the upper limit of the trophic

state; or (iii) Conduct lake-specific study to determine and

propose to adopt appropriate criteria as described in (c) of

this subsection. (c) The following actions are recommended

if ambient monitoring of a lake shows total phosphorus to

exceed the action value for an ecoregion shown in Table 1 of

this section or where recommended ecoregional action

values do not exist:

(i) Conduct a lake-specific study to evaluate the

characteristic uses of the lake. A lake-specific study

may vary depending on the source or threat of

impairment. Phytoplankton blooms, toxic

phytoplankton, or excessive aquatic plants, are

examples of various sources of impairment. The

following are examples of quantitative measures that a

study may describe: Total phosphorus, total nitrogen,

chlorophyll-a, dissolved oxygen in the hypolimnion if

thermally stratified, pH, hardness, or other measures of

existing conditions and potential changes in any one of

these parameters.

(ii) Determine appropriate total phosphorus concentrations

or other nutrient criteria to protect characteristic lake

uses. If the existing total phosphorus concentration is

protective of characteristic lake uses, then set criteria at

existing total phosphorus concentration. If the existing

total phosphorus concentration is not protective of the

existing characteristic lake uses, then set criteria at a

protective concentration. Proposals to adopt appropriate

total phosphorus criteria to protect characteristic uses

must be developed by considering technical information

and stakeholder input as part of a public involvement

process equivalent to the Administrative Procedure Act

(chapter 34.05 RCW).

(iii) Determine if the proposed total phosphorus criteria

necessary to protect characteristic uses is achievable. If

the recommended criterion is not achievable and if the

characteristic use the criterion is intended to protect is

not an existing use, then a higher criterion may be

proposed in conformance with 40 CFR part 131.10.

(d) The department will consider proposed lake-specific

nutrient criteria during any water quality standards rule

making that follows development of a proposal. Adoption by

rule formally establishes the criteria for that lake.
▼D-12

appendix D • Criteria Adopted as Water Quality Standards by States Across the U.S. 

State Water Body Type or Designated Use Numeric or Narrative Chlorophyll a Criteria

Wyoming Surface water classes and uses.

1) Class 1 – Those surface waters in which

no further water quality degradation by

point source discharge other than from

dams will be allowed.

2) Class 2 – Those surface waters, other than

those classified as Class 1, which are

determined to support game fish.

3) Class 3 –Those surface waters, other than

those classified as Class 1, which are

determined to be presently supporting

non-game fish only.

4) Class 4 – Those surface waters, other than

those classified as Class 1, which are

determined to not have the hydrologic or

natural water quality potential to support

fish and include all intermittent and

ephemeral streams. Class 4 waters shall

receive protection for agricultural uses and

wildlife watering.

(i) USES

(a) Agriculture;

(b) Protection and propagation of fish

and wildlife;

(c) Industry;

(d) Human consumption;

(e) Recreation;

(f) Scenic value.

Section 28. Undesirable Aquatic Life.

All Wyoming surface waters shall be free from

substances and conditions or combinations thereof which

are attributable to municipal, industrial or other

dischargers or agricultural practices, in concentrations

which produce undesirable aquatic life.

Source: http://www.epa.gov/ost/standards/wqslibrary/
HISTORICAL DATA SETS

The earliest water quality data in the Chesapeake Bay Program data base date from

the early 1950s. Thus, the historical era referred to here extends from the early 1950s

to 1984, when the coordinated baywide Chesapeake Bay Monitoring Program

began. Most of the early studies focused on the physical and chemical characterization of tidal waters. Sometimes measurements of phosphorus species, usually

orthophosphate, and chlorophyll a were taken. The impetus for more nutrient measurements came during the 1960s (possibly exacerbated by the severe drought in that

decade) and 1970s with the increasing awareness of the Chesapeake Bay’s eutrophication and other signs of degradation. Nitrate measurements were collected more

frequently, and measurements of a larger suite of phosphorus and nitrogen species

began to be collected. Estimates of total phosphorus and total nitrogen are infrequent

in the historical data, however. 

Data from the Johns Hopkins University Chesapeake Bay Institute and the U.S.

Environmental Protection Agency’s Annapolis Field Office constitute the largest

contributions to the historical database. Maryland and Virginia state monitoring

programs provided data from various state waters. In Virginia, other major contributors to the historical database were the Virginia Institute of Marine Science and the

Virginia State Water Control Board slack water surveys. The database also includes

many smaller data sets including, among others, data from University of Maryland

researchers and from environmental impact studies of electric power generation in

Maryland.

Historical to present chlorophyll a concentration data are presented by Chesapeake

Bay Program segment within decades (1950s-1990s) in Table E-1. Table E-2

presents the same chlorophyll a data by Chesapeake Bay Program segment across

the same decades.

▼E-1

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

appendixE 

1950s–1990s Chesapeake Bay and Tidal

Tributary Chlorophyll a Concentrations 

by Chesapeake Bay Program Segment
BENCHMARK CHLOROPHYLL A DATA ASSESSMENT

The historical and current monitoring data sets (through 1999) were pooled, and the

surface (sampling depth < 1.5 meters) values of the parameters were retained. Each

data point was associated with a segment (from the original Chesapeake Bay

Program segmentation scheme) and a salinity regime. Salinity regimes were defined

as: tidal-fresh 0-0.5 ppt; oligohaline >0.5-5.0 ppt; mesohaline > 5-18 ppt and polyhaline >18 ppt.

If a salinity measurement was associated with the value, then that measurement

determined the regime. Otherwise, the regime was assumed from the median salinity

of the segment in which the measurement was taken. Values were further identified

according to decade (1950s through 1990s) and season. The seasons that were

included were: annual (January through December), spring (March, April and May)

and summer (June, July, August and September).

The individual data values were assessed using the Chesapeake Bay Program

method for calculating relative status (Alden and Perry 1997). The method uses the

logistic distribution of values in a reference data set to assess values in a test data set.

The procedure yields a score between 0 and 100 for each test value. The reference

data, in this case, were Chesapeake Bay Program Water Quality Monitoring data

from 1985 through 1990, which includes the largest number of stations and greatest

seasonal coverage of the monitoring program’s history to date. It thus provided the

best available spatial and temporal coverage of the historical record. The time period

also represented a relatively wide variety of flow and other climatic conditions,

although none was particularly extreme. 

The reference and test data sets were similarly partitioned by depth, segment, salinity

zone and season. For each reference grouping, the logistic distribution of values was

obtained and cutoff points representing the upper, middle and lower thirds of the

distribution were determined. For nitrogen, phosphorus, chlorophyll a and

suspended solids, high values are undesirable, therefore, the cutoff points represented ‘poor’, ‘fair’ and ‘good’ quality conditions, respectively, in this context. The

status procedure scored each test value between 0 and 100, based on the distribution

of the complementary reference distribution. Then, for each parameter/segment/

salinity zone/decade/season, the median score was calculated for each calendar

month, from which the median score for the season was obtained. The season

median scores were categorized as ‘good’, ‘fair’ or ‘poor’ by using the reference

cutoff points and adjusted slightly for the number of observations in the test data.

Each segment/zone/decade/season was then evaluated as representing ‘healthy’

nutrient and sediment levels. To qualify, none of the critical parameters—total

nitrogen, total phosphorus, chlorophyll a or total suspended solids—could have a

‘poor’ assessment; only one parameter could have a ‘fair’ assessment and one or

more parameters had to be ‘good’. Benchmark levels for each parameter were then

derived from this set of reference locations by extracting the values only from the

▼E-2

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
reference locations in which the parameter of interest was assessed as ‘good’. These

values were then pooled by salinity regime and decade and, ultimately, by salinity

regime alone. 

LITERATURE CITED

Alden, R. W. III and E. S. Perry 1997. Presenting Measurements of Status: Report to the

Chesapeake Bay Program Monitoring Subcommittee’s Data Analysis Workgroup.

Chesapeake Bay Program, Annapolis, Maryland.

▼E-3

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations
▼E-4

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by Chesapeake Bay Program segment within decade: 1950s–1990s.

1950 Northern Chesapeake Bay – – – – 1.4 1

Upper Chesapeake Bay 1.1 1 – – 2.2 7

Upper Central Chesapeake Bay – – 1.7 1 3.2 10

Middle Central Chesapeake Bay 3.1 3 2.1 1 4.0 13

Lower Chesapeake Bay 14.1 3 5.6 1 7.0 16

Western Lower Chesapeake Bay – – – – 0.7 8

Eastern Lower Chesapeake Bay 7.9 3 – – 4.2 19

Mouth of the Chesapeake Bay – – – – 1.6 8

Outside of Ches. Bay Mouth – – 2.0 1 2.2 2

Northeast River – – – – – –

Elk/Bohemia Rivers – – – – – –

Sassafras River – – – – – –

Chester River – – – – – –

Eastern Bay – – 0.5 1 1.5 3

Choptank River 2.4 2 3.4 3 2.8 7

Lower Choptank River 6.9 1 1.7 3 2.6 5

Nanticoke River – – – – – –

Wicomico River – – – – – –

Manokin River – – – – – –

Big Annemessex River – – – – – –

Tangier Sound – – 11.8 1 4.3 8

Pocomoke River – – – – – –

Bush River – – – – – –

Gunpowder River – – – – – –

Middle River – – – – – –

Chesapeake Bay Spring (N) Summer (N) Annual (N)

Decade Program Segment Mean Mean Mean
▼E-5

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by Chesapeake Bay Program segment within decade: 1950s–1990s (continued).

1950 Back River – – – – – –

Patapsco River – – – – 7.5 1

Magothy River – – – – – –

Severn River – – – – – –

South/Rhode/West Rivers – – – – – –

Upper Patuxent River 2.6 1 1.7 2 1.7 4

Middle Patuxent River – – 2.1 2 2.9 3

Lower Patuxent River 5.3 3 3.3 4 2.6 14

Upper Potomac River – – – – – –

Middle Potomac River – – 26.7 1 26.7 1

Lower Potomac River 10.8 2 5.0 12 6.1 23

Upper Rappahannock River – – – – – –

Middle Rappahannock River – – – – 3.7 2

Lower Rappahannock River 8.2 1 – – 4.3 6

Upper York River – – – – – –

Middle York River – – – – 2.0 1

Lower York River 4.5 1 – – 1.8 3

Mobjack Bay – – – – 0.6 2

Upper James River – – – – – –

Middle James River – – – – – –

Lower James River – – 3.3 19 2.3 28

1960 Northern Chesapeake Bay 6.1 8 18.2 11 12.4 31

Upper Chesapeake Bay 7.0 10 25.9 15 15.9 42

Upper Central Chesapeake Bay 6.9 29 18.2 59 11.5 122

Middle Central Chesapeake Bay 3.9 18 11.1 25 7.4 69

Lower Chesapeake Bay 2.4 7 10.9 12 9.7 28

Chesapeake Bay Spring (N) Summer (N) Annual (N)

Decade Program Segment Mean Mean Mean
▼E-6

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by Chesapeake Bay Program segment within decade: 1950s–1990s (continued).

1960 Western Lower Chesapeake Bay – – – – – –

Eastern Lower Chesapeake Bay 5.5 1 1.2 2 2.0 5

Mouth of the Chesapeake Bay – – – – – –

Outside the Ches. Bay Mouth 1.1 2 0.8 4 1.0 8

Northeast River – – – – – –

Elk/Bohemia Rivers – – – – – –

Sassafras River – – 18.1 3 20.8 5

Chester River 5.2 11 8.7 14 5.6 36

Eastern Bay 5.3 27 9.2 39 6.5 94

Choptank River – – – – – –

Lower Choptank River – – – – – –

Nanticoke River – – – – – –

Wicomico River – – – – – –

Manokin River – – – – – –

Big Annemessex River – – – – – –

Tangier Sound – – – – – –

Pocomoke River – – – – – –

Bush River – – – – – –

Gunpowder River – – – – – –

Middle River – – – – – –

Back River – – 7.7 1 30.9 3

Patapsco River 18.7 17 47.1 41 41.9 64

Magothy River 8.6 13 12.5 21 11.5 56

Severn River 7.1 12 15.9 22 10.8 60

South/Rhode/West Rivers 6.3 17 15.4 38 11.1 73

Chesapeake Bay Spring (N) Summer (N) Annual (N)

Decade Program Segment Mean Mean Mean
▼E-7

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by Chesapeake Bay Program segment within decade: 1950s–1990s (continued).

1960 Upper Patuxent River 20.1 18 32.0 43 22.5 65

Middle Patuxent River 15.0 2 24.8 4 21.5 6

Lower Patuxent River 19.9 2 20.5 4 20.3 6

Upper Potomac River 24.3 50 59.1 81 38.7 176

Middle Potomac River 8.1 26 29.3 35 23.6 83

Lower Potomac River 8.5 24 18.7 33 13.7 76

Upper Rappahannock River – – – – – –

Middle Rappahannock River – – – – – –

Lower Rappahannock River – – – – – –

Upper York River – – – – – –

Middle York River – – – – – –

Lower York River – – – – – –

Mobjack Bay – – – – – –

Upper James River – – – – – –

Middle James River – – – – – –

Lower James River 12.8 2 – – 12.8 2

1970 Northern Chesapeake Bay 11.7 28 19.3 66 12.1 116

Upper Chesapeake Bay 9.6 26 15.4 66 10.6 125

Upper Central Chesapeake Bay 14.2 156 20.7 266 14.8 589

Middle Central Chesapeake Bay 11.5 99 10.5 142 9.7 325

Lower Chesapeake Bay 11.5 29 7.7 35 8.1 94

Western Lower Chesapeake Bay – – 11.0 1 11.0 1

Eastern Lower Chesapeake Bay 14.7 13 4.8 17 7.3 45

Mouth of the Chesapeake Bay 14.8 4 7.7 14 8.7 29

Outside the Ches. Bay Mouth 5.1 7 3.5 8 4.2 31

Northeast River 40.0 11 54.9 35 49.0 53

Chesapeake Bay Spring (N) Summer (N) Annual (N)

Decade Program Segment Mean Mean Mean
▼E-8

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by Chesapeake Bay Program segment within decade: 1950s–1990s (continued).

1970 Elk/Bohemia Rivers 27.3 62 28.8 136 25.9 248

Sassafras River 42.2 26 43.1 61 46.8 106

Chester River 18.2 42 25.6 84 22.7 159

Eastern Bay 6.5 84 21.7 89 14.0 226

Choptank River 18.4 99 17.1 121 18.8 276

Lower Choptank River 11.1 37 21.5 60 17.2 103

Nanticoke River 32.5 37 22.9 80 26.7 168

Wicomico River 36.7 31 41.9 42 31.4 101

Manokin River 15.5 3 7.2 5 12.2 8

Big Annemessex River – – 18.2 6 18.2 6

Tangier River 20.3 37 16.6 57 27.6 113

Pocomoke River 23.1 43 19 63 19.9 146

Bush River 7.3 4 13.2 12 10.1 25

Gunpowder River 7.6 24 7.3 39 9.7 94

Middle River 14.7 8 28.2 8 17.7 19

Back River 55.7 115 61.5 167 58.3 392

Patapsco River 14.1 36 40.9 77 23.4 162

Magothy River 33.8 40 37.8 50 32.7 129

Severn River 22.2 12 32.1 43 24.8 75

South/Rhode/West Rivers 25.2 31 29.7 84 29.4 157

Upper Patuxent River 10.9 37 15.8 68 14.3 147

Middle Patuxent River 31.3 2 18.1 8 16.8 14

Lower Patuxent River 10.9 4 15.7 5 11.5 12

Upper Potomac River 17.9 142 31.0 286 18.0 559

Middle Potomac River 20.0 78 19.3 142 16.6 288

Chesapeake Bay Spring (N) Summer (N) Annual (N)

Decade Program Segment Mean Mean Mean
▼E-9

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by Chesapeake Bay Program segment within decade: 1950s–1990s (continued).

1970 Lower Potomac River 8.0 40 8.9 65 11.2 140

Upper Rappahannock River 2.1 66 9.4 142 5.7 313

Middle Rappahannock River 6.4 13 6.6 29 5.6 65

Lower Rappahannock River 6.8 14 8.0 35 7.5 76

Upper York River 3.9 18 9.8 107 7.2 170

Middle York River 5.0 24 9.8 109 7.2 167

Lower York River 7.8 8 5.7 21 5.8 35

Mobjack Bay 8.3 16 7.4 42 6.5 69

Upper James River 5.5 55 8.9 187 5.2 345

Middle James River 7.7 19 4.6 75 4.6 137

Lower James River 7.6 9 3.8 43 3.6 73

1980 Northern Chesapeake Bay 7.6 20 10.9 28 7.8 68

Upper Central Chesapeake Bay 8.4 38 10.1 55 7.3 135

Upper Central Chesapeake Bay 11.5 87 14.7 152 10.7 362

Middle Central Chesapeake Bay 10.4 155 10.7 225 9.4 590

Lower Chesapeake Bay 10.3 111 9.0 158 8.6 454

Western Lower Chesapeake Bay 7.2 60 8.7 80 7.6 236

Eastern Lower Chesapeake Bay 6.2 140 5.8 187 6.5 543

Mouth of the Chesapeake Bay 5.8 45 4.9 62 5.5 181

Outside the Ches. Bay Mouth 6.0 1 2.5 2 4.0 5

Northeast River 23.7 11 54.3 17 31.9 44

Elk/Bohemia Rivers 18.1 34 9.9 52 10.1 141

Sassafras River 34.3 12 70.2 15 47.9 45

Chester River 8.1 46 16.0 83 10.5 205

Eastern Bay 4.3 14 10.2 23 6.6 58

Choptank River 7.0 34 17.4 57 11.2 138

Chesapeake Bay Spring (N) Summer (N) Annual (N)

Decade Program Segment Mean Mean Mean
▼E-10

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by Chesapeake Bay Program segment within decade: 1950s–1990s (continued).

