United States

Environmental Protection

Agency

Office of Water

4304

EPA-822-R-00-012

November 2000

Ambient Aquatic Life 

Water Quality Criteria 

for Dissolved Oxygen

(Saltwater): Cape Cod to

Cape Hatteras 
Ambient Aquatic Life Water Quality Criteria 

for Dissolved Oxygen (Saltwater): 

Cape Cod to Cape Hatteras

November 2000

U.S. Environmental Protection Agency 

Office of Water

Office of Science and Technology

Washington, DC

Office of Research and Development

National Health and Environmental Effects Research Laboratory

Atlantic Ecology Division

Narragansett, Rhode Island 
iii

Notices

This document has been reviewed by the Atlantic Ecology Division, Narragansett, RI

(Office of Research and Development) and the Office of Science and Technology (Office

of Water), U.S. Environmental Protection Agency, and approved for publication.

Mention of trade names or commercial products does not constitute endorsement or

recommendation for use.

Acknowledgments

This document was written by Glen Thursby, Don Miller, Sherry Poucher (Science

Applications International Corporation), Laura Coiro, Wayne Munns, and Timothy

Gleason. Comments on two earlier versions of this document by Richard Batiuk (EPA's

Chesapeake Bay Program), Charles Delos, Keith Sappington (both from EPA's Office of

Water), and Walter Berry, Wayne Davis, and Diane Nacci (all from EPA's Atlantic

Ecology Division) improved the contents of the current version. The current version also

addresses comments by six peer reviewers. These include Larry Brooke, Daniel Call,

Gary Chapman, William Collins and Tyler Linton of the Great Lakes Environmental

Center (GLEC), Traverse City, MI, and Stephan Jordan from the Maryland Department of

Natural Resources. Useful discussions on several aspects of the final criteria also were

held with David J. Hansen of GLEC. Several individuals were involved with the

successful completion of many of the bioassays conducted at EPA's Atlantic Ecology

Division. These include Steven Rego, Kathy Simmonin, and Nan Hayden. Kenneth A.

Rahn provided valuable editorial comments for the final version.
v

Executive Summary

This document recommends an approach to deriving the lower limits of dissolved

oxygen (DO) necessary to protect coastal and estuarine animals in the Virginian Province

(Cape Cod, MA, to Cape Hatteras, NC). The information on hypoxic effects used here

was obtained from studies conducted by the USEPA's Atlantic Ecology Division

specifically for this purpose, and from all other available reports applicable to hypoxic

issues of the Virginian Province. Hypoxia is defined here as concentrations of DO that are

below saturation. Literature on the effects of anoxia, while applicable to certain

ecological risk analyses, was not included in this document. This approach combines

features of traditional water quality criteria with a new biological framework that

integrates time (replacing the concept of an averaging period) and establishes separate

criteria for different life stages (larvae versus juveniles and adults). Where practical, data

were selected and analyzed in a manner consistent with the Guidelines for Deriving

Numerical National Water Quality Criteria for the Protection of Aquatic Organisms and

Their Uses (Stephan et al., 1985). This document considers how to protect three aspects

of biological health: survival of juveniles and adults, growth, and larval recruitment

(estimated with a generic model).

The recommended criteria described here apply to both continuous (persistent) and

cyclic (diel, tidal, or episodic) hypoxia. If the DO exceeds the chronic protective value for

growth (4.8 mg/L), the site meets objectives for protection. If the DO is below the limit

for juvenile and adult survival (2.3 mg/L), the site does not meet objectives for protection. 

When the DO is between these values, the site requires evaluation of duration and

intensity of hypoxia to determine suitability of habitat for the larval recruitment objective. 

The limits identified are based entirely on laboratory findings but are supported in

part by field observations. For example, juvenile and adult animals showed field acute

effects at <2.0 mg/L, below the limit of 2.3 mg/L for juveniles and adults. Also,

behavioral effects were generally seen in the range of laboratory sublethal effects. 

Unfortunately, however, no field observations are available for survival and growth of

larvae that are sensitive to hypoxia. This type of information is critical because two of the

three criteria are derived from laboratory responses of larvae.

Hypoxia as a stressor differs from chemical toxicants in that it can occur naturally

and because it is not controlled directly, whereas toxic chemicals are. Instead, hypoxia is

regulated primarily by controlling nutrients (largely nitrogen) and other oxygen-demanding

wastes. Criteria for DO may be used appropriately in a risk assessment framework. The

limits presented by the approach outlined here can be easily used to compare the abilities

of different areas to support aquatic life. Environmental managers can determine which

sites need the most attention, and how hypoxic problems vary in time and space from one

year to the next. Finally, environmental planners can make better cost-benefit decisions by

using this approach to evaluate how various management scenarios will improve

conditions.
vii

Contents 

Executive Summary .................................................... v

Lists of Tables and Figures ............................................. viii

Introduction .......................................................... 1

Overview of the Problem ................................................ 3

Biological Effects of Low Dissolved Oxygen ................................. 4

Overview of the Approach ............................................... 5

Persistent Exposure to Low Dissolved Oxygen ................................ 6

Juvenile and Adult Survival .......................................... 6

Growth Effects ................................................... 7

Larval Recruitment Effects .......................................... 11

Application of Persistent Exposure Criteria ................................. 17

Less Than 24 Hr Episodic and Cyclic Exposure to Low Dissolved Oxygen .......... 19

Cyclic Juvenile and Adult Survival .................................... 19

Cyclic Growth Effects ............................................. 20

Cyclic Larval Recruitment Effects .................................... 25

Other Laboratory Bioassay Data ......................................... 28

Laboratory Observed Behavioral Effects of Hypoxia .......................... 30

Observed Field Effects ................................................. 32

Data Not Used ....................................................... 35

Virginian Province Criteria .............................................. 36

Implementation ...................................................... 39

References .......................................................... 43

List of Appendices

Appendix A. Comparison of 24 Hr and 96 Hr Acute Sensitivity to 

Low Dissolved Oxygen for Saltwater Animals ................... A-1

Appendix B. Acute Sensitivity of Juvenile and Adult Saltwater 

Animals to Low Dissolved Oxygen ........................... B-1

Appendix C. "Chronic" Sensitivity of Saltwater Animals to 

Low Dissolved Oxygen .................................... C-1

Appendix D. Acute Sensitivity of Larval Saltwater Animals to Low 

Dissolved Oxygen at 24 Hr and 96 Hr ......................... D-1

Appendix E. Explanation of Larval Recruitment Model and 

How It Is Used .......................................... E-1

Appendix F. Sensitivity Analysis of Larval Recruitment Model ................ F-1

Appendix G. Time-to-Death Curves Used to Generate the Regressions 

in Figures 9A and 9B ..................................... G-1

Appendix H. Growth Data for Constant Versus Cyclic Exposure to 

Low Dissolved Oxygen .................................... H-1

Appendix I. Comparison of American Lobster Growth Effects with

Other Saltwater Species ................................... I-1

Appendix J. Other Data on the Sensitivity of Saltwater Animals to 

Low Dissolved Oxygen .................................... J-1
viii

List of Tables 

Table 1. Acute sensitivity of juvenile and adult saltwater animals to low 

dissolved oxygen .............................................. 8

Table 2. Effects of low dissolved oxygen on growth of saltwater animals ......... 10

Table 3. Dissolved oxygen and duration data from a hypothetical persistent 

time series (Figure 8) .......................................... 19

Table 4. Dissolved oxygen and duration data from a hypothetical cyclic time 

series (Figure 13) ............................................. 25

Table 5. Dissolved oxygen and duration data from the intervals selected from the

hypothetical cyclic time series in Figure 15 ......................... 27

Table 6. Summary of Virginian Province saltwater dissolved oxygen criteria ....... 37

List of Figures

Figure 1. Relationship between 24 and 96 hr LC50 values for juvenile 

saltwater animals exposed to continuous low DO .................... 6

Figure 2. Plot of low DO effect (GMAVs for LC50s) against percentile rank of each

value in the data set ........................................... 9

Figure 3. Plot of low DO effect (GMCVs for growth) against percentile rank of each

value in the data set .......................................... 12

Figure 4. Plot of the GMAV data from Figure 2 along with 24 hr and 96 hr

LC50 values for larval life stages of various saltwater animals .......... 13

Figure 5. Twenty-four hr dose-response curves for nine genera used in the larval

recruitment model ........................................... 15

Figure 6. Plot of model outputs that protect against greater than 5% 

cumulative impairment of recruitment ............................ 16

Figure 7. Plot of the final criteria for saltwater animals continuously 

exposed to low DO .......................................... 17

Figure 8. A hypothetical representative DO time series for one site ............. 18

Figure 9. Slope (A) and intercept (B) versus low DO effect values at 24 hr from

time-to-death (TTD) curves ................................... 21

Figure 10. Criterion for juvenile saltwater animals exposed to low DO for 

24 hr or less ............................................... 22

Figure 11. Plot of test results from growth experiments pairing constant low

DO exposure with exposures to various cycles of low DO and 

concentrations above the CCC ................................. 22

Figure 12. Plot of dose-response data for growth reduction in American lobster

(Homarus americanus) exposed to various continuous low DO 

concentrations .............................................. 24

Figure 13. A hypothetical representative DO time series for one cycle ............ 24

Figure 14. Time-to-death (TTD) curves generated for the Final Survival 

Curve "genus" .............................................. 26

Figure 15. The same hypothetical DO time series as Figure 13 .................. 26

Figure 16. The DO minima and the durations listed in Table 5 superimposed 

on Figure 14 ............................................... 27
ix

Figure 17. A plot that combines the information from Figures 5 and 6 into

a single cyclic translator to convert expected daily mortality from

cyclic exposures into allowable number of days of those cycles ......... 28

Figure 18. A plot of the other juvenile/adult mortality data from Appendix J 

along with the proposed DO criteria for juvenile/adult survival ......... 29

Figure 19. A plot of the other larval survival data from Appendix J .............. 31
1

Introduction

This document provides guidance to States and Tribes authorized to establish water

quality standards under the Clean Water Act (CWA) concerning dissolved oxygen (DO)

values that protect aquatic life from acute and chronic effects. Under the CWA, States

and Tribes are to establish water quality criteria to protect designated uses. While this

document constitutes the U.S. Environmental Protection Agency’s (EPA’s) scientific

recommendations regarding ambient concentrations of dissolved oxygen that protect

saltwater aquatic life in the Virginian Province, this document does not substitute for the

CWA or EPA’s regulations, nor is it a regulation itself. Thus, it cannot impose legally

binding requirements on EPA, States, Tribes, or the regulated community, and may not

apply to a particular situation based upon the circumstances. State and Tribal

decisionmakers retain the discretion to adopt approaches on a case-by-case basis that

differ from this guidance when appropriate. EPA may change this guidance in the future.

Section 304 (a)(2) of the CWA calls for information on the conditions necessary “to

restore and maintain biological integrity of all . . . waters, for the protection and

propagation of shellfish, fish and wildlife, to allow recreational activities in and on the

water, and to measure and classify water quality.” EPA has not previously issued

saltwater criteria for DO because the available information on effects was insufficient. 

This document is the result of a 10-year research effort to produce the required

information to support the development of saltwater DO criteria. During that effort there

were several technical work group meetings involving stakeholders and external scientists

that helped to guide the process. The criteria presented herein represent the best

estimates, based on the available data, of DO concentrations necessary to protect aquatic

life and its uses. 

These water quality criteria recommendations apply to coastal waters (waters within

territorial seas, defined as within 3 miles from shore under Section 502(8) of the CWA) of

the Virginian Province (southern Cape Cod to Cape Hatteras). However, with

appropriate modification, they may be applied to other coastal regions of the United

States. The document provides the information necessary for environmental planners and

regulators in the Virginian Province to decide whether the DO at a given site can protect

coastal or estuarine aquatic life. The approach can be used to evaluate existing localized

DO goals (e.g., Jordan et al., 1992) or to establish new ones. This document does not

address direct behavioral responses (i.e., avoiding low DO) or the ecological

consequences of behavioral responses such as changes in predation rates or in community

structures. The document also does not address the issue of spatial extent of a DO

problem. A given site may have DO conditions expected to cause a significant effect on

aquatic life, however; the environmental manager will have to judge whether the spatial

extent of the low DO area is sufficient to warrant concern. The approach presented here

for deriving criteria is expected to work for other regions. However, additional regionally

specific data may be required in order to amend the database for use in other regions. 