1980 Lower Choptank River 6.4 26 9.3 44 7.0 107

Nanticoke River 11.4 23 18.0 32 13.1 90

Wicomico River 6.6 11 19.6 16 11.3 44

Manokin River 8.2 12 13.8 16 9.0 43

Big Annemessex River 5.0 12 10.0 16 6.5 43

Tangier Sound 9.5 65 10.7 86 8.2 237

Pocomoke River 4.2 12 11.2 15 8.9 45

Bush River 17.6 13 42.9 22 25.3 53

Gunpowder River 22.3 11 20.5 24 17.5 53

Middle River 14.8 11 24.2 19 19.8 48

Back River 105.5 13 101.8 38 83.7 87

Patapsco River 17.5 22 50.3 44 29.3 95

Magothy River 10.0 13 22.1 19 15.0 51

Severn River 13.0 10 22.8 18 16.8 47

South/Rhode/West Rivers 14.9 42 23.8 58 16.5 157

Upper Patuxent River 4.7 94 18.4 160 9.2 414

Middle Patuxent River 15.5 13 14.2 26 17.1 65

Lower Patuxent River 14.7 52 11.4 95 11.4 245

Upper Potomac River 4.5 95 15.9 121 7.9 336

Middle Potomac River 7.4 62 7.4 79 5.8 224

Lower Potomac River 18.2 31 10.3 43 10.7 120

Upper Rappahannock River 4.1 30 15.2 53 8.4 124

Middle Rappahannock River 22.1 24 10.8 39 12.5 103

Lower Rappahannock River 10.9 78 8.9 120 8.3 324

Upper York River 3.1 24 5.1 40 3.8 102

Chesapeake Bay Spring (N) Summer (N) Annual (N)

Decade Program Segment Mean Mean Mean
▼E-11

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by Chesapeake Bay Program segment within decade: 1950s–1990s (continued).

1980 Middle York River 5.4 36 11.0 60 7.0 152

Lower York River 13.5 36 8.3 59 9.7 151

Mobjack Bay 6.3 60 8.4 80 6.9 236

Upper James River 10.2 65 20.7 114 11.2 283

Middle James River 13.8 24 17.3 40 13.7 100

Lower James River 13.8 88 6.2 140 9.6 349

1990 Northern Chesapeake Bay 6.6 27 8.6 40 5.8 102

Upper Chesapeake Bay 5.0 58 6.3 79 4.2 220

Upper Central Chesapeake Bay 7.6 147 14.3 200 9.0 487

Middle Central Chesapeake Bay 7.5 300 9.9 400 8.1 929

Lower Chesapeake Bay 9.5 210 8.5 279 8.0 819

Western Lower Chesapeake Bay 7.1 118 7.5 159 6.7 475

Eastern Lower Chesapeake Bay 6.6 264 6.8 359 6.5 1059

Mouth of the Chesapeake Bay 6.3 88 5.6 120 5.8 354

Outside the Ches. Bay Mouth – – – – – –

Northeast River 23.0 27 53.5 38 31.4 105

Elk/Bohemia Rivers 6.9 88 6.5 113 5.9 326

Sassafras River 39.6 29 66.9 35 46.1 113

Chester River 10.6 89 20.3 117 13.2 350

Eastern Bay 8.3 30 12.8 39 9.2 117

Choptank River 13.3 60 19.9 78 12.8 234

Lower Choptank River 7.4 60 8.4 78 7.4 229

Nanticoke River 10.4 60 26.9 74 15.5 226

Wicomico River 8.1 29 14.3 36 10.6 112

Manokin River 11.8 30 11.2 36 9.8 111

Big Annemessex River 7.5 30 9.6 35 7.4 112

Chesapeake Bay Spring (N) Summer (N) Annual (N)

Decade Program Segment Mean Mean Mean
▼E-12

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-1. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by Chesapeake Bay Program segment within decade: 1950s–1990s (continued).

1990 Tangier Sound 10.8 147 10.6 189 9.3 566

Pocomoke River 2.1 30 7.5 39 4.6 113

Bush River 26.4 28 50.9 37 31.0 106

Gunpowder River 21.5 29 18.6 38 17.0 106

Middle River 20.1 29 12.8 38 13.5 107

Back River 104.2 29 82.4 38 75.7 107

Patapsco River 15.5 29 36.1 39 22.3 113

Magothy River 12.2 29 18.3 37 13.6 110

Severn River 13.2 30 19.4 35 14.4 109

South/Rhode/West Rivers 12.4 89 18.4 110 13.0 315

Upper Patuxent River 5.9 234 15.9 307 8.7 863

Middle Patuxent River 17.8 30 15.6 39 15.6 118

Lower Patuxent River 10.7 120 13.0 156 10.4 472

Upper Potomac River 6.0 174 20.3 233 9.8 655

Middle Potomac River 5.0 93 8.4 121 5.6 350

Lower Potomac River 10.8 60 9.4 80 8.7 228

Upper Rappahannock River 3.6 149 14.1 209 7.3 563

Middle Rappahannock River 9.0 66 11.0 85 8.5 250

Lower Rappahannock River 8.2 187 7.9 250 7.1 727

Upper York River 1.5 64 4.4 79 2.5 240

Middle York River 3.5 92 13.3 118 7.4 349

Lower York River 10.3 97 7.6 125 7.6 371

Mobjack Bay 7.3 125 8.5 167 7.3 502

Upper James River 6.3 210 16.3 284 8.9 813

Middle James River 13.3 64 14.1 85 11.1 245

Lower James River 10.7 331 7.9 447 7.7 1295

Chesapeake Bay Spring (N) Summer (N) Annual (N)

Decade Program Segment Mean Mean Mean
▼E-13

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by segment across decades: 1950s–1990s. 

 CB1 1950 - - - - 1.4 1

 CB1 1960 6.1 8 18.2 11 12.4 31

 CB1 1970 11.7 28 19.3 66 12.1 116

 CB1 1980 7.6 20 10.9 28 7.8 68

 CB1 1990 6.6 27 8.6 40 5.8 102

 CB2 1950 1.1 1 - - 2.2 7

 CB2 1960 7.0 10 25.9 15 15.9 42

 CB2 1970 9.6 26 15.4 66 10.6 125

 CB2 1980 8.4 38 10.1 55 7.3 135

 CB2 1990 5.0 58 6.3 79 4.2 220

 CB3 1950 - - 1.7 1 3.2 10

 CB3 1960 6.9 29 18.2 59 11.5 122

 CB3 1970 14.2 156 20.7 266 14.8 589

 CB3 1980 11.5 87 14.7 152 10.7 362

 CB3 1990 7.6 147 14.3 200 9.0 487

 CB4 1950 3.1 3 2.1 1 4.0 13

 CB4 1960 3.9 18 11.1 25 7.4 69

 CB4 1970 11.5 99 10.5 142 9.7 325

 CB4 1980 10.4 155 10.7 225 9.4 590

 CB4 1990 7.5 300 9.9 400 8.1 929

 CB5 1950 14.1 3 5.6 1 7.0 16

 CB5 1960 2.4 7 10.9 12 9.7 28

 CB5 1970 11.5 29 7.7 35 8.1 94

 CB5 1980 10.3 111 9.0 158 8.6 454

 CB5 1990 9.5 210 8.5 279 8.0 819

 CB6 1950 - - - - 0.7 8

Spring

Mean

Summer

Mean

Annual

Segment Decade (N) (N) Mean (N)
▼E-14

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by segment across decades: 1950s–1990s (continued).

 CB6 1960 - - - - - -

 CB6 1970 - - 11.0 1 11.0 1

 CB6 1980 7.2 60 8.7 80 7.6 236

 CB6 1990 7.1 118 7.5 159 6.7 475

 CB7 1950 7.9 3 - - 4.2 19

 CB7 1960 5.5 1 1.2 2 2.0 5

 CB7 1970 14.7 13 4.8 17 7.3 45

 CB7 1980 6.2 140 5.8 187 6.5 543

 CB7 1990 6.6 264 6.8 359 6.5 1059

 CB8 1950 - - - - 1.6 8

 CB8 1960 - - - - -

 CB8 1970 14.8 4 7.7 14 8.7 29

 CB8 1980 5.8 45 4.9 62 5.5 181

 CB8 1990 6.3 88 5.6 120 5.8 354

 MOUTH 1950 - - 2.0 1 2.2 2

 MOUTH 1960 1.1 2 0.8 4 1.0 8

 MOUTH 1970 5.1 7 3.5 8 4.2 31

 MOUTH 1980 6.0 1 2.5 2 4.0 5

 MOUTH 1990 - - - - - -

 ET1 1950 - - - - - -

 ET1 1960 - - - - - -

 ET1 1970 40.0 11 54.9 35 49.0 53

 ET1 1980 23.7 11 54.3 17 31.9 44

 ET1 1990 23.0 27 53.5 38 31.4 105

 ET2 1950 - - - - - -

 ET2 1960 - - - - - -

Spring

Mean

Summer

Mean

Annual

Segment Decade (N) (N) Mean (N)
▼E-15

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by segment across decades: 1950s–1990s (continued).

 ET2 1970 27.3 62 28.8 136 25.9 248

 ET2 1980 18.1 34 9.9 52 10.1 141

 ET2 1990 6.9 88 6.5 113 5.9 326

 ET3 1950 - - - - - -

 ET3 1960 - - 18.1 3 20.8 5

 ET3 1970 42.2 26 43.1 61 46.8 106

 ET3 1980 34.3 12 70.2 15 47.9 45

 ET3 1990 39.6 29 66.9 35 46.1 113

 ET4 1950 - - - - - -

 ET4 1960 5.2 11 8.7 14 5.6 36

 ET4 1970 18.2 42 25.6 84 22.7 159

 ET4 1980 8.1 46 16.0 83 10.5 205

 ET4 1990 10.6 89 20.3 117 13.2 350

 EE1 1950 - - 0.5 1 1.5 3

 EE1 1960 5.3 27 9.2 39 6.5 94

 EE1 1970 6.5 84 21.7 89 14.0 226

 EE1 1980 4.3 14 10.2 23 6.6 58

 EE1 1990 8.3 30 12.8 39 9.2 117

 ET5 1950 2.4 2 3.4 3 2.8 7

 ET5 1960 - - - - - -

 ET5 1970 18.4 99 17.1 121 18.8 276

 ET5 1980 7.0 34 17.4 57 11.2 138

 ET5 1990 13.3 60 19.9 78 12.8 234

 EE2 1950 6.9 1 1.7 3 2.6 5

 EE2 1960 - - - - - -

 EE2 1970 11.1 37 21.5 60 17.2 103

Spring

Mean

Summer

Mean

Annual

Segment Decade (N) (N) Mean (N)
▼E-16

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by segment across decades: 1950s–1990s (continued).

 EE2 1980 6.4 26 9.3 44 7.0 107

 EE2 1990 7.4 60 8.4 78 7.4 229

 EE3 1950 - - 11.8 1 4.3 8

 EE3 1960 - - - - - -

 EE3 1970 20.3 37 16.6 57 27.6 113

 EE3 1980 9.5 65 10.7 86 8.2 237

 EE3 1990 10.8 147 10.6 189 9.3 566

 ET6 1950 - - - - - -

 ET6 1960 - - - - - -

 ET6 1970 32.5 37 22.9 80 26.7 168

 ET6 1980 11.4 23 18.0 32 13.1 90

 ET6 1990 10.4 60 26.9 74 15.5 226

 ET7 1950 - - - - - -

 ET7 1960 - - - - - -

 ET7 1970 36.7 31 41.9 42 31.4 101

 ET7 1980 6.6 11 19.6 16 11.3 44

 ET7 1990 8.1 29 14.3 36 10.6 112

 ET8 1950 - - - - - -

 ET8 1960 - - - - - -

 ET8 1970 15.5 3 7.2 5 12.2 8

 ET8 1980 8.2 12 13.8 16 9.0 43

 ET8 1990 11.8 30 11.2 36 9.8 111

 ET9 1950 - - - - - -

 ET9 1960 - - - - - -

 ET9 1970 - - 18.2 6 18.2 6

 ET9 1980 5.0 12 10.0 16 6.5 43

Spring

Mean

Summer

Mean

Annual

Segment Decade (N) (N) Mean (N)
▼E-17

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by segment across decades: 1950s–1990s (continued).

 ET9 1990 7.5 30 9.6 35 7.4 112

 ET10 1950 - - - - - -

 ET10 1960 - - - - - -

 ET10 1970 23.1 43 19.0 63 19.9 146

 ET10 1980 4.2 12 11.2 15 8.9 45

 ET10 1990 2.1 30 7.5 39 4.6 113

 WT1 1950 - - - - - -

 WT1 1960 - - - - - -

 WT1 1970 7.3 4 13.2 12 10.1 25

 WT1 1980 17.6 13 42.9 22 25.3 53

 WT1 1990 26.4 28 50.9 37 31.0 106

 WT2 1950 - - - - - -

 WT2 1960 - - - - - -

 WT2 1970 7.6 24 7.3 39 9.7 94

 WT2 1980 22.3 11 20.5 24 17.5 53

 WT2 1990 21.5 29 18.6 38 17.0 106

 WT3 1950 - - - - - -

 WT3 1960 - - - - - -

 WT3 1970 14.7 8 28.2 8 17.7 19

 WT3 1980 14.8 11 24.2 19 19.8 48

 WT3 1990 20.1 29 12.8 38 13.5 107

 WT4 1950 - - - - - -

 WT4 1960 - - 7.7 1 30.9 3

 WT4 1970 55.7 115 61.5 167 58.3 392

 WT4 1980 105.5 13 101.8 38 83.7 87

 WT4 1990 104.2 29 82.4 38 75.7 107

Spring

Mean

Summer

Mean

Annual

Segment Decade (N) (N) Mean (N)
▼E-18

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by segment across decades: 1950s–1990s (continued).

 WT5 1950 - - - - 7.5 1

 WT5 1960 18.7 17 47.1 41 41.9 64

 WT5 1970 14.1 36 40.9 77 23.4 162

 WT5 1980 17.5 22 50.3 44 29.3 95

 WT5 1990 15.5 29 36.1 39 22.3 113

 WT6 1950 - - - - - -

 WT6 1960 8.6 13 12.5 21 11.5 56

 WT6 1970 33.8 40 37.8 50 32.7 129

 WT6 1980 10.0 13 22.1 19 15.0 51

 WT6 1990 12.2 29 18.3 37 13.6 110

 WT7 1950 - - - - - -

 WT7 1960 7.1 12 15.9 22 10.8 60

 WT7 1970 22.2 12 32.1 43 24.8 75

 WT7 1980 13.0 10 22.8 18 16.8 47

 WT7 1990 13.2 30 19.4 35 14.4 109

 WT8 1950 - - - - - -

 WT8 1960 6.3 17 15.4 38 11.1 73

 WT8 1970 25.2 31 29.7 84 29.4 157

 WT8 1980 14.9 42 23.8 58 16.5 157

 WT8 1990 12.4 89 18.4 110 13.0 315

 TF1 1950 2.6 1 1.7 2 1.7 4

 TF1 1960 20.1 18 32.0 43 22.5 65

 TF1 1970 10.9 37 15.8 68 14.3 147

 TF1 1980 4.7 94 18.4 160 9.2 414

 TF1 1990 5.9 234 15.9 307 8.7 863

 RET1 1950 - - 2.1 2 2.9 3

Spring

Mean

Summer

Mean

Annual

Segment Decade (N) (N) Mean (N)
▼E-19

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by segment across decades: 1950s–1990s (continued).

 RET1 1960 15.0 2 24.8 4 21.5 6

 RET1 1970 31.3 2 18.1 8 16.8 14

 RET1 1980 15.5 13 14.2 26 17.1 65

 RET1 1990 17.8 30 15.6 39 15.6 118

 LE1 1950 5.3 3 3.3 4 2.6 14

 LE1 1960 19.9 2 20.5 4 20.3 6

 LE1 1970 10.9 4 15.7 5 11.5 12

 LE1 1980 14.7 52 11.4 95 11.4 245

 LE1 1990 10.7 120 13.0 156 10.4 472

 TF2 1950 - - - - - -

 TF2 1960 24.3 50 59.1 81 38.7 176

 TF2 1970 17.9 142 31.0 286 18.0 559

 TF2 1980 4.5 95 15.9 121 7.9 336

 TF2 1990 6.0 174 20.3 233 9.8 655

 RET2 1950 - - 26.7 1 26.7 1

 RET2 1960 8.1 26 29.3 35 23.6 83

 RET2 1970 20.0 78 19.3 142 16.6 288

 RET2 1980 7.4 62 7.4 79 5.8 224

 RET2 1990 5.0 93 8.4 121 5.6 350

 LE2 1950 10.8 2 5.0 12 6.1 23

 LE2 1960 8.5 24 18.7 33 13.7 76

 LE2 1970 8.0 40 8.9 65 11.2 140

 LE2 1980 18.2 31 10.3 43 10.7 120

 LE2 1990 10.8 60 9.4 80 8.7 228

 TF3 1950 - - - - - -

 TF3 1960 - - - - - -

Spring

Mean

Summer

Mean

Annual

Segment Decade (N) (N) Mean (N)
▼E-20

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by segment across decades: 1950s–1990s (continued).