Animals may have adapted to lower oxygen in locations where high temperatures have

historically reduced concentrations, or in systems with natural high demands for oxygen.
1Hypoxia is defined in this document as the reduction of DO concentrations below air saturation.

2Guidelines for Deriving Numerical National Water Quality Criteria for the Protection of

Aquatic Organisms and Their Uses (Stephan et al., 1985—hereafter referred to as the Guidelines). 

2

In addition, effects of hypoxia1 may vary latitudinally, or site-specifically, particularly as

reproductive seasons determine risks of exposure for sensitive early life stages. 

As with the freshwater DO document (U.S. EPA, 1986), all data and criteria are

expressed in terms of the actual amount of DO available to aquatic organisms in

milligrams per liter (mg/L). Unlike the freshwater document, which provides limits for

DO in both warm and cold water, criteria are presented for warm saltwater only because

hypoxia in Virginian Province coastal waters is restricted primarily to the warm water of

summer. However, these warm-water limits can be considered protective for colder times

of the year. Also, the freshwater criteria are based almost entirely on fish data even

though insects were often more sensitive than fish. The saltwater limits, on the other

hand, use data from fish and invertebrates.

The saltwater DO criteria described herein were derived using the Guidelines2 and

are intended to maintain and support aquatic life communities and their designated uses. 

Although the criteria are intended to protect aquatic communities, they rely primarily on

data generated at the organism level, and emphasize data for the most sensitive life stage. 

But a population of a given species can potentially withstand some mortality to certain life

stages without a significant long-term effect on the population. Hence, an assessment of

criteria should preferably include population-level considerations. One nuance of

population-level assessment is the fact that a population's sensitivity to hypoxia may

depend on which stages have been exposed. For example, many populations of marine

organisms may be more impacted by mortality occurring during the juvenile and adult

stages than during the larval stage(s). In this regard, a particular individual larva is not as

important to the population as a particular individual juvenile or adult. With this in mind,

the saltwater criteria for DO segregate effects on juveniles and adults from those on

larvae. The survival data on the sensitivity of the former are handled in a traditional

Guidelines manner. The cumulative effects of low DO on larval recruitment to the

juvenile life stage, on the other hand, address survival effects on larvae. The DO approach

presented here uses a mathematical model to evaluate the effect on larvae by tracking

intensity and duration effects across the larval recruitment season. The model is used to

generate a DO criterion for larval survival as a function of time. It is recommended that

the parameters for this model be evaluated and adjusted where necessary to meet

site-specific conditions, especially those for length of recruitment season and larval

development time.

For the reasons listed above, the approach recommended in this document to derive

DO criteria for saltwater animals deviates from EPA's traditional approach for toxic

chemicals outlined in the Guidelines. Where practical, however, data selection and

analytical procedures are consistent with the Guidelines. Therefore, some of the 
3Although in the case of dissolved oxygen, CMC is more appropriately defined as the criterion

minimum concentration.

4

The pycnocline is the region of density discontinuity in a stratified water column between

surface and bottom waters. The density difference between the two is primarily due to differences in

temperature and salinity.

3

terminology and the calculation procedures are the same. Thus, knowing the Guidelines

is useful (but not essential) for better understanding how the limits were derived.

Terminology from the Guidelines used here includes species mean acute value (SMAV),

genus mean acute value (GMAV), final acute value (FAV), genus mean chronic value

(GMCV), and final chronic value (FCV). Procedures from the Guidelines include those

for calculating FAVs, criterion maximum concentration3 (CMC), and criterion continuous

concentration (CCC). 

Overview of the Problem

EPA's Environmental Monitoring and Assessment Program (EMAP) for the estuaries

in the Virginian Province has shown that 25% of its area is exposed to some degree to DO

concentrations less than 5 mg/L (Strobel et al., 1995). EMAP has also generated field

observations that correlate biological degradation in many benthic areas with low DO in

the lower water column (Paul et al., 1997). The two reports serve to 

emphasize that low DO is a major concern within the Virginian Province. Even though

hypoxia is a major concern, a strong technical basis for developing benchmarks for effects

of low DO have been lacking.

Hypoxia in the Virginian Province is essentially a warm-water phenomenon. In the

southern portions of the Province, such as the Chesapeake Bay and its tributaries, DO may

be reduced any time between May and October; in the more northern coastal and estuarine

waters, any time from late June into September. Hypoxic events may be seasonal or diel. 

Seasonal hypoxia often develops as stratified water prevents the oxygenated surface water

from mixing downward. Low DO then appears in the lower waters when respiration in

the water and sediment depletes oxygen faster than it can be replenished. As summer

progresses, the areas of hypoxia expand and intensify, then disappear as the water cools in

the fall. The cooler temperatures eliminate the stratification and allow the surface and

bottom waters to mix. Diel cycles of hypoxia often appear in unstratified shallow habitats

where nighttime respiration can temporarily deplete DO. 

Although the primary fauna at risk from exposure to hypoxia in the Virginian

Province are summer inhabitants of subpycnocline4

 (i.e., bottom) waters, hypoxia can

occur in other habitats as well. For example, upwelling may permit subpycnocline,

oxygen-poor water to intrude into shallow areas. Hypoxia also may appear in the upper

water of eutrophic water bodies on calm, cloudy days, when more oxygen is consumed

than is produced by photosynthesis and when atmospheric reaeration is limited. In spite of

this tendency, however, minima in DO are generally less severe above the pycnocline 
4

than below it. Hypoxia above the pycnocline also tends to be more transient because it

largely depends on weather patterns.

Hypoxia may persist more or less continuously over a season (with or without a

cyclic component) or be episodic (i.e., of irregular occurrence and indefinite duration). 

Continuous hypoxia without a cyclic component is exemplified in the subpycnocline

waters of western Long Island Sound and off the New Jersey coast (Armstrong, 1979). 

Hypoxia in Long Island Sound may be interrupted temporarily by major storms, but

returns 1 or 2 weeks later, when the waters again become stratified (Welsh et al., 1994).

Hypoxia may oscillate with tidal, diel, or lunar frequencies. Tidal hypoxia is common

in subpycnocline waters of the mesohaline Chesapeake Bay main stem and the mouth of

the adjacent tributaries during summer (Sanford et al., 1990; Diaz et al., 1992). In this

case, DO concentrations oscillate as the tides alternately advect poorly oxygenated

subpycnocline water from the mid-bay trough or tributaries and better oxygenated water

from the lower bay. Diel cycles of hypoxia are found in small eutrophic embayments and

harbors all along the coast of the Virginian Province, where oxygen is depleted overnight

by respiration and replenished by photosynthesis after dawn. The Childs River is an 

example of diel hypoxia (D'Avanzo and Kremer, 1994). Lunar cycles of oxygen may

occur in various systems but have been documented most clearly at the mouths of some

Chesapeake Bay tributaries, where destratification from spring tides saturates the water

with oxygen and stratification afterward depletes the oxygen (Haas, 1977; Kuo et al.,

1991; Diaz et al., 1992).

Episodic hypoxia has been noted in shoal waters of mid-Chesapeake Bay (Breitburg,

1990) and in adjacent tributaries (Sanford et al., 1990). Persistent winds tilt the

pycnocline laterally and displace low DO water onto the shoals or tributaries indefinitely. 

As noted above, DO may also be reduced episodically in eutrophic surface waters,

particularly during calm and cloudy weather, when photosynthesis is slow and daytime

reoxygenation is reduced.

Biological Effects of Low Dissolved Oxygen

Oxygen is essential in aerobic organisms for the electron transport system of

mitochondria. Oxygen insufficiency at the mitochondria results in reduction in cellular

energy and a subsequent loss of ion balance in cellular and circulatory fluids. If oxygen

insufficiency persists, death will ultimately occur, although some aerobic animals also

possess anaerobic metabolic pathways, which can delay lethality for short time periods

(minutes to days). Anaerobiosis is well developed in some benthic animals, such as

bivalve molluscs and polychaetes, but not in other groups, like fish and crustaceans

(Hammen, 1976). There is no evidence that any free-living animal inhabiting coastal or

estuarine waters can live without oxygen indefinitely.

Many aquatic animals have adapted to short periods of hypoxia and anaerobiosis by

taking up more oxygen and transporting it more effectively to cells and mitochondria, that

is, by ventilating its respiratory surfaces more intensely and increasing its heart rate. If 
5

these responses are insufficient to maintain the blood's pH, the oxygen-carrying capacity

of the respiratory pigment will decrease. An early behavioral response might be moving

faster toward better oxygenated water. However, if the hypoxia persists, the animal may

reduce its swimming and feeding, which will reduce its need for energy and hence oxygen. 

Such reduced motor activity may make the animal more tolerant over the short term, but

will not solve its long-term problem. For example, even the modest reductions in

locomotion required by mild hypoxia may make the animal more vulnerable to predators,

and the reduced feeding may decrease its growth.

Compensatory adaptations are well developed in marine animals that commonly

experience hypoxia, for example, intertidal and tide pool animals (McMahon, 1988) and

burrowing animals, which partly explains their reported high tolerance to low DO. In

contrast, compensatory adaptations are poorly developed in animals that inhabit

well-oxygenated environments such as the upper water column. The animals most

sensitive to hypoxia are among this latter group. Details on compensatory adaptations to

hypoxia are provided in reviews for marine animals (Vernberg, 1972), aquatic

invertebrates (Herreid, 1980), and fish (Holeton, 1980; Hughes, 1981; Kramer, 1987;

Rombough, 1988a; Heath, 1995).

Overview of the Approach

The approach to determine the limits of DO that will protect saltwater animals within

the Virginian Province considers both continuous (i.e., persistent) and cyclic (e.g., diel)

exposures to low DO. The continuous situation is covered first, and deals with exposures

longer than 24 hr. It is followed by sections on criteria for exposures of less than 24 hr

but that may be repeated for days. Both scenarios cover three areas of protection

(summarized here, and explained in more detail in the sections that follow):

1. Juvenile and adult survival—A lower limit is calculated for continuous

exposures by using FAV calculation procedures outlined in the Guidelines

(Stephan et al., 1985), but with data for only juvenile or adult stages. Limits for

cyclic exposures are derived from an appropriate time-to-death curve for

exposures less than 24 hr. 

2. Growth effects—A threshold above which long-term, continuous exposures

should not cause unacceptable effects is derived from growth data (mostly from

bioassays using larvae). This FCV is calculated in the same manner as the FAV

for juvenile and adult survival. This threshold limit as currently presented has

no time component (it can be applied to exposures of any duration). Cyclic

exposures are evaluated by comparing reductions in laboratory growth from

cyclic and continuous exposures.

3. Larval recruitment effects—A larval recruitment model was developed to

project cumulative loss caused by low DO. The effects depend on the intensity

and the duration of adverse exposures. The maximum acceptable reduction in

seasonal recruitment was set at 5% (although other percentages also may be 
6

Juveniles Only

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

24 hr LC50 (mg/L)

96 hr LC50 (mg/L)

Figure 1. Relationship between 24 and 96 hr LC50 values for juvenile saltwater animals exposed to

continuous low DO. Each point represents a paired set of values calculated from the same test run. The

line drawn represents a one-to-one relationship. Data for the plot are summarized by species in

Appendix A. Appendix A also contains data for test runs with larvae.

appropriate on a site-specific basis), which is equivalent to the protective limit

for juvenile and adult survival. The number of acceptable days of seasonal

exposure to low DO decreases as the severity of the hypoxic condition

increases. The severity of cyclic exposure is evaluated with a time-to-death

model (as in the protective limit for juveniles and adults).