 TF3 1970 2.1 66 9.4 142 5.7 313

 TF3 1980 4.1 30 15.2 53 8.4 124

 TF3 1990 3.6 149 14.1 209 7.3 563

 RET3 1950 - - - - 3.7 2

 RET3 1960 - - - - - -

 RET3 1970 6.4 13 6.6 29 5.6 65

 RET3 1980 22.1 24 10.8 39 12.5 103

 RET3 1990 9.0 66 11.0 85 8.5 250

 LE3 1950 8.2 1 - - 4.3 6

 LE3 1960 - - - - - -

 LE3 1970 6.8 14 8.0 35 7.5 76

 LE3 1980 10.9 78 8.9 120 8.3 324

 LE3 1990 8.2 187 7.9 250 7.1 727

 TF4 1950 - - - - - -

 TF4 1960 - - - - - -

 TF4 1970 3.9 18 9.8 107 7.2 170

 TF4 1980 3.1 24 5.1 40 3.8 102

 TF4 1990 1.5 64 4.4 79 2.5 240

 RET4 1950 - - - - 2.0 1

 RET4 1960 - - - - - -

 RET4 1970 5.0 24 9.8 109 7.2 167

 RET4 1980 5.4 36 11.0 60 7.0 152

 RET4 1990 3.5 92 13.3 118 7.4 349

 LE4 1950 4.5 1 - - 1.8 3

 LE4 1960 - - - - - -

 LE4 1970 7.8 8 5.7 21 5.8 35

Spring

Mean

Summer

Mean

Annual

Segment Decade (N) (N) Mean (N)
▼E-21

appendix E • 1950s–1990s Chesapeake Bay and Tidal Tributary Chlorophyll a Concentrations

Table E-2. Chesapeake Bay and tidal tributaries chlorophyll a concentrations (μg liter-1) 

by segment across decades: 1950s–1990s (continued).

 LE4 1980 13.5 36 8.3 59 9.7 151

 LE4 1990 10.3 97 7.6 125 7.6 371

 WE4 1950 - - - - 0.6 2

 WE4 1960 - - - - - -

 WE4 1970 8.3 16 7.4 42 6.5 69

 WE4 1980 6.3 60 8.4 80 6.9 236

 WE4 1990 7.3 125 8.5 167 7.3 502

 TF5 1950 - - - - - -

 TF5 1960 - - - - - -

 TF5 1970 5.5 55 8.9 187 5.2 345

 TF5 1980 10.2 65 20.7 114 11.2 283

 TF5 1990 6.3 210 16.3 284 8.9 813

 RET5 1950 - - - - - -

 RET5 1960 - - - - - -

 RET5 1970 7.7 19 4.6 75 4.6 137

 RET5 1980 13.8 24 17.3 40 13.7 100

 RET5 1990 13.3 64 14.1 85 11.1 245

 LE5 1950 - - 3.3 19 2.3 28

 LE5 1960 12.8 2 - - 12.8 2

 LE5 1970 7.6 9 3.8 43 3.6 73

 LE5 1980 13.8 88 6.2 140 9.6 349

 LE5 1990 10.7 331 7.9 447 7.7 1295

Spring

Mean

Summer

Mean

Annual

Segment Decade (N) (N) Mean (N)
This appendix describes various analyses performed with the 1984-2001 Chesapeake Bay Program water quality and plankton monitoring data that supported

determination of the phytoplankton reference community chlorophyll a concentrations reported in Chapter V. 

REFERENCE PHYTOPLANKTON COMMUNITIES AND

WATER QUALITY CONDITION CLASSIFICATIONS

Biological populations found in pristine or minimally impaired habitats provide

essential information about how restoration efforts might improve ecosystem structure and function. Called ‘reference communities,’ these populations serve as

benchmarks for measuring ecosystem impairment. Ecosystem impairment is

assessed with a suite of physical, chemical and biological performance indicators

which are measurable attributes of the ecosystem linked directly to restoration objectives. The properties of the performance indicators in biological reference

communities furnish the evaluation (scoring) criteria needed to quantify ecosystem

impairment at other sites (National Research Council 1992). Chlorophyll a has long

been used as a surrogate measure of phytoplankton biomass and as a performance

indicator of nutrient enrichment across a wide spectrum of aquatic systems (see

Chapter V). Chlorophyll a as an indicator is directly linked to a restoration objective

of the Chesapeake Bay Program, namely the reduction of excess, uneaten phytoplankton that accumulates in the water column and contributes to reduced water

clarity and summer oxygen depletion in bottom waters, ultimately stressing the food

webs the phytoplankton support.

Chlorophyll a concentrations for season- and salinity-specific phytoplankton reference communities for Chesapeake Bay tidal waters are described in this appendix

and elsewhere (Buchanan et al., in review). The reference communities are based on

phytoplankton populations currently found in waters least impaired by poor water

clarity and nutrients in excess of phytoplankton growth requirements. Water quality

▼F-1

appendix F • Phytoplankton Reference Community Data Analyses

appendixF 

Phytoplankton Reference 

Community Data Analyses
condition classifications were determined with three parameters crucial to phytoplankton growth: light penetration (measured as Secchi depth), dissolved inorganic

nitrogen (DIN) and ortho-phosphate (PO4). 

ANALYSIS APPROACH

Chesapeake Bay water quality and phytoplankton data collected at Chesapeake Bay

Program biomonitoring stations between 1984 and 2001 were first analyzed to identify samples that were least impaired by poor water clarity and excess nutrients.

Seasonal and salinity-specific phytoplankton ‘reference’ communities for the Chesapeake Bay were then derived from the populations in those samples. The reference

communities are used in this analysis to quantify chlorophyll a concentrations in the

least-impaired water quality conditions currently found in the Chesapeake Bay and

its tidal tributaries. 

The Chesapeake Bay Monitoring Program has coordinated the year-round collection

of plankton and water quality data at more than 26 stations for all salinity zones in

the Chesapeake Bay mainstem and its major tidal tributaries since August 1984. Data

for some parameters were collected over shorter periods or only by one state. The

primary data and data documentation are available at http://www.chesapeakebay.net/

data. Phytoplankton parameters that are measured (primary data) or derived from

measured data include chlorophyll a, pheophytin, species abundances, biomasses of

individual species in the nano (2–20 micron) and micro (20–200 micron) size fractions, phytoplankton biomass in pico (<2 micron) size fractions, average cell size of

the nano-micro phytoplankton and the ratio of phytoplankton biomass (as carbon) to

chlorophyll a. Productivity cannot be used for baywide analyses because Maryland

and Virginia methodologies are different. In this study, water quality and phytoplankton data from the mixed upper layer of the water column (usually identified as

‘above-pycnocline,’ or AP) were analyzed, with the exception of a few tidal-fresh

stations where samples were from the whole water column (WC). Data from each

sampling event at an individual station were sorted into two seasons and four salinity

zones for examination: spring (March, April and May) and summer (July, August

and September); and tidal-fresh (0.0 to 0.5 ppt), oligohaline (>0.5 to 5.0 ppt), mesohaline (>5.0 to18.0 ppt) and polyhaline (>18.0 ppt). This minimizes the influence of

season and salinity regime on the analysis.

Phytoplankton and water quality data within each season-salinity group were binned

(further grouped) into six categories using Secchi depth, DIN and PO4 thresholds

shown in tables F-1 and F-2. The thresholds classify the Secchi depth, DIN, and PO4

values of each data record as ‘worst,’ ‘poor,’ ‘better,’ or ‘best’. The DIN and PO4

thresholds separating ‘better’ and ‘poor’ values in tables F-1 and F-2 have been

experimentally shown to be resource limitation thresholds for natural Chesapeake

Bay phytoplankton populations (Fisher et al. 1988, 1999; Thomas Fisher personal

communication). The Secchi depth thresholds separating ‘better’ and ‘poor’ values

were empirically determined from the monitoring data using the Relative Status,

or benchmark, method (Olson 2002). The ‘better’ water clarity levels are those

▼F-2

appendix F • Phytoplankton Reference Community Data Analyses
associated with the least impaired stations currently monitored in the Chesapeake

Bay. They also approximate the light levels required for growth of underwater bay

grasses (Batiuk et al. 2000). For the purpose of establishing phytoplankton reference

communities, a water quality parameter classification of ‘worst or ‘poor’ is considered impaired while a water quality parameter classification of ‘better’ or ‘best’ is

considered unimpaired.

When all three parameters were classified as ‘worst,’ the data record was placed in

the ‘worst’ water quality category. When all three parameters classified as ‘poor’ or

‘worst’ (includes all ‘worst’), the data record was placed in the ‘poor’ water quality

category. ‘Poor’ and ‘worst’ water quality conditions are characterized by low levels

of light, and concentrations of DIN and PO4 that exceed phytoplankton nutrient

requirements. ‘Worst’ is an extreme subset of ‘poor.’ Similarly, when all three parameters classified as ‘best,’ the data record was placed in the ‘best’ water quality

category. When all three classified as ‘best’ or ‘better’ (includes all ‘best’), the data

record was placed in the ‘better’ water quality category. ‘better’ and ‘best’ water

quality conditions had high levels of light and limiting (low) concentrations of DIN

and PO4. ‘Best’ is an extreme subset of ‘better’. Data records were placed in a

▼F-3

appendix F • Phytoplankton Reference Community Data Analyses

Table F-1. Spring (March through May) classification criteria for determining ‘worst’, ‘poor’, ‘better’ and ‘best’ water quality parameter conditions. Key: Secchi-Secchi depth (meters);

DIN-average dissolved organic nitrogen in surface mixed layer (mg liter1); PO4—average orthophosphate (SRP) in surface mixed layer (mg liter1); TF—tidal fresh salinities 

(0 to 0.5 ppt); OH—oligohaline salinities (>0.5 to 5 ppt); MH—mesohaline salinties

(>5 to 18 ppt); PH—polyhaline (>18 ppt). The 25th percentile, median and 75th percentile of the parameter’s values at stations identified as ‘good’ by the Relative Status

Method are given for comparison purposes. See Buchanan et al. (in review) for details.

Parameter Selected Spring Classification Criteria Relative Status Method

Worst Poor Better Best 25th%/median/75th%

Secchi TF <0.7 =<0.9 >0.9 >1.1 0.7 | 0.9 | 1.10

Secchi OH <0.5 =<0.7 >0.7 >1.1 0.5 | 0.7 | 1.10

Secchi MH <1.35 =<1.8 >1.8 >2.25 1.35 | 1.80 | 2.25

Secchi PH <1.6 =<2.15 >2.15 >2.55 1.6 | 2.15 | 2.55

Worst Poor Better Best 75th%/median/25th%

DIN TF >.585 >0.070 =<0.070 <0.030 .585 | .434 | .290

DIN OH >.885 >0.070 =<0.070 <0.030 .885 | .680 | .464

DIN MH >.265 >0.070 =<0.070 <0.030 .265 | .150 | .070

DIN PH >.070 >0.070 =<0.070 <0.030 .063 | .020 | .011

Worst Poor Better Best 75th%/median/25th%

PO4 (SRP) TF >0.020 >0.003 =<0.003 =<0.003 .020 | .136 | .010

PO4 (SRP) OH >0.010 >0.003 =<0.003 =<0.003 .010 | .005 | .004

PO4 (SRP) MH >0.003 >0.002 =<0.002 =<0.002 .003 | .002 | .0006

PO4 (SRP) PH >0.005 >0.003 =<0.003 =<0.003 .005 | .004 | .0007
‘mixed poor light’ category if Secchi depth classified as ‘poor’ or ‘worst’ and one or

both of the nutrient parameters classified as ‘better’ or ‘best’. Data records were

placed in a ‘mixed better light’ category if Secchi depth classified as ‘better’ or ‘best’

and one or both of the nutrient parameters classified as ‘poor’ or ‘worst’.

SUMMARY OF CHLOROPHYLL A RESULTS

The ‘better’ water quality conditions (includes ‘best’) occurred in 1.6 percent

(spring) and 5.8 percent (summer) of the mesohaline biomonitoring records, and

21.1 percent (spring) and 10.4 percent (summer) of the polyhaline biomonitoring

records collected between 1984 and 2001. Therefore, reference communities could

be characterized directly from the phytoplankton associated with these leastimpaired water quality data. Because values of most phytoplankton parameters in the

mesohaline and polyhaline ‘mixed better light’ categories, including chlorophyll a,

closely resembled those in ‘better’ categories, ‘mixed better light’ data were used to

augment the small number of spring mesohaline ‘better’ data records. Median

chlorophyll a concentrations were 5.6 (spring) and 7.1 (summer) μg liter-1 in the

▼F-4

appendix F • Phytoplankton Reference Community Data Analyses

Table F-2. Summer (July through September) classification criteria for determining ‘worst’, ‘poor,’

‘better,’ and ‘best’ water quality parameter conditions. Key: Secchi-Secchi depth

(meters); DIN-average dissolved organic nitrogen in surface mixed layer (mg liter1);

PO4—average orthophosphate (SRP) in surface mixed layer (mg liter1); TF—tidal fresh

salinities (0 to 0.5 ppt); OH—oligohaline salinities (>0.5 to 5 ppt); MH—mesohaline

salinties (>5 to 18 ppt); PH—polyhaline (>18 ppt). The 25th percentile, median and

75th percentile of the parameter’s values at stations identified as ‘good’ by the

Relative Status Method are given for comparison purposes. See Buchanan et al. 

(in review) for details.

Parameter Selected Summer Classification Criteria Relative Status Method

Worst Poor Better Best 25th%/median/75th%

Secchi TF <0.6 =<0.8 >0.8 >1.0 0.6 | 0.8 | 1.0

Secchi OH <0.55 =<0.6 >0.6 >0.7 0.55 | 0.6 | 0.7

Secchi MH <1.2 =<1.45 >1.45 >1.7 1.2 | 1.45 | 1.7

Secchi PH <1.55 =<1.85 >1.85 >2.35 1.55 | 1.85 | 2.35

Worst Poor Better Best 75th%/median/25th%

DIN TF >.390 >0.070 =<0.070 <0.030 .390 | .240 | .125

DIN OH >.090 >0.070 =<0.070 <0.030 .090 | .050 | .028

DIN MH >.074 >0.070 =<0.070 <0.030 .074 | .035 | .014

DIN PH >.070 >0.070 =<0.070 <0.030 .028 | .011 | .008

Worst Poor Better Best 75th%/median/25th%

PO4 (SRP) TF >0.025 >0.003 =<0.003 =<0.003 .025 | .020 | .010

PO4 (SRP) OH >0.010 >0.003 =<0.003 =<0.003 .010 | .009 | .004

PO4 (SRP) MH >0.008 >0.002 =<0.002 =<0.002 .008 | .005 | .0035

PO4 (SRP) PH >0.010 >0.003 =<0.003 =<0.003 .010 | .008 | .005
mesohaline reference communities, and 2.9 (spring) and 4.4 (summer) μg liter-1 in

the polyhaline reference communities. Reference community chlorophyll a values

are within the 2-7 μg liter-1 range identified by Molvaer et al. (1997) for mesotrophic

marine waters, but are slightly higher than the 1-3 μg liter-1 chlorophyll a range identified as mesotrophic by Smith et al. (1990). They can be considered high

mesotrophic. The reference community medians are 50 percent (spring) and 58

percent (summer) of the Poor category median concentrations in mesohaline waters

and 32 percent (spring) and 72 percent (summer) of the ‘poor’ category median

concentrations in polyhaline waters. These differences are significant (Wilcoxon

test, p<0.01). Chlorophyll a concentrations in the ‘poor’ categories classify as

eutrophic in mesohaline waters and borderline eutrophic in polyhaline waters.

Tidal-fresh and oligohaline reference community chlorophyll a concentrations are

based primarily on phytoplankton in the ‘mixed better light’ water quality category,

which is the least impaired category commonly found in low salinity waters of the

Chesapeake Bay. ‘Better’ water quality conditions occurred in less than 1 percent of

all samples. The combined ‘mixed better light’ and ‘better’ categories occurred in 4.7

percent (spring) and 21.5 percent (summer) of the tidal fresh biomonitoring records

and in 18.7 percent (spring) and 29.9 percent (summer) of the oligohaline biomonitoring records collected between 1984 and 2001. Median chlorophyll a

concentrations were 4.3 (spring) and 8.6 (summer) μg liter-1 in the tidal fresh reference communities, and 9.6 (spring) and 6.0 (summer) μg liter-1 in the oligohaline

reference communities. Reference community chlorophyll a values are within the

ranges identified by Wetzel (2001) and Novotny and Olem (1994) for mesotrophic

fresh waters, but sometimes exceed the ranges identified by Smith et al. (1998) and

Ryding and Rast (1989). These values can be considered high mesotrophic. Median

chlorophyll a concentrations of the reference community are 64 percent (spring) and

34 percent (summer) of those in tidal fresh ‘poor’ category waters, and 52 percent

(spring) and 35 percent (summer) of those in oligohaline ‘poor’ category waters.