Persistent Exposure to Low Dissolved Oxygen

Juvenile and Adult Survival

Data were used from tests with exposure ranging from 24 to 96 hr. This maximized

the number of genera for the FAV calculation. Data for juveniles show that LC50 values

calculated for 24 and 96 hr observations are very similar (Figure 1); therefore, all values

are applied as 24 hr data. The restriction of the data set to tests of 96 hr duration or less

was somewhat arbitrary; however, 96 hr is the duration used for most acute tests for

traditional water quality criteria (Stephan et al., 1985). In addition, there are insufficient

test data to compare 24 hr exposures versus those longer than 96 hr. Juvenile and adult

mortality data from exposures longer than 96 hr are compared to the final criterion in the

section, Other Laboratory Bioassay Data. 
5

The standard calculation for toxicants in the Guidelines uses the fifth percentile. The 95th

percentile is used here because, unlike toxicants, DO effects decrease as the concentration of DO

increases.

6

The use of a ratio to adjust the FAV to a CMC is designed to estimate a negligible lethal effect

concentration corresponding to the 5th percentile species. It may in fact represent an adverse effect

concentration for species more sensitive than the 5th percentile. The Guidelines use a factor of 2;

however, there were sufficient data available for low DO to use a factor specific to this stressor. There

was not a significant relationship between genus sensitivity and the LC5/LC50 ratio; therefore, all ratios

were included in the calculation of the final ratio. 

7

Data on the acute sensitivity of juvenile and adult saltwater animals to low DO are

available for 12 invertebrate and 11 fish species (almost all of the data are for juveniles). 

The values are summarized in Table 1 and Appendix B. Overall GMAVs range from

<0.34 mg/L for the green crab, Carcinus maenas, to 1.63 mg/L for the pipe fish,

Syngnathus fuscus, a factor greater than 4.8. Juvenile fish are somewhat more sensitive

than juvenile crustaceans (Table 1; Figure 2). In fact, the four most sensitive genera are

all fish, and the range of values for these is 1.32 to 1.63 mg/L, a ratio of only 1.2. 

As stated previously, the criterion for juveniles and adults exposed to continuous low

DO was calculated using the Guidelines procedures for derivation of an FAV (Stephan et

al., 1985). However, the procedures outlined in the Guidelines were created for

toxicants. Since DO behaves in a manner opposite to that of toxicants (i.e., the greatest

response is associated with the lowest concentrations), the calculation is reversed. The

FAV calculation is essentially a linear regression using the LC50 values for the four most

sensitive genera and their respective percentile ranks. The final FAV is the value

representing the 95th percentile genus,5 which for DO is 1.64 mg/L. This value is adjusted

to a criterion of 2.27 mg DO/L by multiplying by 1.38, the average LC5 to LC50 ratio6 for

juveniles (Table 1). This value is analogous to the CMC in traditional Water Quality

Criteria for toxicants.

Growth Effects

A threshold above which long-term, continuous exposures to low DO should not

cause unacceptable effects was calculated with growth data (mostly from bioassays using

larvae). Sublethal effects were evaluated with only growth data for two reasons. First,

growth is generally more sensitive than survival to low DO. There were only two

exceptions where survival was more sensitive to low DO than growth. One test was with

Dyspanopeus sayi; however, growth was the more sensitive endpoint in eight other tests

with this species (Appendix C). The results from this one test were not included in Table

2. The other exception was a 28-day early life stage test using the Atlantic silverside,

Menida menidia (Appendix C). There was no effect at 4.8 mg/L DO, but there were 40%

mortality and a 24% reduction in growth at a DO concentration of 3.9 mg/L. This 24%

reduction in growth, however, was not statistically significant. There was essentially no

growth of surviving M. menidia at a DO concentration of 2.8 mg/L. Only the growth data

were summarized in Table 2.
8

Table 1. Acute sensitivity of juvenile and adult saltwater animals to low dissolved oxygen. Exposure durations ranged from 24 to 96 hr.

Data from individual tests are presented in Appendix B.

Species Common Name Life Stage

SMAV

LC50a

SMAV

LC5

SMAV LC5/

LC50

GMAV 

LC50 GMAV LC5

GMAV

LC5/LC50 GMAV Rankb

Carcinus maenus green crab Juvenile/Adult < 0.34 < 0.34 1

Spisula solidissima Atlantic surfclam Juvenile 0.43 0.70 1.63 0.43 0.70 1.63 2 

Rithropanopeus harrisii Harris mud crab Juvenile 0.51 0.51 3 

Prionotus carolinus northern sea robin Juvenile 0.55 0.80 1.45 0.55 0.80 1.45 4 

Eurypanopeus depressus flat mud crab Juvenile 0.57 0.57 5 

Leiostomus xanthurus spot Juvenile 0.70 0.81 1.16 0.70 0.81 1.16 6 

Tautoga onitis tautog Juvenile 0.82 1.15 1.40 0.82 1.15 1.40 7 

Palaemonetes vulgaris marsh grass shrimp Juvenile 1.02 1.4 1.37 0.86 1.24 1.45 8 

Palaemonetes pugio daggerblade grass shrimp Juvenile 0.72 1.1 1.53

Ampelisca abdita amphipod Juvenile < 0.9 < 0.9 9 

Scopthalmus aquosus windowpane flounder Juvenile 0.81 1.20 1.48 0.90 1.20 1.48 10 

Apeltes quadracus fourspine stickleback Juvenile/Adult 0.91 1.20 1.32 0.91 1.20 1.32 11 

Homarus americanus American lobster Juvenile 0.91 1.6 1.76 0.91 1.6 1.76 12 

Crangon septemspinosa sand shrimp Juvenile/Adult 0.97 1.6 1.65 0.97 1.6 1.65 13 

Callinectes sapidus blue crab Adult < 1.0 < 1.0 14

Brevoortia tyrannus Atlantic menhaden Juvenile 1.12 1.72 1.53 1.12 1.72 1.53 15

Crassostrea virginica eastern oyster Juvenile < 1.15 < 1.15 16

Stenotomus chrysops scup Juvenile 1.25 1.25 17

Americamysis bahia mysid Juvenile 1.27 1.50 1.16 1.27 1.50 1.16 18

Paralichthys dentatus summer flounder Juvenile 1.32 1.57 1.19 1.32 1.57 1.19 19

Pleuronectes americanus winter flounder Juvenile 1.38 1.65 1.20 1.38 1.65 1.20 20