These differences are significant (Wilcoxon test, p<0.01). Chlorophyll a concentrations in the tidal-fresh and oligohaline ‘poor’ categories classify as eutrophic to

highly eutrophic.

Reference communities were also distinguishable from ‘poor’ category phytoplankton populations by their smaller chlorophyll a ranges (Figure F-1). Typically,

ranges of chlorophyll a concentrations in the reference communities were 1/5 to 1⁄2

the span of those in ‘poor’ water quality conditions. The large ranges of chlorophyll

a concentrations found in the ‘worst,’ ‘poor,’ and ‘mixed poor light’ water quality

categories of all salinity zones demonstrate the occurrence of frequent algal blooms

in these categories. Marshall et. al. (in draft) show that the species compositions of

phytoplankton associated with the lowest quartile (minimum—25th percentile) of

chlorophyll a values in ‘worst’, ‘poor’ and ‘mixed poor light’ water quality conditions are generally mixed, while species compositions in the highest quartile of

chlorophyll values (75th percentile–maximum) are dominated by ‘bloom-forming’

species. Mesohaline and polyhaline bloom-forming species include the diatoms

▼F-5

appendix F • Phytoplankton Reference Community Data Analyses
▼F-6

appendix F • Phytoplankton Reference Community Data Analyses

Figure F-1: Chlorophyll a concentrations (μg liter-1) for six water quality conditions in eight season-salinity groups (see text for details). Symbols: median (•),

average (°), and 5th, 25th, 75th and 95th percentiles (—). Median and 95th

percentile values are shown. A blank indicates <10 data points were available 

in the water quality category.
Chaetoceros spp., Cyclotella spp. and (at times) the small, unidentified centric

diatom, and the dinoflagellates Gymnodinium spp., Katodinium rotundatum and

Prorocentrum minimum. Tidal-fresh and oligohaline bloom-forming species include

colonial bluegreens such as Microcystis aeruginosa, filamentous bluegreen genera

such as Oscillatoria and Raphidiopsis, diatoms such as Coscinodiscus spp., Leptocylindrus minimus, small unidentified centrics and Melosira varians, greens such as

Coelastrum spp., and the dinoflagellate Gymnodinium spp. Coincident water quality

data suggests the high chlorophyll a groups in ‘worst,’ ‘poor,’ and ‘mixed poor light’

conditions may represent blooms at their peak, while the low chlorophyll a groups

may represent populations unable to use the available nutrients and blooming due to

low light levels. Specifically, DIN concentrations in the high chlorophyll a groups

are sometimes as little as half of those in the low chlorophyll a groups, indicating

increased nitrogen utilization in the high chlorophyll a groups. 

The ranges of chlorophyll a concentrations (5th percentile–95th percentile) observed

in the phytoplankton reference communities indicate the peak concentrations that

should be expected in populations currently inhabiting unimpaired Chesapeake

waters. Chlorophyll a concentrations above these peak values constitute excess

phytoplankton production fueled by high nutrient concentrations and are potentially

harmful to the Chesapeake ecosystem. Peak chlorophyll a concentrations of the

reference communities, expressed as μg liter-1, are 13.5 (tidal-fresh), 24.3 (oligohaline), 24.6 (mesohaline) and 6.7 (polyhaline) in spring, and 15.9 (tidal-fresh), 25.2

(oligohaline), 14.0 (mesohaline) and 8.7 (polyhaline) in summer. 

LITERATURE CITED

Batiuk, R. A., P. Bergstrom, M. Kemp, E. Koch, L. Murray, J. C. Stevenson, R. Bartleson, V.

Carter, N. B. Rybicki, J. M. Landwehr, C. Gallegos, L. Karrh, M. Naylor, D. Wilcox, K. A.

Moore, S. Ailstock and M. Teichberg. 2000. Chesapeake Bay Submerged Aquatic Vegetation

Water Quality and Habitat-Based Requirements and Restoration Targets: A Second Technical

Synthesis. CBP/TRS 245/00 EPA 903-R-00-014. U.S. EPA Chesapeake Bay Program,

Annapolis, Maryland.

Buchanan, C., R. V. Lacouture, H. G. Marshall, M. M. Olson and J. Johnson. In review. 

Phytoplankton reference communities for Chesapeake Bay. 

Fisher, T. R., L. W. Harding, D. W. Stanley and L. G. Ward. 1988. Phytoplankton, nutrientsand turbidity in the Chesapeake, Delaware and Hudson estuaries. Estuarine, Coastal and

Shelf Science 27:61-93.

Fisher, T. R., A. B. Gustafson, K. Sellner, R. Lacouture, L. W. Haas, R. L. Wetzel, R.

Magnien, D. Everitt, B. Michaels and R. Karrh. 1999. Spatial and temporal variation of

resource limitation in Chesapeake Bay. Marine Biology 133:763-778.

Marshall, H. G., R. V. Lacouture, C. Buchanan, and J. Johnson. In preparation. Phytoplankton assemblages associated with water quality conditions during spring and summer in

salinity regions of Chesapeake Bay derived from a long-term monitoring program.

▼F-7

appendix F • Phytoplankton Reference Community Data Analyses
Molvaer. J., J. Knutzen, J. Magnusson, B. Rygg, J. Skei and J. Sorensen. 1997. Environmental quality classification in fjords and coastal areas. Statens Forurensningstilsyn TA-1467,

Norway. 36 pp.

National Research Council. 1992. Restoration of Aquatic Systems. National Academy Press.

Washington, D.C. 552 pp.

Novotny V. and Olem H. 1994. Water Quality: Prevention, Identification and Management of

Diffuse Pollution. Van Nostrand Reinhold. New York, New York.

Olson, M. 2002. Benchmarks for Nitrogen, Phosphorus, Chlorophyll and Suspended Solids

in Chesapeake Bay. Chesapeake Bay Program Technical Report Series, Chesapeake Bay

Program, Annapolis, Maryland.

Ryding, S. O. and W. Rast. 1989. The Control of Eutrophication of Lakes and Reservoirs.

Man and the Biosphere Series, Volume 1, Parthenon Publication Group, Park Ridge, New

Jersey.

Smith, V. H. 1998. Cultural eutrophication of inland, estuarine and coastal waters. In:

Successes, Limitation and Frontiers in Ecosystem Science. Pace, M. L. and P. M. Groffman

(eds.) Pp. 7-49 Springer-Verlag, New York, New York.

Wetzel, R. G. 2001. Limnology–Lake and River Ecosystems, 3rd Edition. Academic Press,

New York, New York.

▼F-8

appendix F • Phytoplankton Reference Community Data Analyses
MICROCYSTIS AERUGINOSA EFFECTS TRESHOLD

A substantial body of literature deals with the negative effects of toxic cyanobacteria

on the feeding, growth, behavior and survival of micro- and mesozooplankton.

Numerous studies have documented the avoidance of ingestion of toxic and nontoxic

strains of Microcystis aeruginosa by specific taxa of zooplankton (Clarke 1978;

Lampert 1981; Gilbert and Bogdan 1984; Fulton and Paerl 1987, 1988; DeMott and

Moxter 1991) while others indicate physiological and behavioral problems associated with the ingestion of Microcystis aeruginosa (Lampert 1981, 1982; Nizan et al.

1986; Fulton and Paerl 1987; DeMott et al. 1991; Henning et al. 1991).

Fulton and Paerl’s study (1987) indicated that a unicellular strain of Microcystis

aeruginosa (concentrations of 100,000 cells milliliter-1) was toxic to and failed to

support populations of Keratella mixta, Diaphanosoma bracyurum, Daphnia

ambigua and Bosmina longirostris (a rotifer and three cladocerans, respectively).

Other studies have shown additional evidence of inhibitory effects of Microcystis

aeruginosa. For instance, Penaloza et al. (1990), showed that water-soluble fractions of Microcystis aeruginosa were toxic to several rotifers, a copepod and a

cladoceran. De Mott et al. (1991) showed that a calanoid copepod was more sensitive to purified microcystin than the cladoceran that he used in his experiments.

Nutritionally, many zooplankton have been shown to grow poorly on Microcystis

aeruginosa because it lacks certain fatty acids (Ahlgren et al. 1990). The results of

these studies indicate a deleterious effect exerted by blooms of Microcystis aeruginosa on zooplankton communities. Two studies were chosen in the context of

deriving thresholds for impairment because they used densities of cells that could be

used to evaluate data from the Chesapeake Bay Monitoring Program and ultimately

translated into chlorophyll a concentrations. Without doing direct experiments on

inhibitory effects of the Chesapeake Bay strains of Microcystis aeruginosa on

zooplankton populations, certain assumptions of comparable toxicity were made for

the purposes of setting thresholds.

▼G-1

appendix G • Data Supporting Determination of Adverse Affect Thresholds 

appendixG 

Data Supporting Determination of

Adverse Affect Thresholds for

Potentially Harmful Algal Bloom Species
Numerous laboratory studies also have documented the acute effects of toxins from

the cyanobacterium Microcystis aeruginosa on fish (Erickson et al. 1986; Rabergh

et al. 1991; Keshavanath et al. 1994; Beveridge et al. 1993; Tencalla et al. 1994; Bury

et al. 1995). Several instances of fish kills resulting from cyanobacterial blooms also

have been documented (Erickson et al. 1986; Penaloza et al. 1990; Rabergh et al.

1991). These studies indicate a variety of negative effects, including inhibition of

filtering rate, liver damage, disturbed ionic regulation, behavioral changes and

mortality. However, these studies addressed the potential damage from the standpoint of toxin concentrations, not actual cell densities of the phytoplankton species

itself. Therefore, it was not possible to deduce a specific quantitative chlorophyll a

threshold whereby fish can be assessed as being negatively affected by blooms of

Microcystis aeruginosa.

Two laboratory studies were chosen to determine the threshold at which a negative

impact on the zooplankton community occurs–an impact in which the zooplankton

community structure is altered by the poor food quality, large particle size of the

colonies, increased density of particles in the water column or directly by the toxin.

Lampert (1981) conducted a laboratory feeding study in which densities as low as

1,400 cells milliliter-1 of Microcystis aeruginosa resulted in the feeding inhibition of

zooplankton. Similarly, Fulton and Paerl (1987) conducted grazing experiments in

which the inhibitory threshold of Microcystis aeruginosa ranged from 10,000-

100,000 cells milliliter-1, but was most clearly demonstrated at concentrations of

100,000 cells milliliter-1. Since there is a difference of two orders of magnitudes

between the two studies, an intermediate concentration of 10,000 cells milliliter-1

was chosen for exhibiting an inhibitory effect on zooplankton feeding.

It should be noted that a third study has been identified which documented negative

impacts on zooplankton at Microcystis aeruginosa cell densities of 50,000 cells

milliliter-1 which is an intermediate value compared to the two previously cited

studies (Smith and Gilbert 1995).

PROROCENTRUM MINIMUM EFFECTS THRESHOLD

Certain strains of Prorocentrum minimum are toxic. In Japan in 1942, Prorocentrum

minimum was attributed as the cause of a shellfish poisoning in Japan in which 114

people died (Nagazima 1965, 1968). Prorocentrum minimum isolated from a 1998

bloom in the Choptank River and subsequently grown in the laboratory was found

toxic to scallops (Wickfors, personal communication). Blooms of Prorocentrum

minimum in the source intake water to Virginia and Maryland oyster hatcheries were

suspected to have caused oyster larvae mortality at the two hatcheries in 1998 (Luckenbach and Merritt, personal communication). There has been no documented case

of shellfish toxicity or mortality as a result of the 1998 Prorocentrum minimum

bloom in the Chesapeake Bay, but clearly the potential exists for toxic repercussions

to shellfish and other organisms as a result of this bloom.

▼G-2

appendix G • Data Supporting Determination of Adverse Affect Thresholds 
Embryonic development of the Eastern oyster (Crassostrea virginica) was not

affected by living cells or extracts of Prorocentrum minimum, however, larvae

showed poor growth and poor development of the digestive system when fed Prorocentrum minimum (approximately 4,000 cells milliliter-1) (Wickfors and Smolowitz

1995). Juvenile oysters adapted to digesting Prorocentrum minimum, but only after

a two-week period. The study concludes that feeding Prorocentrum minimum to

oyster larvae resulted in clear detrimental effects, but it was not apparent whether the

effects were from toxicity or starvation. In addition, it was concluded that some

component of the Prorocentrum minimum cell interfered with cellular digestive

processes in oyster larvae and spat. 

The Wickfors and Smolowitz (1995) study also showed detrimental effects of

various diets containing different proportions of Prorocentrum minimum to oyster

larvae and newly set spat. The larvae showed consistently poorer survival and

growth in the different experimental diets and only those fed the diet with no Prorocentrum minimum or one-third the maximum concentration developed into

pedi-veligers and set. Both life stages showed difficulties in the digestive system

after Prorocentrum minimum became a major component of their diet. The study

concludes that Prorocentrum minimum blooms impaired the survival, growth and

development of oyster larvae. That the study did not reveal whether the cause of

these detrimental effects was toxicity or starvation is important to the derivation of

numeric chlorophyll a criteria or target concentrations. The highest density used in

the study was 3,900 cells milliliter-1 and detrimental effects were seen at densities of

~ 2,600 cells milliliter-1 in a mixed diet. The study is, however, useful in establishing

that 1) Prorocentrum minimum is detrimental to oyster life stages and 2) specific

densities of cells cause impairment.

Another laboratory study indicated more intense impairment of Eastern oyster life

stages when they were subjected to bloom concentrations of Prorocentrum minimum

(Luckenbach et al. 1993). Growth rates were minimal at cell densities of 3,000 cells

milliliter-1, as an inverse relationship was documented between grazing rate and cell

density. Ultimately, mortality resulted for 43 percent of the juvenile oysters that

were subjected to this same density of Prorocentrum minimum cells.

The 1993 Luckenbach study was designed to test the effects of Prorocentrum

minimum on the growth and survival of the Eastern oyster. The momentum for this

study came from observations over many years made at the Virginia Institute of

Marine Science oyster hatchery over many years on the impact of dinoflagellate

blooms on the oyster populations in the hatchery. These observations are unpublished but still noteworthy. They include the observation that adult oysters do not

spawn in the presence of bloom densities of Prorocentrum minimum and that early

larval development is impaired and high mortalities occur in the presence of high

densities of this dinoflagellate. The study used densities between 8,900-25,000 cells

milliliter-1 for the 100 percent bloom density and 2,964-8,250 cells milliliter-1 for a

33 percent bloom density. Mortalities of 100 percent for juvenile oysters took place

▼G-3

appendix G • Data Supporting Determination of Adverse Affect Thresholds 
in the 100 percent bloom diet, while 43 percent mortality was observed in the 33

percent bloom diet.

The density of 3,000 cells milliliter-1 that was chosen as a threshold for the chlorophyll a criteria analysis is based on the results of these two studies, whereby

detrimental effects were documented at cell densities of 2,600 cells milliliter-1 in one

study and 2,964-8,250 cells milliliter-1 in the other study. Neither study was aimed

specifically at determining the threshold of impairment for Prorocentrum minimum,

but impairments took place in both studies at a bloom density of around 3,000 cells

milliliter-1. The fact that two different strains of Prorocentrum minimum were used

in the two studies and negative effects occurred at a very similar density, gives

credence to using 3,000 cells milliliter-1 as a threshold for impairment. 

COCHLODINIUM HETEROLOBATUM

EFFECTS THRESHOLD

This species forms intense blooms in warm months at the mouth of the York River and

in the lower Chesapeake Bay (Mackiernan 1968; Zubkoff and Warriner 1975;

Zubkoff et al. 1979; Marshall 1995). Laboratory studies indicated a threshold concentration of ~ 500 cells milliliter-1 whereby calcium uptake was depressed and

mortality of larvae was significantly elevated (Ho and Zubkoff 1979). Above densities of ~ 1000 cells milliliter-1, calcium uptake was negligible and mortality extremely

high. Mortality was attributed to ‘spatial competition’ rather than a ‘toxic secretion’

(although this chain-forming dinoflagellate produces copious amounts of mucilage;

Lacouture, personal communication). The densities of this organism during bloom

conditions far exceeds these values and the extent of these densities can cover tens of

square miles (Mackiernan 1968; Zubkoff et al. 1979; Marshall 1995).

LITERATURE CITED

Ahlgren, G., L. Lundstedt, M. Brett and C. Forsberg. 1990. Lipid composition and food

quality of some freshwater phytoplankton for cladoceran zooplankters. Journal of Plankton

Research 12:809-818.

Beveridge, M. C. M., D. J. Baird, S. M. Rahmattulah, L. A. Lawton, K. A. Beattie and G. A.

Codd. 1993. Grazing rates on toxic and non-toxic strains of cyanobacteria by Hypohthalmictys molitrix and Oreochromis niloticus. Journal of Fish Biology 43:901-907.

Clarke, N. V. 1978. The food of adult copepods from Lake Kainji, Nigeria. Freshwater

Biology 8:321-326.

DeMott, W. R. and F. Moxter. 1991. Foraging on cyanobacteria by copepods: Responses to

chemical defenses and resource abundance. Ecology 72:1820-1834.

DeMott, W. R., Q. Z. Zhang and W. W. Carmichael. 1991. Effects of toxic cyanobacteria

and purified toxins on the survival and feeding of a copepod and three species of Daphnia.

Limnology and Oceanography 36:1346-1357.

▼G-4

appendix G • Data Supporting Determination of Adverse Affect Thresholds 
Eriksson, J. E., J. A. O. Meriluoto and T. Lindholm. 1986. Can cyanobacterial peptide toxins

accumulate in aquatic food chains? Proceedings from the IV International Symposium of

Microbial Ecology. Ljubljana, Jugoslavia. Pp. 655-658.

Fulton III, R. S. and H. W. Paerl. 1987. Toxic and inhibitory effects of the blue-green alga

Microcystis aeruginosa on herbivorous zooplankton. Journal of Plankton Research 9

(5):837-855.

Fulton III, R. S. and H. W. Paerl. 1988. Effects of the blue-green alga Microcystis aeruginosa on zooplankton competitive relations. Oecologia 76:383-389.

Gilbert, J. J. and K. G. Bogdan. 1984. Rotifer grazing: In situ studies on selectivity and

rates. In: Trophic interactions within aquatic ecosystems. Meyers, D. G. and J. R. Strickler

eds. American Association for the Advancement of Science Selected Symposium. Vol.

85:97-133. Boulder, Colorado.

Henning, M., H. Hertel, H. Wall and J. G. Kohl. 1991. Strain-specific influence of Microcystis aeruginosa on food ingestion and assimilation of some cladocerans and copepods. Int.

Rev. Ges. Hydrobiol. 76:37-45.

Ho, M. S. and P. L. Zubkoff. 1979. The effects of a Cochlodinium heterolobatum bloom on

the survival and calcium uptake by larvae of the American oyster, Crassostrea virginica. In:

Toxic Dinoflagellate Blooms. Taylor, F. J. R. and H. H. Seliger, eds. Elsevier North Holland,

New York

Keshavanath, P., M. C. M. Beveridge, D. J. Baird, L. A. Lawton, A. Nimmo and G. A. Codd.

1994. The functional grazing response of a phytoplanktivorous fish Oreochromis niloticus to

mixtures of toxic and non-toxic strains of the cyanobacterium Microcystis aeruginosa.

Journal of Fish Biology 45:123-129.

Lampert, W. 1981. Inhibitory and toxic effects of blue-green algae on Daphnia.

International Review Gesamten Hydrobiologic 66:285-298.

Lampert, W. 1982. Further studies on the inhibitory effect of the toxic blue-green Microcystis

aeruginosa on the filtering rate of zooplankton. Archives of Hydrobiology 95:207-220.

Luckenbach, M. W., K. G. Sellner, S. E. Shumway and K. Greene. 1993. Effects of two

bloom- forming dinoflagellates, Prorocentrum minimum and Gyrodinium uncatenum, on the

growth and survival of the Eastern oyster, Crassostrea virginica (Gmelin 1791). Journal of

Shellfish Research 12 (2):411-415.

Mackiernan, G. B. 1968. Seasonal distribution of dinoflagellates in the lower York River,

Virginia. Masters thesis. School of Marine Science, College of William and Mary, Gloucester

Point, Virginia. 104 pp.

Marshall, H. G. 1995. Succession of dinoflagellate blooms in the Chesapeake Bay, U.S.A.

In: Harmful Marine Algal Blooms. Lassus, P., G. Arzul, E. Erard, P. Gentien and C.

Marcaillou (eds.) Lavoisier, Intercept Ltd., Paris. 

Nagazima, M. 1965. Studies on the source of shellfish poison in Lake Hamana I. Relation

of the abundance of a species of dinoflagellate, Prorocentrum sp. to shellfish toxicity.

Bulletin of Japanese Society of Scientific Fisheries 31:198-203.

Nagazima, M. 1968. Studies on the source of shellfish poison in Lake Hamana IV. Identification and collection of the noxious dinoflagellate. Bulletin of Japanese Society of Scientific

Fisheries 43:130-131.

▼G-5

appendix G • Data Supporting Determination of Adverse Affect Thresholds 
Nizan, S., C. Dimentman and M. Shilo. 1986. Acute toxic effects of the cyanobacterium

Microcystis aeruginosa on Daphnia magna. Limnology and Oceanography 31:497-502.

Penaloza, R., M. Rojas, I. Vila and F. Zambrano. 1990. Toxicity of a soluble peptide from

Microcystis sp. to zooplankton and fish. Freshwater Biology 24:233.

Rabergh, C. M. I., G. Bylund and J. E. Eriksson. 1991. Histopathological effects of microcystis- LR, a cyclic peptide toxin from the cyanobacterium (blue-green alga) Microcystis

aeruginosa on common carp (Cyprinius carpio L.) Aquatic Toxicology 20:131-146. 

Smith, A. D. and J. J. Gilbert. 1995. Relative susceptibilities of rotifers and cladocerans to

Microcystis aeruginosa. Archives of Hydrobiology 132:309-336.

Tencalla, F. G., D. R. Dietrich and C. Schlatter. 1994. Toxicity of Microcystis aeruginosa

peptide toxin to yearling rainbow trout (Oncorhyncus mykiss). Aquatic Toxicology 30:215-

224.

Wickfors, G. H. and R. M. Smolowitz. 1995. Experimental and histological studies of four

life- history stages of the Eastern oyster, Crassostrea virginica, exposed to a cultured strain

of the dinoflagellate, Prorocentrum minimum. Biological Bulletin 188:313-328.

Zubkoff, P. L., J. C. Munday, R. G. Rhodes and J. E. Warriner. 1979. Mesoscale features of

summer (1975 to 1977) dinoflagellate blooms in the York River, Virginia (Chesapeake Bay

Estuary). In: Toxic Dinoflagellate Blooms. F. J. R. Taylor and H. H. Seliger (eds.). Elsevier

North Holland, Inc. New York, New York.

Zubkoff, P. L. and J. E. Warriner. 1975. Synoptic sightings of red waters of the lower Chesapeake Bay and its tributary rivers (May 1973 to September 1974). In: Proceedings of the

First International Conference on Toxic Dinoflagellate Blooms, LoCicero, V. R. (ed.). Massachusetts Science and Technology Foundation. Wakefield, Massachusetts. 

▼G-6

appendix G • Data Supporting Determination of Adverse Affect Thresholds 
Building from the descriptions of reference curves in Chapter VI, this appendix

provides more detailed description of the process and options considered in deriving

the open-water and deep-water dissolved oxygen reference curves and the water

clarity criteria reference curves.

DISSOLVED OXYGEN REFERENCE CURVES 

The Chesapeake Bay dissolved oxygen criteria have several duration components:

30-day mean, 7-day mean, 1-day mean and instantaneous minimum. At this time,

reference curves have been developed only for the 30-day mean component.

OPEN-WATER CRITERIA REFERENCE CURVES

The open-water designated use includes surface and surface-mixed water above a

pycnocline. It also includes waters deeper in the water column where there is no

vertical density barrier (pycnocline) or where a vertical barrier is present but does not

prevent exchange with oxygenated water horizontally. 

The dissolved oxygen criteria are based primarily on target species that are ecologically and commercially valuable and have high oxygen requirements. If the criteria

are protective of these species, then by default they are protective of other species

with lower oxygen requirements. Ideally, a reference curve for the open-water criteria

would be based on dissolved oxygen data collected in this habitat at times and places

where these sensitive species are known to thrive. Unfortunately, there is a lack of

open-water column estuarine fish/shellfish-based indices of biotic integrity, or similar

biological indicator, in addition to the lack of adequate fisheries-independent data

over the necessary geographic area and time period. Therefore, surrogate indicators

of ‘healthy’ open-water water quality conditions were employed. To validate the

reference areas, the same indicators were used to identify ‘unhealthy conditions.’

Criteria attainment curves were derived for both groups for comparison.

▼H-1

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

appendixH 

Derivation of Cumulative Frequency 

Distribution Criteria Attainment 

Reference Curves
Four approaches to defining ‘healthy’ locations by Chesapeake Bay Program

segment were examined for the open-water designated use. Approach 1 identified

Chesapeake Bay Program segments with ‘good’ and ‘poor’ water quality conditions

using water quality parameters not including dissolved oxygen. Reference and validation curves were derived using interpolated dissolved oxygen concentration data

from the reference and validation segments. Approach 2 ranked the Chesapeake Bay

Program segments in order based on seasonal median dissolved oxygen concentration (spring and summer, separately). Criteria attainment curves of the highest and

lowest 14 segments (10 percent and 10 percent, respectively for a total of 20 percent)

were used to derive reference and validation curves using the interpolated dissolved

oxygen monitoring data. In Approach 3, all the polyhaline Chesapeake Bay Program

segments were selected and similarly processed for comparison with the other

approaches, given these segments were the most likely to have the highest dissolved

oxygen values and least impaired biological communities. Approach 4 involved

selecting open-water CBP interpolator cells from locations (segment, year and

season) where healthy and stressed benthic communities were found (see “DeepWater Reference Curves,” below, for more details). All the data for this analysis

came from the Chesapeake Bay Water Quality Monitoring Program database, with

the years 1985 through 1994 selected to reflect the years of hydrology currently evaluated through the Chesapeake Bay water quality model (see Chapter VI).

Approach 1: Reference and Validation Curves 

Using Water Quality Status

The Chesapeake Bay Program’s Tidal Monitoring and Analysis Workgroup developed

a procedure to assess relative status for situations in which an absolute point of reference for a water quality parameter is not available (Alden and Perry 1997). That

procedure uses the (logistic) distribution of the parameter in a ‘benchmark’ data set as

a standard against which individual data points are assessed. The assessments are done

separately within salinity classification and generally within depth layers. The median

score of the individual data points is then calculated for any user-specified time and

space grouping. In the present context, the benchmark distribution is divided roughly

into thirds, which are defined as ‘good,’‘fair’and ‘poor’. These terms relate only to each

other, not necessarily to actual water quality requirements of living resources. 

For this analysis, the combined status assessments for total nitrogen, total phosphorus, chlorophyll a and total suspended solids were used to select reference and

validation locations. Using the above procedure, surface concentrations of the four

parameters for each Chesapeake Bay Program segment, year and season (spring and

summer) were assessed to yield an assessment of ‘good’, ‘fair’ or ‘poor’ for each

parameter. Each segment/year/season was further evaluated. To qualify as a reference location, at least three out of four water quality parameters had to be ‘good’ and

only one parameter could be ‘fair’. To qualify as a validation location, at least three

parameters had to be ‘poor,’ the other could be ‘fair’ and none could be ‘good.’ The

lists of reference and validation locations using this approach are found in Tables 

H-1 through H-4. 

▼H-2

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
▼H-3

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Table H-1. Reference locations for spring open-water, dissolved oxygen criteria

reference curve based on water quality parameters (Approach 1).

Segment Years

BOHOH 1994

CB2OH 1985 1986 1988 1989 1990 1991 1992 1993 1994

CB4MH 1985 1989 1992

CB5MH 1985 1986 1989 1991 1992

CB6PH 1989

CB7PH 1989

CB8PH 1989 1991 1992

CHKOH 1985 1986 1987

CRRMH 1985 1986 1988 1989 1992

EASMH 1987

ELKOH 1991

JMSOH 1985 1986 1987

JMSTF 1992 1993

MIDOH 1993

MPNOH 1985

MPNTF 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

PAXMH 1992

PIAMH 1985 1986 1989 1992 1994

PMKTF 1985 1986 1987 1988 1989 1990 1991 1993 1994

RPPMH 1989

RPPOH 1985 1986 1987

RPPTF 1985 1986 1987 1991 1992

TANMH 1986

Source: Chesapeake Bay Water Quality Monitoring Program database

http://www.chesapeakebay.net/data
▼H-4

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Table H-2. Validation locations for spring open water, dissolved oxygen criteria

reference curve based on water quality parameters (Approach 1).

Segment Years

BIGMH 1990

BOHOH 1986 1990 1992 1993

C&DOH 1986 1987

CB3MH 1990

CB6PH 1993

CB7PH 1993

CB8PH 1987 1993

CHOMH2 1986 1989 1990 1994

CHOOH 1985 1987 1988 1993 1994

CHSMH 1985

CHSOH 1985 1986 1988 1990 1991 1992 1993 1994

EBEMH 1989 1991 1993 1994

ELIPH 1987 1988 1989 1990 1991 1992 1993

ELKOH 1986 1987

FSBMH 1986 1987 1990 1991 1993

GUNOH 1988 1991

JMSMH 1988 1989 1991 1992 1993

JMSOH 1990

JMSPH 1985 1986 1987 1988 1990 1991 1992 1993 1994

LAFMH 1989 1990

MAGMH 1985 1986 1989 1990

MANMH 1987 1990 1994

MOBPH 1987 1993 1994

NANMH 1986 1987 1988 1990 1991 1992 1993 1994

NANTF 1986 1988 1990 1992 1993

PAXMH 1986 1990

PAXOH 1986 1988

PAXTF 1986 1989 1990 1991 1994

POCMH 1993 1994

POTMH 1990 1991

RHDMH 1991

RPPMH 1990 1991

SBEMH 1989 1991 1993 1994

SEVMH 1991 1993

SOUMH 1985 1990 1992

TANMH 1987

WBEMH 1989 1990 1991 1992 1993 1994

WICMH 1986 1987 1988 1989 1990 1991 1992 1993 1994

WSTMH 1986 1988 1991

YRKMH 1989 1991 1992

YRKPH 1986 1987 1988 1990 1991 1993 1994

Source: Chesapeake Bay Water Quality Monitoring Program database

http://www.chesapeakebay.net/data
▼H-5

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Table H-3. Reference locations for summer open-water, dissolved oxygen criteria

reference curve based on water quality parameters (Approach 1).

Segment Years

BIGMH 1993

CB1TF 1985 1986 1987 1990 1991 1992 1993 1994

CB2OH 1985 1986 1987 1988 1990 1991 1992 1993 1994

CB3MH 1992 1993

CB4MH 1985 1986 1987 1988 1990 1991 1992 1993 1994

CB5MH 1985 1986 1987 1988 1990 1991 1992 1993 1994

CB7PH 1986 1987

CB8PH 1986 1987 1988 1990 1991

CHKOH 1985 1992

CRRMH 1987 1988 1991 1992

EASMH 1986

ELKOH 1991 1992 1994

GUNOH 1985

JMSOH 1985 1986 1987 1990 1994

JMSTF 1991 1992

LCHMH 1986

MATTF 1987

MIDOH 1990 1991 1993 1994

MPNOH 1985 1986

MPNTF 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

PIAMH 1985 1986 1987 1992 1993

PISTF 1986 1987

PMKOH 1985

PMKTF 1985 1986 1987 1988 1989 1990 1991 1994

POTMH 1985 1986 1987 1991

POTOH 1986 1987 1988 1989 1990

POTTF 1987 1989 1990

RPPMH 1985 1986 1987

RPPOH 1985 1986 1987 1988 1991 1992 1994

RPPTF 1992 1994

TANMH 1986

Source: Chesapeake Bay Water Quality Monitoring Program database

http://www.chesapeakebay.net/data
▼H-6

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Table H-4. Validation for summer open-water, dissolved oxygen criteria reference

curve based on water quality parameters (Approach 1).

CBP

Segment Years

APPTF 1988 1990 1991 1992 1993

BOHOH 1986 1987 1988 1989 1992 1994

BSHOH 1985 1989

CB6PH 1989

CHOMH2 1989 1990 1991 1994

CHOOH 1985 1986 1987 1990 1991 1994

CHSMH 1989 1990 1993

CHSOH 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

ELIPH 1985 1986 1987 1988 1989 1990 1991 1992 1993

FSBMH 1988 1989

GUNOH 1993

JMSMH 1989

JMSPH 1987 1989 1991 1992 1993 1994

LAFMH 1989 1990

LCHMH 1989

MAGMH 1986 1987 1988 1989 1990 1991 1994

MANMH 1986 1987 1988 1989 1990 1991 1993 1994

MOBPH 1986 1989 1990 1991 1993

NANMH 1986 1987 1988 1989 1990 1991 1993 1994

NANTF 1989 1990 1992 1993 1994

NORTF 1989

PATMH 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

PAXMH 1988 1989 1993

PAXOH 1986 1989 1992 1994

PAXTF 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

POCMH 1989 1994

POTTF 1994

RHDMH 1985 1986 1987 1988 1989 1990 1991 1992

SASOH 1986 1987 1988 1989 1990 1991 1993

SBEMH 1992 1993 1994

SEVMH 1985 1987 1988 1989 1990 1991 1992 1994

SOUMH 1987 1988 1989 1990 1991 1994

WBEMH 1989 1990 1991 1992

WICMH 1986 1987 1988 1989 1990 1991 1993 1994

WSTMH 1985 1987 1988 1989 1990 1991 1994

YRKMH 1987 1989 1990 1991 1993

YRKPH 1986 1988 1989 1990 1991 1992 1993

Source: Chesapeake Bay Water Quality Monitoring Program database

http://www.chesapeakebay.net/data
Monthly mean dissolved oxygen concentration data were then interpolated basinwide

for each month, 1985 to 1994. In addition, a basinwide ‘master’ interpolated 3-dimensional grid file was created in which each cell has a Chesapeake Bay segment

assignment and a static Designated Use assignment (open-water [OW], deep-water

[DW] and deep-channel [DC]) based on proposed tidal water designated use boundaries

(U.S. EPA 2003). Each cell could thus be identified by the appropriate dissolved oxygen

concentration(s) associated with its respective designated use.