Morone saxatilis striped bass Juvenile 1.58 1.95 1.23 1.58 1.95 1.23 21

Syngnathus fuscus pipe fish Juvenile 1.63 1.9 1.17 1.63 1.9 1.17 22

Final Acute Value= 1.64 mg/L

Mean LC5/LC50 Ratio= 1.38

CMC = 1.64 mg/L x 1.38 = 2.27 mg/L

aSMAVs (Species Mean Acute Values) and GMAVs (Genus Mean Acute Values) are all geometric means (Stephan et al., 1985).

bRanked by LC50 GMAV.
9

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60 70 80 90 100

% Rank of GMAV

Dissolved Oxygen GMAV (mg/L)

Fish

Invertebrates

1.64

95%

Figure 2. Plot of low DO effect (GMAVs for LC50s) against percentile rank of

each value in the data set. Values for each genera are listed in Table 1. Results

from individual tests for each species are listed in Appendix B. The value

highlighted on the y-axis is the calculated FAV. This value is the LC50 that is

higher than the values for 95% of the tested genera. The LC50 values for the four

most sensitive genera are the only values used in the FAV calculation other than

the total number (“n”) of values. Arrows refer to those values that are less thans.

The second reason for restricting sublethal effects to growth is that results are

available from only one saltwater test that measured reproductive effects. Data are

presented in Appendix C from a 28-day life cycle test using the mysid, Americamysis

bahia. Although growth was reduced 25% at 3.17 mg/L and was technically the most

sensitive endpoint in this test, the percentage reduction in growth was essentially the same

at 2.76 and 2.17 mg/L as it was at 3.17 mg/L (20% and 27%, respectively). Reproduction

was reduced by 76% at 2.17 mg/L, the first treatment that resulted in a significant effect

on this endpoint. Although this test suggests that growth is more sensitive than

reproduction, there are insufficient data to confirm this conclusion for saltwater species. 

Data from two standardized freshwater tests, however, indicate that growth is more

sensitive than reproduction for both fathead minnows (Brungs, 1971) and Daphnia magna

(Homer and Waller, 1983). Thus, DO limits that protect against growth effects also may

be protective for reproductive effects.
10

Table 2. Effects of low dissolved oxygen on growth of saltwater animals. Data from individual tests are presented in Appendix C.

Species Common Name Life Stage

Duration

(days) NOECa HOECa

Chronic

Value

GeoMean Rankb

Cyprinodon variegatus sheepshead minnow larval 14 2.5 1.5 1.94 > 1.97 12

Cyprinodon variegatus sheepshead minnow larval 7 7.5 2.0 > 2.00

Americamysis bahia mysid <48 hr old juvenile 10 2.4 1.6 1.96 2.67 13

Americamysis bahia mysid <48 hr old juvenile 28 4.17 3.17 3.64

Morone saxatilis striped bass juvenile 21 2.8 < 2.8 < 2.8 14

Cancer irroratus Atlantic rock crab larval stage 5 to megalopa 7 3.42 2.41 2.87 2.87 15

Palaemonetes vulgaris marsh grass shrimp newly hatched 8 6.71 3.42 4.79 3.15 16

Palaemonetes vulgaris marsh grass shrimp <16 hr old 7 5.40 3.77 4.51

Palaemonetes vulgaris marsh grass shrimp <16 hr old 8 6.94 3.20 4.71

Palaemonetes vulgaris marsh grass shrimp larval stage 1 to 3 7 2.30 1.56 1.89

Palaemonetes vulgaris marsh grass shrimp postlarval 14 3.57 2.59 3.04

Palaemonetes vulgaris marsh grass shrimp postlarval 14 3.42 2.17 2.72

Palaemonetes vulgaris marsh grass shrimp postlarval 14 2.5 1.51 1.94

Mercenaria mercenaria northern quahog embryo 14 4.2 2.4 3.17 3.17 17

Menidia menidia Atlantic silverside embryo to larva 28 3.9 2.8 3.30 3.30 18

Paralichthys dentatus summer flounder newly metamorphosed juvenile 14 4.53 3.53 4.00 3.97 19

Paralichthys dentatus summer flounder newly metamorphosed juvenile 14 4.39 3.39 3.86

Paralichthys dentatus summer flounder newly metamorphosed juvenile 14 7.23 4.49 5.70

Paralichthys dentatus summer flounder newly metamorphosed juvenile 10 4.4 1.8 2.81

Homarus americanus American lobster larval stage 2 to 3 4 5.4 3.9 4.59 4.47 20

Homarus americanus American lobster larval stage 2 to 3 4 5.0 3.7 4.30

Homarus americanus American lobster larval stage 3 to 4 4 7.7 5.45 6.48

Homarus americanus American lobster larval stage 3 to 4 4 4.9 3.8 4.32

Homarus americanus American lobster larval stage 3 to 4 6 5.25 4.22 4.71

Homarus americanus American lobster postlarval stage 4 to 5 20 7.51 3.45 5.09

Homarus americanus American lobster juvenile stage 5 to 6 27 3.50 1.53 2.31

Homarus americanus American lobster juvenile stage 5 to 6 29 7.61 3.54 5.19

Dyspanopeus sayi Say mud crab <48 hr old 8 6.81 4.21 5.35 4.67 21

Dyspanopeus sayi Say mud crab larval stage 1 to 3 7 3.31 2.45 2.85

Dyspanopeus sayi Say mud crab larval stage 1 to 3 7 7.65 3.39 5.09

Dyspanopeus sayi Say mud crab larval stage 1 to 3 7 4.46 3.51 3.96

Dyspanopeus sayi Say mud crab larval stage 3 to 4 7 6.27 5.00 5.60

Dyspanopeus sayi Say mud crab larval stage 3 to megalopa 4 5.44 4.40 4.89

Dyspanopeus sayi Say mud crab larval stage 3 to megalopa 8 5.78 4.68 5.20

Dyspanopeus sayi Say mud crab larval stage 3 to megalopa 10 5.47 4.40 4.91

Dyspanopeus sayi Say mud crab larval stage 3 to megalopa 11 7.54 3.23 4.93

Labinia dubia longnose spider crab larval stage 1 to 2 7 5.30 4.11 4.67 4.67 22

aNOEC= no observed effect concentration; HOEC=highest observed effect concentration.

bRanked by geometric means.
7However, the CCC represents the potential for an approximate 25% reduction in growth. The

CCC for growth is based on statistically significant differences that result in chronic values similar to

IC25s for growth of many organisms. IC25 values are listed as a part of Appendix C for four species of

crustaceans and two species of fish. The geometric mean of these values (by species) correlates with the

geometric mean of the chronic values. In fact, a CCC calculated using IC25 values is similar to the CCC

calculated using statistically significant differences.