For each monthly baywide interpolation, the dissolved oxygen concentration in each

open-water designated use cell was compared to the appropriate criteria concentration for the season, and the percent of cells passing/failing the criteria calculated for

each segment/designated use. Using the respective lists of ‘good’ and ‘bad’ locations

(segment_years), the data for the reference and validation segments were extracted

and pooled in separate groups. For example, segment POCMH in spring 1993 and

1994 were identified as validation locations. The percent volume failing the criterion

in POCMH was calculated for each month—February, March, April and May of

1993 and 1994—and pooled with the percent-volume-failing data from other similarly identified locations. Then, the cumulative frequency distribution attainment

curves were derived for each pooled group. Figures H-1 and H-2 show the openwater designated use dissolved oxygen criteria reference and validation curves for

the spring and summer seasons generated applying water quality status approach.

It is clear that both reference (hatched line) and validation (solid line) areas meet the

spring 30- day 5 mg liter-1 criterion almost all if not 100 percent of the time. If there

are areas that do not meet this criterion in spring, this method does not detect them.

There also is little apparent distinction between the illustrated reference and validation curves in summer (Figure H-2).

However, when the summer data are separated by salinity zone (figures H-3 and 

H-4), there are distinct differences between the reference and validation curves. In

tidal fresh and oligohaline segments, overall exceedance is low, but reference areas

have more apparent exceedance than validation areas. The reverse is true for meso-

▼H-7

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-1. Spring open-water reference (hatched line) and validation (solid line) curves:

water quality status approach.
▼H-8

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-2. Summer open-water reference (hatched) and validation (solid line) curves:

water quality status.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-3: Lower salinity summer open water reference (hatched line) and validation

(solid line) curves: water quality status approach.

0 10 20 30 40 50 60 70 80 90 100

10

20

30

40

50

60

70

80

90

100

0

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-4. Higher salinity summer open water reference (hatched line) and validation

(solid line) curves: water quality status approach.
haline-polyhaline segments where exceedance is generally greater. An important

point to remember is that, while we usually think of the open-water habitat as the

surface-mixed layer, the open-water criteria are applicable throughout the water

column in areas that do not experience chronic vertical stratification. There are such

areas in the basin that are quite deep but usually do not have a pycnocline. These

areas are more commonly found in mesohaline and polyhaline segments than in the

tidal-fresh and oligohaline waters. This is one likely factor in the difference between

the reference and validation curves. Another factor could be that surface waters in

the validation segments in the tidal-fresh and oligohaline zones are more affected by

the oxygen-generating processes of algal blooms, whereas the mesohaline and polyhaline validation segments are more affected by oxygen-consumptive processes

occurring in the deep water layers beneath them.

The cumulative frequency distribution curves for reference locations using the water

quality status method show that areas with low nutrients, chlorophyll a and

suspended solids levels also have dissolved oxygen levels that do not greatly exceed

the applicable criterion. On the other hand, the validation curves suggest these

parameters are not good indicators of locations with dissolved oxygen criteria attainment levels. The mesohaline and polyhaline curve shows some nonattainment, but

conditions are far better than those reflected in the validation curve derived from the

ranking exercise described below. This result is essentially as expected since, in most

of the Chesapeake Bay and tidal tributaries, the link between the water quality

parameters and dissolved oxygen has a number of intermediate steps and the

dissolved oxygen response to water quality parameters is often displaced in time or

space or both.

Approach 2: Segments with Highest and Lowest Long-term

Dissolved Oxygen Concentrations

The ranking procedure for selecting reference and validation segments was based on

observed (i.e., not interpolated) data. Dissolved oxygen measurements are available

for each monitoring station at 1- to 2-meter intervals from surface to bottom. The

depth of the pycnocline, if one existed, also is available. For this analysis, all

dissolved oxygen measurements above the pycnocline, or the shallower of all measurements above 7 meters or above the bottom if there was no pycnocline, were

assumed to be in open-water designated use habitats. To control for supersaturating

conditions, dissolved oxygen concentrations that were above saturation levels

(calculated from temperature and salinity measured concurrently) were set down to

the saturation level.

Spring (March through May) and summer (June through September) data were averaged first by date and station; then by month and segment; then the 10th, 50th and 90th

percentiles of the monthly segment averages were calculated for spring and summer

seasons over the 1985-1994 period. The seasonal median, i.e., 50th percentile, was

used to rank the segments (tables H-5 and H-6). Some segments were excluded,

resulting in 67 segments that were ranked. The excluded Chesapeake Bay Program

▼H-9

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves
▼H-10

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Table H-5. Chesapeake Bay Program segments listed in order of spring open-water designated use,

seasonal median dissolved oxygen concentration.

CBP 10th 90th

Segment Median Mean percentile percentile

WICMH 8.0 8.4 6.2 11.5

PMKTF 8.0 8.1 6.3 10.1

YRKMH 8.0 8.1 6.3 10.3

POCTF 8.1 7.9 5.4 10.2

MPNOH 8.1 8.1 6.1 10.3

PMKOH 8.1 8.2 6.2 10.3

CHOOH 8.6 8.9 7.2 10.9

MPNTF 8.7 8.6 6.9 10.5

PAXOH 8.7 8.8 7.0 10.8

YRKPH 8.7 8.7 7.0 10.3

ELIMH 8.8 8.8 6.5 11.0

ELIPH 8.8 8.8 7.1 10.7

JMSMH 8.9 9.0 7.4 10.7

FSBMH 8.9 9.3 7.4 11.7

RPPTF 9.0 9.3 7.3 11.3

CHSOH 9.1 9.1 7.3 10.8

MANMH 9.1 9.1 7.4 10.9

JMSPH 9.1 9.2 7.5 11.1

NANMH 9.1 9.3 7.4 11.1

MOBPH 9.1 9.2 7.7 10.7

RPPOH 9.2 9.2 7.4 11.0

POTOH 9.2 9.4 7.8 11.4

APPTF 9.2 9.4 8.0 11.2

POCMH 9.2 9.3 7.9 10.9

BIGMH 9.3 9.3 7.5 11.1

CB8PH 9.3 9.3 8.0 10.9

PAXTF 9.3 9.2 7.3 10.6

TANMH 9.4 9.3 7.4 11.2

PATMH 9.4 9.4 7.7 11.0

RPPMH 9.4 9.2 7.3 10.9

CHOMH2 9.4 9.4 7.6 11.4

CRRMH 9.4 9.2 7.1 10.9

CHKOH 9.4 9.2 7.1 11.1

JMSOH 9.5 9.4 7.9 11.0

CBP 10th 90th

Segment Median Mean percentile percentile

CB7PH 9.5 9.4 7.9 10.9

PIAMH 9.6 9.6 8.0 11.2

PAXMH 9.6 9.3 6.9 11.2

MAGMH 9.6 9.7 7.5 11.5

NANTF 9.6 9.5 7.5 11.4

CHSMH 9.6 9.8 8.1 11.7

POTTF 9.7 9.8 7.9 11.8

RHDMH 9.7 9.6 7.8 11.6

WSTMH 9.7 9.7 7.4 11.9

CB6PH 9.7 9.7 8.0 11.2

JMSTF 9.7 9.6 8.2 10.8

CB3MH 9.7 9.4 7.6 11.2

SOUMH 9.7 9.2 5.8 11.5

CB2OH 9.7 9.8 7.5 11.8

POTMH 9.7 9.7 7.9 11.7

BSHOH 9.8 10.0 8.1 12.0

C&DOH 9.8 10.0 8.0 12.0

BACOH 9.9 9.9 8.0 12.0

EASMH 9.9 9.9 8.0 11.5

PISTF 9.9 10.1 7.9 11.9

SEVMH 9.9 9.8 7.7 11.7

LCHMH 9.9 9.8 8.3 11.4

CB5MH 10.0 10.0 8.3 11.5

MATTF 10.0 10.0 8.5 11.7

ELKOH 10.0 10.1 8.3 12.1

CB4MH 10.1 9.9 8.1 11.4

CHOMH1 10.1 9.8 7.8 11.3

SASOH 10.1 10.1 8.4 11.9

MIDOH 10.3 10.3 8.7 12.2

NORTF 10.4 10.5 9.2 12.3

BOHOH 10.4 10.2 8.6 12.4

GUNOH 10.5 10.4 8.6 12.1

CB1TF 11.0 10.7 8.6 12.5

Source: Chesapeake Bay Water Quality Monitoring Program database

http://www.chesapeakebay.net/data
▼H-11

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Table H-6. Chesapeake Bay segments listed in order of summer dissolved oxygen designated use,

seasonal median dissolved oxygen concentration. 

CBP 10th 90th

Segment Median Mean percentile percentile

SOUMH 4.2 4.5 2.7 7.1

MAGMH 4.7 4.8 3.3 6.9

PMKTF 4.9 5.0 4.4 5.8

MPNOH 4.9 4.9 4.0 5.6

POCTF 4.9 5.1 3.7 7.1

PMKOH 5.0 4.9 4.1 5.7

YRKMH 5.2 5.2 4.5 5.8

PAXMH 5.4 5.4 4.8 6.1

YRKPH 5.5 5.5 4.9 6.2

MPNTF 5.5 5.5 4.6 6.4

ELIMH 5.6 5.7 4.4 7.1

ELIPH 5.6 5.7 4.6 6.8

WICMH 5.8 5.7 4.6 6.8

CRRMH 5.9 5.8 4.5 6.8

SEVMH 5.9 5.9 5.1 7.6

PAXOH 5.9 5.9 4.9 7.0

CHOMH2 6.0 6.0 5.3 6.8

PATMH 6.1 6.0 4.8 7.0

POTMH 6.1 6.1 5.4 6.8

WSTMH 6.1 6.1 5.0 7.5

JMSMH 6.2 6.2 5.6 6.7

RPPMH 6.2 6.2 5.5 6.7

CB3MH 6.2 6.1 5.4 6.8

CHOOH 6.3 6.3 5.4 7.2

POTOH 6.3 6.3 5.6 .1

CB2OH 6.4 6.3 5.6 6.8

NANMH 6.4 6.4 5.7 7.4

CHKOH 6.5 6.4 5.3 7.4

C&DOH 6.5 6.5 5.8 7.2

TANMH 6.5 6.5 5.9 7.1

MOBPH 6.5 6.4 5.6 7.0

JMSPH 6.5 6.6 6.0 7.2

MANMH 6.6 6.6 5.9 7.2

RPPOH 6.6 6.5 5.7 7.3

CBP 10th 90th

Segment Median Mean percentile percentile

FSBMH 6.6 6.5 5.8 7.3

ELKOH 6.6 6.5 5.8 7.2

CHSMH 6.6 6.5 5.8 7.3

RHDMH 6.6 6.6 5.5 7.9

POTTF 6.6 6.6 6.0 7.2

JMSOH 6.7 6.7 6.1 7.2

CB6PH 6.7 6.8 6.2 7.5

POCMH 6.7 6.7 6.3 7.3

LCHMH 6.7 6.7 5.9 7.5

PIAMH 6.7 6.6 5.7 7.4

CB4MH 6.7 6.6 6.0 7.2

CB7PH 6.8 6.9 6.4 7.3

CHOMH1 6.8 6.9 6.3 7.6

SASOH 6.8 6.5 4.1 8.0

CB5MH 6.9 6.9 6.4 7.5

BIGMH 6.9 6.9 6.3 7.4

EASMH 6.9 6.9 6.2 7.5

CB8PH 6.9 6.9 6.4 7.5

BOHOH 7.0 7.1 5.7 8.3

CB1TF 7.0 7.0 6.4 7.8

JMSTF 7.0 7.0 6.4 7.5

PISTF 7.0 6.7 5.4 7.6

APPTF 7.2 7.1 5.8 8.0

RPPTF 7.2 7.2 6.6 8.0

PAXTF 7.2 7.1 6.0 8.1

MIDOH 7.3 7.1 5.7 8.0

CHSOH 7.3 7.2 6.1 8.3

NANTF 7.4 7.1 5.9 8.3

GUNOH 7.5 7.3 6.1 8.5

BACOH 7.7 7.3 5.5 8.5

BSHOH 7.8 7.4 5.5 8.7

NORTF 7.9 7.8 7.2 8.4

MATTF 7.9 7.9 7.4 8.6

Source: Chesapeake Bay Water Quality Monitoring Program database

http://www.chesapeakebay.net/data
segments were: the Western Branch of the Patuxent River (WBRTF) because it is a

small water body dominated by a waste water treatment plant; the mesohaline tributaries of the Elizabeth River (SBEMH, EBEMH, and WBEMH) because the

dissolved oxygen interpolations did not extend to those segments or the data record

was too short; and the Lafayette River (LAFMH) because it contains no water

quality monitoring station. Within season, the highest ranked 14 segments made up

the list of reference locations and the lowest ranked 14 segments constituted the list

of validation locations.

Monthly mean dissolved oxygen concentration data were then interpolated basinwide for each spring and summer month, 1985 to 1994. For each interpolation, the

dissolved oxygen concentration in each cell qualifying as open-water was compared

to the appropriate criteria concentration for the month, and the percent of cells

passing/failing the criteria was calculated for each segment or designated use. Using

the respective lists of ‘good’ and ‘bad’ locations, the data for the reference and validation segments were extracted, pooled and plotted (Figure H-5).

The reference curve (hatched line) using this approach looks too good to be true. The

Chesapeake Bay Program segments with the highest dissolved oxygen levels include

a number of segments known to be eutrophic, with high chlorophyll a concentrations. These segments are likely to have elevated daytime dissolved oxygen

concentrations due to the addition of oxygen from photosynthesis, but these are also

frequently associated with nighttime dissolved oxygen sags when photosynthesis

stops and respiration increases. This curve is, therefore, not a valid reference curve.

▼H-12

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Percentage of Area/Volume Exceeding the Criteria

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Figure H-5. Open water reference and validation curves for summer based on the best

and worst ~20 percent of all segments approach.
▼H-13

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Approach 3: Using only Polyhaline Segments

In this exercise, the interpolated data sets from Chesapeake Bay Program segments

western lower Chesapeake Bay (CB6PH), eastern lower Chesapeake Bay (CB7PH),

mouth of the Chesapeake Bay (CB8PH), mouth of the York River (YRKPH),

Mobjack Bay (MOBPH), mouth of the James River (JMSPH) and Elizabeth River

(ELIPH) were processed as described above. The percent attainment for each month

in spring and summer seasons was calculated for the open-water designated use cells

in each polyhaline segment. These data were pooled and a cumulative frequency

distribution curve generated for each season. The cumulative frequency distribution

curve for summer is shown below (Figure H-6). In the ranking exercise above, the

York (YRKPH) and Elizabeth (ELIPH) river segments fell in the lowest ranked group

of 14 segments while the other polyhaline segments were scattered in the middle

range in both spring and summer seasons (see tables H-5 and H-6, respectively). 

With regard to this reference curve and all of the validation curves, it should be noted

that summer temperature and salinity conditions, particularly in the Elizabeth River

and occasionally elsewhere, can be such that oxygen saturation concentrations are

below the open-water dissolved oxygen criterion concentration, and it is impossible

to meet the criteria due to natural physical conditions. According to proposed implementation guidance, nonattainment is forgiven under those conditions. In this

analysis, nonattainment for this reason was not taken into account and, depending on

how often such conditions occurr, this and the other curves may be more accurate.

Approach 4: Reference and Validation Curves using 

Benthic Community Health

Benthic community health is the reference and validation site identifier for the deepwater reference curves. In the absence of other biologically-based indicators for

open-water, open-water reference curves based on benthic health were explored for

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-6. Summer open water reference curve: polyhaline segments only approach.
▼H-14

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

comparison with the other approaches. The logic was that Chesapeake Bay benthic

organisms have a high tolerance for low dissolved oxygen concentrations, thus

healthy benthos in open-water habitat would not necessarily indicate that the 30-day

mean of 5 mg liter-1 was met. On the other hand, a stressed benthic community in an

open-water designated use habitat could indicate that dissolved oxygen criteria in the

habitat zone were not met. 

Reference and validation locations (tables H-7 and H-8, respectively) were identified

by methods described below (see section titled “Deep Water Criteria Reference

Curves”) and the frequency and extent of criterion attainment were processed as

described below and similar to the other approaches for open-water. Figure H-7

shows the curves resulting from pooling all reference and validation segments in

their respective groups. Figures H-8 and H-9 show the results further segregating

segments by salinity zone.

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-7. Summer open-water dissolved oxygen criteria reference (hatched line) and

validation (solid line) curves based on the benthic index of biotic integrity.

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-8. Lower salinity summer open-water reference (hatched line) and validation

(solid line) curves: benthic community health approach.
▼H-15

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Table H-7. Reference locations based on benthic index ≥3 for summer 

open-water dissolved oxygen criteria reference curve (Approach 4).