11

Data on the effects of hypoxia on growth are presented for 4 species of fish and 7

species of invertebrates from a total of 36 tests. Sensitivity of growth to low DO has been

determined in only two standard 28-day tests that meet Guidelines requirements; the

above life cycle test with A. bahia and the above early life stage test with M. menidia. 

Therefore, growth data from nonstandard tests (i.e., not life cycle, partial life cycle, or

early life stage tests) were used to augment the chronic database. These nonstandard tests

ranged from 4 to 29 days long. Data from short duration tests were included because

effects of oxygen deprivation are assumed to be instantaneous. Oxygen is required

continuously for the efficient production of cellular energy. Therefore, even modest

reductions in DO may result in the redirection of energy use from growth to compensatory

mechanisms. In addition, data from larval growth of two bivalves (Morrison, 1971; Wang

and Widdows, 1991) and several fish and crustaceans (Appendix C) show that chronic

values for DO do not change substantially for exposures ranging from a few days to

several weeks for most of the species tested. The Mercenaria mercenaria (Morrison,

1981) and Mytilis edulis (Wang and Widdows, 1991) studies show that the effect on

larval bivalve growth within the same test run is the same over a series of days (13 days

for M. mercenaria and 6 to 10 days for M. edulis).

Overall GMCVs for effects on growth range from >1.97 for the sheepshead minnow,

Cyprinodon variegatus, to 4.67 mg/L for the longnose spider crab, Labinia dubia, a ratio

of <2.4. Three of the most sensitive species were crustaceans (Figure 3; Table 2). The

range of chronic values for the four most sensitive genera is 3.97 to 4.67 species in the

Virginian Province.7 The consequences of reduced growth in the field, however, are

uncertain.

Larval Recruitment Effects

A generic model has been developed that evaluates the cumulative effects of stresses

on early life stages of aquatic organisms. Early life history information and

exposure-response relationships are integrated with duration and intensity of exposure to

provide an ecologically relevant measure of larval recruitment. There are existing

recruitment models for marine organisms (e.g., Ricker, 1954; Beverton and Holt, 1957). 

However, these models address other processes such as parental stock size, population

fecundity, and density-dependent processes such as cannibalism and intraspecific

competition. These existing models therefore are not appropriate for the needs of the DO

document, which requires incorporation of abiotic stressor effects. 

Larvae are more acutely sensitive to low DO than juveniles (Figure 4). A method is

provided that estimates how many days a given DO concentration can be tolerated 
8Once the larvae are “recruited” into the juvenile life stage, the juvenile survival criterion

established above is applied.

12

Figure 3. Plot of low DO effect (GMCVs for growth) against percentile rank of each

value in the data set. Percentile rank was adjusted based on the total “n” from the acute

data set (see text for explanation). Specific values for each genus included are listed in

Table 2. Results from individual tests for each species are listed in Appendix C. The

value highlighted on the y-axis is the calculated FCV. This value is the chronic value

that is higher than the values for 95% of the species represented. The chronic values for

the four most sensitive genera are the only values used in the FCV calculation other than

the total number (“n”) of values. Arrows refer to less than and greater than

without causing unacceptable effects on total larval survival for the entire recruitment

season. This is accomplished with a larval recruitment model8 and applying biological and

hypoxic effect parameters for each species for which sufficient data are available. The

level of impairment to cumulative seasonal larval recruitment that has been selected 

as acceptable is 5%. This does not mean that a population cannot withstand a greater 

percentage effect with no significant effect on recruitment. Rather, the 5% means that this

level of effect should be insignificant relative to recruitment in the absence of hypoxic

events. Many juveniles will eventually be eaten as prey or otherwise harvested as adults. 

The 5% impairment is intended to minimize the effect of hypoxia on the ultimate fate of

juveniles. On the other hand, this may not be the case for certain highly sensitive species

or populations that are already highly stressed, for example an endangered species. This
13

may also not be the case where there are other important natural or anthropogenic

stressors that contribute to a loss of the larval life stage. In such situations, it may be that

a 5% loss in larval recruitment from DO alone is not protective enough, and environmental

risk managers may need to evaluate the province-wide 5% protection goal in light of their

site-specific factors that may contribute to a cumulative loss in seasonal larval recruitment. 

States and authorized Tribes may choose a different level of acceptable impairment, but

they must justify doing so and show that the new level of impairment still protects and

maintains designated uses.

The equations that compose the model and the major assumptions used in its

application are presented and explained in detail in Appendix E. The life history

parameters in the model include larval development time, larval season, attrition rate, and

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 with the interval of low DO 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

stress. The third life history variable, natural attrition rate, gages the impact of slower

growth and development of the larvae in response to low DO 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 vertical distribution of larvae in
14

the water column determines the percentage of larvae that would be exposed to reduced

DO under stratified conditions.

For the purpose of the Virginian Province criterion, certain simplifying assumptions

have been made. The recruitment model assumes that the period of low DO 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. The Province-wide application

of the model also assumes that a new cohort occurs every day of the spawning season, and

that each cohort is equal in size. These assumptions can be easily modified and the model

rerun using site-specific information. The model does not require that a fresh cohort be

available every day. If the model is run "longhand" as presented in Appendix E, then its

use is very flexible. Successful calculation of the 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. If necessary, one also could use cohorts of various initial sizes. 

Assuming a fixed rate of cohort introduction and size simplifies the calculation of the total

number of cohorts and the calculation of hypoxic effects on larval survival. The model

application for the Virginian Province is further simplified by assuming that none of the life

history parameters change in response to hypoxia. These parameters are only changed

when a different species is modeled, although, as with cohort frequency and size, they can

be easily changed for a site-specific application to adjust for latitudinal changes in life

history requirements.