CBP

Segment Years

CB1TF 1985 1987 1990 1991 1992

CB2OH 1986 1988

CB3MH 1988 1993 1994

CB6PH 1986 1990 1991 1992 1993

CB7PH 1988 1990 1992 1993 1994

CB8PH 1985 1986 1987 1989 1990 1991 1992 1993 1994

CHOMH1 1987

CHOMH2 1986 1993 1994

CHSMH 1986 1987

CHSOH 1992

ELKOH 1986 1992

JMSMH 1985 1988 1990 1991 1992 1994

JMSOH 1988

JMSPH 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

JMSTF 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

NANMH 1986 1988

PAXMH 1987 1988

PAXOH 1986 1987

PAXTF 1987 1994

PMKTF 1991 1992 1993 1994

POTOH 1986 1987 1988

POTTF 1988

RPPMH 1985 1986 1987 1988 1990 1992

RPPOH 1988 1992

SASOH 1992

YRKMH 1985 1986 1987 1988 1990

Source: Chesapeake Bay Water Quality Monitoring Program database

http://www.chesapeakebay.net/data
▼H-16

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Table H-8. Validation locations based on benthic index <3 for summer 

open-water dissolved oxygen criteria reference curve (Approach 4).

CBP

Segment Years

BIGMH 1994

CB1TF 1986 1989

CB2OH 1987

CB5MH 1986 1987 1989 1992

CB6PH 1987 1988

CB8PH 1988

CHOMH1 1986 1988 1994

CHOMH2 1988

CHOOH 1986 1987 1988 1992 1994

CHOTF 1991 1992

CHSMH 1990

EASMH 1994

ELKOH 1987 1989 1990 1994

HNGMH 1994

JMSMH 1986 1987 1989 1993

JMSOH 1985 1986 1987 1989 1991 1992 1994

LCHMH 1985 1994

PATMH 1985 1987

PAXOH 1994

PAXTF 1989

PMKTF 1985 1986 1987 1988 1990

POTTF 1986

RPPMH 1994

RPPOH 1985 1986 1987 1990 1991 1993 1994

SASOH 1991

SBEMH 1989 1990 1991 1992 1993

YRKMH 1993 1994

Source: Chesapeake Bay Water Quality Monitoring Program database

http://www.chesapeakebay.net/data
In the tidal-fresh and oligohaline group, the two curves based on benthic health are

not very different from one another and both show little overall exceedance (Figure

H-8). Nevertheless, the reference curve appears to have slightly more exceedance

than the validation curve. By contrast, the two curves in the higher salinity group are

differentiated from each other and the reference curve shows more attainment (i.e.,

less exceedance) than the validation curve (Figure H-8). 

DEEP-WATER CRITERIA REFERENCE CURVES

Chesapeake Bay benthic communities are relatively tolerant of lower oxygen

concentrations and able to compensate for periodic hypoxia. A dissolved oxygen

concentration of 2 mg liter-1 is considered the lower threshold below which benthic

communities start to become severely stressed. A healthy benthic community, therefore, could indicate that dissolved oxygen conditions are meeting the deep-water

30-day mean 3 mg liter-1 criterion, but would not necessarily indicate that the openwater 30-day mean 5 mg liter-1 criterion was met. 

A baywide, long-term benthic monitoring program has been in place since before

1985. Samples are collected at fixed and random locations in the summer season,

usually in August/September. A benthic index of biological integrity (benthic-IBI)

has been developed to assess the status of benthic communities (Weisberg et al.

1997). The benthic IBI is based on a number of parameters, some depending on

salinity zone. Abundance, biomass, species diversity and pollution sensitivity are

some of the attributes on which the index is based. Each of the attributes is scored

on a scale of 1 to 5 against a benchmark community. The benthic-IBI is the average

of these scores and also ranges from 1 to 5. 

Chesapeake Bay Program segments with benthic communities having an index of 3

were considered healthy. It was further assumed that if the community was not

stressed, then the dissolved oxygen conditions were likely to have been adequate for

▼H-17

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-9. Higher salinity summer open-water reference (hatched line) and validation

(solid line) curves: benthic community health approach.
▼H-18

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

the previous one to two months of the summer. Thus, in this analysis, if a healthy

benthic sample identified a reference location and it was not otherwise disqualified by

a sample indicating stress, then data for the whole season for that segment/designated

use/season_year were included in the reference distribution. Each benthic sample is

identified by latitude/longitude, segment and bottom depth. This analysis identified each

site by year (assuming months June through September), segment and depth. It did not

take into account a station’s specific location within the segment. 

The Chesapeake Bay benthic-IBI results from 1985 through 1994 were assessed as

either 3 (‘healthy’/’good’) or <3 (‘stressed’/‘not good’) and then associated with

season_year (in this case summer), segment and designated use, based on season and

sample depth. ‘Healthy’ locations were accumulated as the reference distribution.

‘Stressed’ locations were accumulated as the validation distribution. If both healthy

and stressed sites occurred within the same segment, designated use and

season_year, the location was excluded from both reference and validation distributions. A listing of the reference and validation locations identified in this way is

attached (tables H-9 and H-10, respectively). 

For this exercise, like those described earlier, a baywide master grid file was used in

which each cell has a Chesapeake Bay Program segment assignment and fixed designated use assignment. In a few segments, both open-water and deep-water

designations occur at the same depth. Because of time limitations,the location of

healthy benthic samples was identified only by segment and depth, not by specific

latitude/longitude, i.e., not by specific grid cell. (Note: Using GIS to locate the

comparable grid cell precisely for each sample would improve the analysis greatly,

but complicate the process.) Thus, when a ‘healthy’ benthic sample was found at a

segment depth where both open-water and deep-water designated uses were defined,

both were included in their respective list of reference or validation locations.

Monthly mean dissolved oxygen concentration data were interpolated basinwide for

each summer month, June through September, from 1985 through 1994. For each

interpolation, each cell’s dissolved oxygen concentration was compared to the

appropriate criteria concentration for the month and designated use, as indicated in

the master grid, and the percent of cells passing/failing the criteria calculated for

Table H-9. Reference locations based on benthic index ≥3 for summer

deep-water dissolved oxygen criteria reference curve.

CBP

Segment Years

CB3MH 1992

CB6PH 1985 1986 1987 1988 1991 1992 1993 1994

CB7PH 1985 1986 1991

CHSMH 1992 1993

PAXMH 1992

Source: Chesapeake Bay Water Quality Monitoring Program database

http://www.chesapeakebay.net/data
each segment/designated use. Using the respective lists of locations/dates, the data

for the reference and validation locations were extracted, pooled and plotted (Figure

H-10). This approach illustrates a substantial difference between the attainment

curves of healthy and stressed sites. The curves would likely be different (i.e., likely

reduce nonattainment in the reference curve and increasing nonattainment in the

validation curve) if the location selection process were made more specific as

described earlier.

▼H-19

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Table H-10. Validation locations based on benthic index <3 for summer

deep-water dissolved oxygen criteria reference curve.

CBP

Segment Years

CB3MH 1989 1990 1991 1993

CB4MH 1986 1988 1989 1990 1991 1992 1993 1994

CB5MH 1989 1990 1991 1992 1993 1994

CB6PH 1990

CB7PH 1987

CHSMH 1989

EASMH 1986

PATMH 1989 1990 1991 1992 1993 1994

PAXMH 1987 1988 1989 1990 1993 1994

POTMH 1985 1986 1987 1988 1990 1991 1993 1994

RPPMH 1985 1986 1988 1990 1991 1993 1994

YRKPH 1988 1990 1991 1992 1993 1994

Source: Chesapeake Bay Water Quality Monitoring Program database

http://www.chesapeakebay/net/data

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-10. Summer deep-water reference (hatched line) and validation (solid line)

curves: benthic community healt approach.
WATER CLARITY CRITERIA REFERENCE CURVES

The water clarity criteria were developed to be protective of underwater bay grasses.

The criteria apply to the months within the underwater bay grasses growing seasons

and are specific to salinity zone. Reference areas in each salinity zone were selected

by a team of resource managers and underwater bay grasses scientists based on an

extensive review of the available distribution and abundance data record (over 20

years). Chesapeake Bay Program segments or partial segments were identified where

underwater bay grasses distributions had increased significantly in recent years and

had been present historically (Table H-11). Reference curves were developed for

percent light-through-water (PLW), which is obtained by PLW=100exp(-KdZ),

where Z is the applicaton depth and Kd is a light factor, derived here from Secchi

depth (Kd= 1.45/Secchi depth); see Chapter VI for more detail on implementation of

the water clarity criteria. Application depth (Z) was based on photographic or other

evidence of growth at that depth plus one-half the tide depth in the segment. The

empirical evidence, with the one-half tide height added, provided a range of depths

from which an appropriate depth was selected for inclusion in the PLW calculation.

In some segments, full attainment was achieved at the deepest depth of the range. In

those cases, Z was increased at 0.1 meter increments until exceedance was detected.

▼H-20

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

Table H-11. Chesapeake Bay Program segments or partial segments used to

establish the water clarity criteria reference curves.

Minimum Maximum Selected

Restoration Retoration Restoration Restoration

CBP Target Depth Target Depth Target Depth Target Depth

Segment (meters) (meters) (meters) (meters)

CB1TF 2.0 0.5 1.3 0.9

GUNOH 2.0 0.25 0.8 0.5

MATTF 2.0 0.25 0.8 0.5

PISTF 2.0 0.5 1.5 0.5

POTTF 2.0 0.5 1.4 0.6

POTOH 2.0 0.5 1.2 0.75

CB6PH 2.0 0.5 1.3 1.3

CB7PH 1.0 0.5 1.3 1.3

CHOMH1 2.0 0.5 1.3 1.25

EASMH 2.0 0.25 0.8 1.1

MOBPH 2.0 0.5 1.5 1.2

TANMH 2.0 0.5 1.3 0.9

YRKPH 2.0 0.25 1.0 1.2

Source: Chesapeake Bay Water Quality Monitoring Program database

http://www.chesapeakebay.net/data
Like the methods used to determine attainment of the dissolved oxygen criteria,

ambient light data collected as part of the Chesapeake Bay water quality monitoring

program were averaged monthly and interpolated (using the log transformation).

PLW was calculated for each surface cell using the selected Z depth and the interpolated (back transformed) value for Kd. The PLW value for each cell was compared

to the appropriate criterion for the segment’s salinity zone and the cell area designated as failing or passing the criterion. The spatial extent of attainment, i.e., the

percent area failing the criterion, was tallied for each month in the underwater bay

grass growing season for all years 1985 through 1994, and also for more recent years

through 2000. The monthly figures for percent attainment in each segment were

pooled within salinity classification: tidal-fresh oligohaline and mesohaline polyhaline and the cumulative frequency distribution calculated and plotted. Note that

segment CB7PH was not included in order to balance the relative contributions from

the different salinity zones. The plots were very similar with and without segment

CB7PH. The reference curves from the 1985-94 period (figures H-11 and H-12) are

consistent with the curves developed for the other criteria. The reference curves for

1995-2000 are shown for comparison (figures H-13 and H-14).

It should be noted that the PLW minimum light requirement parameter was originally developed as a seasonal median measure. For assessing the criteria attainment,

light availability is evaluated on a monthly basis, recognizing that available light

could be less than the requirement level (i.e, 13 percent and 22 percent in lower and

higher salinity waters, respectively) about half the time, and that exceedances will be

more frequent than if the criteria were assessed on a seasonal basis. Because both

criteria attainment and reference curves will be assessed in the same way, the additional exceedance should be accounted for by the reference curve. Figures H-15 and

H-16 illustrate the lower and higher salinity water clarity reference curves, respectively, resulting from assessment on a seasonal median basis. 

▼H-21

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

0 10 20 30 40 50 60 70 80 90 100

Percent Area Failed

Seasonal Median

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-11. Lower salinity water clarity reference curve: 1985–1994.
▼H-22

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-14. Higher salinity water clarity reference curve: 1995–2000.

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-12. Higher salinity water clarity reference curve: 1985–1994.

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-13. Lower salinity water clarity reference curve: 1995–2000.
LITERATURE CITED

Alden, R. W. III, and E. S. Perry 1997. Presenting Measurements of Status: Report to the

Chesapeake Bay Program Monitoring Subcommittee’s Data Analysis Workgroup. Chesapeake Bay Program, Annapolis, Maryland.

Weisberg, S. B., J. A. Ranasinghe, D. M. Dauer, L. C. Schaffner, R. J. Diaz, and J. B.

Frithsen. 1997. An estuarine benthic index of biotic integrity (B-IBI) for Chesapeake Bay.

Estuaries 20:149-158. 

▼H-23

appendix H • Derivative of Cumulative Frequency Distribution Criteria Attainment Reference Curves

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-16. Higher salinity water clarity reference curve: seasonal median.

0 10 20 30 40 50 60 70 80 90 100

0

10

20

30

40

50

60

70

80

90

100

Percentage of Time a Specified 

Percentage or More of Area/Volume 

Exceeds the Criteria 

Percentage of Area/Volume Exceeding the Criteria

Figure H-15. Higher salinity water clarity reference curve: seasonal median
The Chesapeake Bay dissolved oxygen criteria have several different durations: 30-

day mean, 7-day mean, daily mean and instantaneous minimum. Users’ ability to

assess these criteria and to have certainty in the results depends on the time scale of

available data and on the ability of models to estimate conditions at those time

scales. At present, long-term, fixed-station, midchannel water quality monitoring in

the Chesapeake Bay and its tidal tributaries provides dissolved oxygen measurements twice monthly at most or approximately every 15 days between April and

August. Proposed enhancements to the tidal water quality monitoring program

include shallow-water monitoring, as well as high-resolution spatial and temporal

monitoring in selected locations. However, these new components are only in the

planning and early implementation stages at this point, and because of financial

constraints or limitations to current technology, direct monitoring at the scales of the

criteria may not be possible in the foreseeable future. Therefore, the assessment of

attainment for some geographic regions and for some short-term criteria elements

must be waived for the time being or must be based on statistical methods that estimate probable attainment. Several approaches to addressing the duration issue are

described below in more detail.

LOGISTIC REGRESSION MODELS USING ROUTINE

FIXED-STATION MONITORING DATA

This method is a modification and significant update of a method developed originally to measure attainment of the 1992 Chesapeake Bay dissolved oxygen

restoration goal (Jordan et al. 1992). The early work demonstrated predictable relationships, on a segment by segment basis, between seasonal mean dissolved oxygen

concentrations and the percent of observations above a target concentration in areas

where dissolved oxygen concentrations ranged above and below goal target concentrations (figures I-1 and I-2). 

▼I-1

appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria

appendixI 

Analytical Approaches for Assessing 

Short-Duration Dissolved Oxygen Criteria
The relationships were then expressed as regression equations, which could be used

to predict the percentage of observations above or below target for any seasonal

mean. By extension, the ‘percentage of observations’ applied both to space and time

within a segment. Most of this pilot work was done for mainstem Chesapeake Bay

segments that were relatively densely populated with fixed monitoring stations laterally and longitudinally. Because of the spatial density of stations, the range of

potential dissolved oxygen exposure that any particular point might experience over

a tidal cycle was captured in the models. Contemporaneous, semicontinuous

dissolved oxygen measurements made with in situ sensors deployed on buoys were

used to validate the model estimates. The models did not predict the extreme minima

▼I-2

appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria

Figure I-1: Percent of Summer dissolved oxygen concentrations above 1.7 mg liter-1 in

segment CB3MH deep-water designated use habitat.

Figure I-2. Percent of Summer dissolved oxygen concentrations above 3 mg liter-1 in

segment CB3MH open-water designated use habitat.
recorded in the continuous record, but were able accurately to predict the frequency

of observations below the mean, 5th and 95th percentile concentrations over a

month’s time (Jordan et al. 1992). 

With benefit of the long (more than 16 yrs) record of the Chesapeake Bay tidal-water

quality monitoring program and the density of measurements (the vertical dissolved

oxygen profile is characterized at 1- to 2-meter intervals), the simple 1992 regression models have been improved. These enhanced models use logistic regression,

which is better suited to percent distributions (i.e., distributions between 0 and 100).

The models are now month- and depth-specific in many segments. These models can

be adapted and applied to estimate attainment of the instantaneous minimum

dissolved oxygen criterion, if the user considers that the minimum criterion is not

met if the dissolved oxygen concentration is below the criterion value at any time

and anywhere in the segment-designated use. 

The first step is to reconstruct the models using the cruise-by-cruise three-dimensional interpolations of dissolved oxygen monitoring data. That is, collect the

percent volume passing/failing the criterion at each depth in a segment month by

month from, for example, 1985 through 1998, and model the relationship of percent

volume failing/passing the criterion as a function of the monthly mean of that

segment/depth as represented by all the cells in the grid. Using the interpolated data

should improve the spatial representation within the segment. 

To assess current attainment, for 1999-2001 for example, the user would interpolate

the monthly average dissolved oxygen concentrations across all tidal waters, as

before, for each month of the season to be evaluated in the assessment period, e.g.,

the summer period including June through September. Then the month/segment/

depth-specific model appropriate for the designated use and cell location would be

applied to estimate the percent of time each cell was likely to be below the instantaneous minimum, based on the cell’s interpolated monthly average. If the model

predicts that the cell is above the minimum dissolved oxygen level less than 99-100

percent of the time, then the cell is not in attainment. Each cell is assessed in this

way and its volume added to the ‘failing’ or ‘passing’ category. Ultimately, the

percent of total volume failing or passing the criterion within the segment and designated use is calculated for each month. The monthly percentages are tallied over all

months in the season in the assessment period and the cumulative frequency distribution is calculated. Except for the use of the logistic regression model, each of the

steps is consistent with assessment methods of the other criteria (see Chapter VI for

details). 

The following is a sample attainment model for Chesapeake Bay Program segment

CB3MH for the open-water instantaneous minimum 3.2 mg liter1 criterion. This

model was based on fixed station data, not on interpolated data as proposed above. 

If the time frame is September through March, attainment is likely to be

100 percent, regardless of depth. For other months, the estimated percent

attainment is estimated by 

▼I-3

appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria
LGT = 1.0757 x (mean monthly dissolved oxygen concentration)

-0.0724 x (depth in meters) 

-1.8576 for April

-2.9219 for May

-2.7982 for June

-2.8341 for July

-2.5443 for August.

Percent attainment = 100 exp(lgt) / (1 + exp(lgt)).

The figures below illustrate attainment curves for segment CB3MH for summer

deep-water designated use, where the instantaneous minimum criterion is 1.7 mg

liter-1 (Figure I-3) and open-water designated use, where the instantaneous minimum

of 3.2 mg liter-1 criterion applies (Fig. I-4). 

At this time the method has not been adequately validated in areas other than the

mainstem Chesapeake Bay. It is, therefore, premature to recommend its implementation for formally assessing criteria attainment. The issue is not so much the method

itself, but how well the midchannel stations represent the flanks and surrounding

waters where station density is low. The models are only as good as the information

on which they are developed. The day-time sampling schedule cannot detect

nocturnal lows in shallow areas or other areas where dissolved oxygen is quickly

regenerated. In the mainstem Chesapeake Bay, however, the dissolved oxygen

‘memory’ in the deep waters represents, to some extent, the night-time dissolved

oxygen sags and the station density captures exposure diversity. In other areas and

designated uses, hypoxia is more ephemeral temporally and the density of fixedmonitoring stations is reduced spatially so the models are likely to be weaker. 

▼I-4

appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria

Figure I-3. Summer instantaneous minimum deep-water dissolved oxygen 1.7 mg liter-1

criterion attainment curve for segment CB3MH based on application of the logistic

regression model.
SYNTHETIC, CLOSE-INTERVAL DATA SETS 

CREATED BY COMBINING SHORT- AND 

LONG-TERM PATTERNS OF VARIATION 

IDENTIFIED THROUGH SPECTRAL ANALYSIS

It is critical to obtain real time information about the ephemeral and episodic events

of low dissolved oxygen. However, it is not possible for the states or other partners

to collect such information over vast regions of the Chesapeake Bay and over the

length of time that would be required to address the short duration (i.e., the 7-day

mean, 1-day mean, instantaneous minimum) criteria directly. The following methodology is proposed to address the short interval criteria by integrating information

from the long-term, low-frequency monitoring program with short-term, highfrequency monitoring that can be accomplished using in-situ semi-continuous data

recorders. 

The method, still in development, is an adaptation of work by Neerchal (1992). The

method combines temporal variability information from the long-term, lowfrequency monitoring data with that from short-term, high-frequency data such as

collected with in situ continuous recording devices. Since these devices are often

deployed using moored buoys, the associated data are referred to as ‘buoy data’.

The method uses spectral analysis to extract the cyclical components of the long- and

short-term dissolved oxygen time series records and combines them to create a

synthesized time-series data set with data synthesized at user-specified time steps.

The synthetic data have the annual and seasonal cyclic and trend characteristics of

the long-term record as well as the tidal, diurnal and other periodic characteristics of

the short-term, high-frequency record. At present, the synthetic data are hourly, with

▼I-5

appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria

Figure I-4. Summer instantaneous minimum open-water dissolved oxygen 3.2 mg liter-1

criterion attainment curve for segment CB3MH based on application of the logistic

regression model.
cyclic components limited to two cycles per day. The synthetic data are then

analyzed like any other data set relative to the specific elements of the criteria. 

The spectral equation for the long-term data (such as the Chesapeake Bay water

quality monitoring program data) is

a) (Equation I-1)

where ltm n/2, flt

k = fourier frequencies, t = time in months, a,b = spectral coefficients, and xlt

t = data.

The spectral equation for the short-term data (for example, from an in situ dissolved

oxygen data recorder) is

b) (Equation I-2)

where stm n/2, fst

k = fourier frequencies, t = time in hours, a,b = spectral

coefficients, and xst

t = data.

The equation for the spectral forecast (for the synthetic data set) is

c)

where t = time scaled to suit f, m1 < ltm, is chosen to exclude high frequencies that

would be duplicated in the short-term equation (>1⁄2 cycle per month), and m2 < stm

is chosen to exclude frequencies too high to be important (>2 cycles per day).

A SAMPLE APPLICATION OF THE METHOD

The example application below uses long-term data from station CB4.2C, a monitoring station in the mid-region of the Chesapeake Bay, and a two-month series of

continuous dissolved oxygen measurements at a buoy deployment in the vicinity of

that station at approximately 9 meters below the surface. Figure I-5 shows the

observed monthly dissolved oxygen concentrations (asterisks) at station CB4.2C (8-

10-meter depth) and the long-term forecast (line) from the spectral equation. 

The synthetic data record is obtained by combining the long- and short-term equations (Figure I- 6). A sample two-month period, August-September, 1987, indicated

by the two vertical parallel reference lines in Figure I-5, is expanded in Figure I-6.

 

▼I-6

appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria

(Equation I-3)
Some major difficulties beyond the cost and labor involved in deploying sensors,

which are substantial to begin with, must be overcome to implement this methodology. The spectral forecasts are essentially temporal interpolations that can be

sampled analytically. The forecasts do not lend themselves easily to an analysis of

spatial extent of criteria attainment. For other criteria assessments, the direct measures of dissolved oxygen in the environment—temporal snap shots—are interpolated

using the Chesapeake Bay Program interpolator, and the spatial extent of attainment

is assessed. Then, the frequencies of the many spatial extent measurements are

collected into a cumulative frequency distribution. The spectral analysis methodology developed thus far has yet to incorporate the assessment of spatial extent.

Three-dimensional spatial interpolations at the 10-, 20- and 30-minute short-interval

frequencies is a computational impracticality. Furthermore, more information is

▼I-7

appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria

Figure I-5. Observed monthly dissolved oxygen concentrations (*) at Chesapeake Bay

Monitoring Program Station CB4.2C (at the 8- to 10-meter depth) from January 1985 to

January 2000 and the long-term ‘forecast’ (—) from application of the spectral equation.

Figure I-6. Expanded view from Figure VI-23 of the two-month period AugustSeptember 1987 synthetic data record obtained by combining the long- and short-term

spectral equations.
needed to determine the sphere of representativeness for the short-term, highfrequency patterns, both vertically and horizontally. 

Some compromises will doubtless be required. New pilot projects, some of which

are already underway, and additional analysis of current data already at hand will

answer some of these questions and provide some basic underpinnings. Also, due to

the rapidly developing technology in this area, changes and new, unforeseen opportunities are likely to present themselves. The importance of the short duration criteria

to the protection of many of the Chesapeake Bay’s target species and communities

has been demonstrated. As Bay scientists and managers move forward with developing assessment tools, it would be prudent to seize new opportunities as technology

evolves. 

LITERATURE CITED

Jordan, S. J., C. Stenger, M. Olson, R. Batiuk and K. Mountford. 1992. Chesapeake Bay

Dissolved Oxygen Goal for Restoration of Living Resource Habitats: A Synthesis of Living

Resource Requirements with Guidelines for Their Use in Evaluating Model Results and

Monitoring Information. CBP/TRS 88/93. Chesapeake Bay Program, Annapolis, MD.

Neerchal, N. K., G. Papush and R. W. Shafer. 1992. Statistical Method for Measuring DO

Restoration Goals by Combining Monitoring Station and Buoy Data. Chesapeake Bay

Program, Annapolis, Maryland.

▼I-8

appendix I • Analytical Approaches for Assessing Short-Duration Dissolved Oxygen Criteria
The amount of ambient surface light required at the leaf surface to support underwater bay grasses survival, growth and propagation was determined by comparing

the results of the following three lines of evidence: application of the 1992 bay grass

habitat requirements; accounting for epiphytic light attenuation; and comparison of

field conditions and bay grass growth gradients.

CALCULATION USING THE 1992 BAY GRASS 

HABITAT REQUIREMENTS

A set of percent light-at-the-leaf (PLL) requirements was derived by applying the

salinity regime-based values for the 1992 Bay grass habitat requirements for Kd,

dissolved inorganic nitrogen, dissolved inorganic phosphorus and total suspended

solids (Table J-1; Batiuk et al. 1992) into the algorithm (Equation J-1) for determining PLL:

PLL=100[exp(-Kd Z)][exp(-KeBe)] (Equation J-1).

See Table VII-1 in Chapter VII for how Ke and Be are calculated in Equation J-1.

Using this algorithm, a PLL value of 8.3 percent was calculated for tidal-fresh and

oligohaline salinity regimes. The calculated PLL value was 17.3 percent in mesohaline regimes and 13.5 percent in polyhaline regimes. The mesohaline and polyhaline

PLL values differed, despite having the same 1992 Kd, total suspended solids and

dissolved inorganic nitrogen habitat requirements, because their dissolved inorganic

phosphorus bay grass habitat requirements for the two regimes differed (Batiuk et al.

1992; Dennison et al. 1993). By applying the 1992 underwater bay grass habitat

requirements, the PLL requirements of 8 percent for tidal- fresh/oligohaline habitats

and 15 percent (the average of 17.3 and 13.5 values) for mesohaline and polyhaline

habitats were derived from this line of evidence.

▼J-1

appendix J • Development of Chesapeake Bay Percent Light-at-the-Leaf Diagnostic Requirements

appendixJ 

Development of Chesapeake Bay 

Percent Light-at-the-Leaf 

Diagnostic Requirements
ACCOUNTING FOR EPIPHYTIC LIGHT ATTENUATION

As noted in Chapter IV, the scientific studies used to derive the percent lightthrough-water (PLW) criteria did not consider the shading effects of epiphytes,

which grow on underwater plant leaves at all depths and on experimentally shaded

plants in the field. Several studies in various estuarine habitats indicate that light

attenuation by epiphytic communities tends to contribute an additional 15 to 50

percent shading on underwater plants (e.g., Bulthuis and Woelkerling 1983; van Dijk

1993). A detailed study of turtlegrass beds in Florida coastal waters (Dixon 2000)

showed that, while light levels at the maximum depth of seagrass colonization averaged about 22 percent of surface irradiance (PLW), epiphytic attenuation reduced

this to approximately 14 percent of the surface light that is actually available for

plant photosynthesis (PLL). This represents an average of approximately 35 percent

more shading by epiphytes.

Light attenuation by epiphytic material appears to be important throughout the

Chesapeake Bay, contributing 20 to 60 percent more attenuation (beyond the PLW)

in the tidal-fresh and oligohaline regions, where nutrient and total suspended solids

concentrations were highest, and 10 to 50 percent in the less turbid mesohaline and

polyhaline regions (Figure J-1). These calculated contributions of epiphyte shading

are consistent with the values derived for PLW and PLL by applying the 1992 bay

grass habitat requirement values (see Table J-1) in equations IV-1 and J-1, respectively, where PLL represents approximately 30 percent additional light reduction

beyond PLW. 

Epiphytic material was assumed to make a 30 percent additional contribution to light

attenuation throughout Chesapeake Bay shallow-water habitats. This figure was

based on literature values for seagrass minimum light requirements, where epiphyte

effects were either avoided with experimental manipulation (e.g., Czerny and

▼J-2

appendix J • Development of Chesapeake Bay Percent Light-at-the-Leaf Diagnostic Requirements

Table J-1. The 1992 underwater bay grasses habitat requirements for the Chesapeake Bay 

and its tidal tributaries.

Salinity

Regime

Bay Grass

Growing

Season

Light Attenuation

Coefficient (meter-1)

Total

Suspended

Solids 

(mg liter-1)

Chlorophyll a

(μg liter-1)

Dissolved

Inorganic

Phosphorus 

(mg liter-1)

Dissolved

Inorganic

Nitrogen 

(mg liter-1)

Tidal-fresh April-October 1.5 <15 <15 <0.02 none

Oligohaline April-October 1.5 <15 <15 <0.02 none

Mesohaline April-October 2.0 <15 <15 <0.01 <0.15

Polyhaline March-May, 

Sept.-November

2.0 <15 <15 <0.02 <0.15

Source: Batiuk et al. 1992.
▼J-3

appendix J • Development of Chesapeake Bay Percent Light-at-the-Leaf Diagnostic Requirements

Figure J-1. Comparing values for percent light-at-the-leaf (PLL) and percent light-throughwater (PLW) calculated for Z=1 meter using equations IV-1 and J-1 for water quality monitoring stations in Virginia portion of the Chesapeake Bay for 1985–1996 in three salinity

regimes. Lines indicate position of points where epiphyte attenuation reduced ambient

light levels at the leaf surface by 0, 25, 50 and 75 percent.

Calculated PAR at SAV Canopy

(PLW, %Io)

Calculated PAR at Leaf Surface (PLL, %Io)
Dunton 1995) or taken into account with direct measurement (e.g., Dixon 2000) and

results from analysis of Chesapeake Bay data.

Accounting for the epiphytic contribution to light attenuation, PLL requirements for

mesohaline/polyhaline and tidal-fresh/oligohaline habitats were calculated to be 

15 percent and 9 percent of surface irradiance, respectively. These values, which

represent the minimum PLL needed to support bay grasses growth, include the

additional 30 percent epiphytic light attenuation beyond the respective PLW requirements. For mesohaline/polyhaline habitats, factoring the additional 30 percent

epiphytic light attenuation into the 22 percent PLW requirement yields a 15 percent

PLL requirement as 30% = 100(22-15)/221. A 9 percent PLL requirement for 

tidal-fresh/oligohaline habitats was derived by factoring the additional 30 percent

epiphytic light attenuation into the 13 percent PLW requirement, as 30% = 

100(13-9)/13. 

The derived underwater bay grass PLW and PLL requirements for the Chesapeake

Bay’s mesohaline and polyhaline habitats (22 percent and 15 percent surface light,

respectively) are remarkably close to the respective values of 22 percent and 14

percent surface light derived through field experimentation for turtlegrass in Florida

(Dixon 2000).

COMPARISON OF FIELD CONDITIONS AND 

BAY GRASSES GROWTH GRADIENTS

Medians of nearshore water quality data (from the Choptank and York rivers) and

Chesapeake Bay Monitoring Program midchannel data were assessed for relationships

between the calculated PLL values, bay grasses growth categories and the proposed

mesohaline/polyhaline and tidal-fresh/oligohaline PLL requirements of 15 percent and

9 percent, respectively. The calculated PLL values from observed water quality conditions associated with ‘persistent’ and ‘fluctuating’ bay grass beds were either very

close or well above the PLL requirements, or the limited set of deviations could be

readily explained (Batiuk et al. 2000). These diagnostic PLL requirements were further

validated through a comprehensive analysis of 14 years (1985-1998) of Chesapeake

Bay water quality monitoring data. The validation results were published in Chapter VII

in Batiuk et al. (2000). From these three lines of evidence, PLL requirements of 

15 percent ambient surface light for mesohaline/polyhaline habitats and 9 percent

surface light for tidal-fresh/oligohaline habitats were established.

16.6 percent represents 30 percent attenuation of the 22 percent light-through-water requirement.

Therefore, light-at-the-leaf requirement is 15.4 percent, which is rounded down to 15 percent.

▼J-4

appendix J • Development of Chesapeake Bay Percent Light-at-the-Leaf Diagnostic Requirements
LITERATURE CITED

Batiuk, R. A., R. Orth, K. Moore, J. C. Stevenson, W. Dennison, L. Staver, V. Carter, N. B.

Rybicki, R. Hickman, S. Kollar and S. Bieber. 1992. Chesapeake Bay Submerged Aquatic

Vegetation Habitat Requirements and Restoration Targets: A Technical Synthesis. CBP/TRS

83/92. U.S. EPA Chesapeake Bay Program, Annapolis, Maryland.

Bulthuis, D. A. and W. J. Woelkerling. 1983. Biomass accumulation and shading effects of

epiphytes on leaves of the seagrass, Heterozostera Tasmanica in Victoria, Australia. Aquatic

Botany 16:137-148.

Czerny, A. B. and K. H. Dunton. 1995. The effects of in situ light reduction on the growth of

two subtropical seagrasses, Thalassia testudinum and Halodule wrightii. Estuaries 18:418-

427.

Dennison, W. C., R. J. Orth, K. A. Moore, J. C. Stevenson, V. Carter, S. Kollar, P. W.

Bergstrom and R. A. Batiuk. 1993. Assessing water quality with submersed aquatic vegetation habitat requirements as barometers of Chesapeake Bay health. Bioscience 43:86-94.

Dixon, L. K. 2000. Establishing light requirements for the seagrass Thalassia testudium: An

example from Tampa Bay, Florida. In: Seagrass Monitoring, Ecology, Physiology and

Management, S.A. Bortone ed., CRC Press, Boca Raton, Florida. Pp. 9-32.

Van Dijk, G. M. 1993. Dynamics and attenuation characteristics of periphyton upon artificial

substratum under various light conditions and some additional observations on periphyton

upon Potamogeton pectinatus L. Hydrobiologia 252:143-161.

▼J-5

appendix J • Development of Chesapeake Bay Percent Light-at-the-Leaf Diagnostic Requirements