The dose-response data used in the model are presented in Figure 5. Data are

available for nine genera and represent 24 hr exposure responses, except for the Say mud

crab (D. sayi). These species were selected based in part on the ability to spawn and test

them in the laboratory. In addition, they represent a range of sensitivities to hypoxia by

water column species. The summary response curve for D. sayi represents the more

sensitive transition from zoea to megalopa. These tests were necessarily longer (7 to 11

days) than the other tests to allow sufficient time for development to megalopa. Although

some enhanced sensitivity in these tests may be from the longer exposures to low DO,

mortality also appeared to be primarily associated with the molt to megalopa (which

occurred over a 24 hr period for a given individual). When the model was run for

Dyspanopeus, the assumption was made that the response of the late larvae in transition to

megalopae could occur following a single day of exposure (i.e., this response is

independent of exposure prior to the day of transition). Thus, the model applies this dose

response as a 24 hr exposure. The model run for Dyspanopeus also includes a second,

less sensitive, dose-response curve for the early life history larval stage for non-megalopa

exposures of this species. Model runs for the other eight larval genera were conducted

using only one life history stage.

Also included in Figure 5 is a final survival curve (FSC). The data points in the FSC

are calculated in the same way that the FAVs and FCVs were calculated, using the data

from the four most sensitive genera (Cancer, Morone, Homarus, and Dyspanopeus).
9

Each genus, except for Palaemonetes, is represented by only one species. Final criteria values

calculated using the 1985 Guidelines are based on genus mean values. Therefore, all references to final

calculated values use genus rather than species.

15

The FSC will be used later for establishing DO limits for larval survival during cyclic

exposures. 

The results of the model runs for each genus9 are summarized in Figure 6. The

complete data along with the biological parameters used for each genus are presented as

part of Appendix E. For the purpose of the Virginian Province, many of the values for the

biological parameters were selected to be deliberately conservative. For example, we have

selected recruitment seasons and larval development times that more likely represent the

northern portion of the Province. To support site-specific applications, Appendix F shows

several examples of how recruitment curves would be expected to change based on

changes to the model’s biological parameters. Lengths of recruitment season and larval

development are particularly important especially because they are expected to change 

0

10

20

30

40

50

60

70

80

90

100

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Dissolved Oxygen (mg/L)

% Survival

Menidia

Palaemonetes

Scianops

Libinia

Erythropanopeus

Cancer

Morone

Homarus

Dyspanopeus

Final Survival Curve

Po = 0.122

L = 100

k = 0.021

Figure 5. Twenty-four hr dose-response curves for nine genera used in the larval recruitment

model. Dark solid line is the regression line of best fit for the FSC. See text for explanation of FSC

and of P0, L, and k. The Solver routine in Microsoft® Excel 97 was used to determine P0 and k.
16

significantly with latitude. Recruitment season gets longer and development time often

shortens as one moves south. This combination can significantly shift a recruitment curve

down and to the right. For this reason, it is expected that the final recruitment curve

(FRC) presented here for the Virginian Province may be overprotective for many sites. 

Therefore, FRCs using site-specific biological parameters are recommended.

An FRC was calculated in the same way as the FSC, using the four most sensitive

recruitment curves out of the nine available curves. The four most sensitive curves were

for the genera Morone, Homarus, Dyspanopeus, and Eurypanopeus. The equation for the

FRC (and the FSC in Figure 5) was derived by an iterative process of fitting the best line

through the points generated by the output of the recruitment model. The equation is a

standard mathematical expression for inhibited growth (logistic function; Bittinger and

Morrel, 1993). This equation is:

P(t) Equation 1 P L

P e (L P )

0

0

Lkt

0

=

+ −

−

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 5 10 15 20 25 30 35 40 45 50 55 60

Time (days)

Dissolved Oxygen (mg/L)

Cancer

Dyspanopeus

Eurypanopeus

Homarus

Libinia

Menidia

Morone

Palaemonetes

Scianops

Final Recruitment Curve

Po = 2.80

L = 4.64

k = 0.0222

Figure 6. Plot of model outputs that protect against greater than 5% cumulative impairment of

recruitment. Input parameters for each genus are explained in Appendix E. The solid line is the

regression line of best fit for the FRC. See text for explanation of FRC and of P0, L, and k. The

Solver routine in Microsoft® Excel 97 was used to determine P0, L, and k.
17

Figure 7. Plot of the final criteria for saltwater animals continuously exposed to low

DO. The upper dashed line is the CCC for growth. The lower dotted line is the CMC for

juvenile (and adult) survival, and the curve between the two is the FRC from Figure 6

representing protective for larval survival. All of the lines are truncated at 1 day. The

cyclic portion of the criteria addresses exposure less than 24 hr.

For Figure 6, P(t) is the DO concentration at time t, P0 is the y-intercept, and L is the

upper DO limit. P0 and L were first estimated by eye from the original plot and then

adjusted higher or lower to minimize the residuals between the real recruitment data and

that estimated from the mathematical fit of the data. The rate constant k was similarly

empirically derived. For Figure 5, the variables t and L represent DO concentration and

the upper limit for survival (100%), respectively. In this latter case, L is always 100%,

because this is always the upper limit for survival.

Application of Persistent Exposure Criteria

The final criteria for saltwater animals in the Virginian Province (Cape Cod to Cape

Hatteras) are indicated in Figure 7 for the case of continuous (i.e., persistent) exposure to

low dissolved oxygen. The most uncertainty with the application of these limits usually

will be when DO conditions are between the juvenile survival and larval growth limits. 

Below the juvenile survival limit, DO conditions do not meet protective goals. Above the

growth limit, conditions are likely to be sufficient to protect most aquatic life and its uses. 

Interpretation of acceptable hypoxic conditions when the DO values are between the 
18

2.3

2.8

3.3

3.8

4.3

4.8

5.3

5.8

1-Jul6-Jul

11-Jul16-Jul21-Jul26-Jul31-Jul

5-Aug

10-Aug15-Aug20-Aug25-Aug30-Aug

Dissolved Oxygen (mg/L)

CCC

a

b

c

d

Figure 8. A hypothetical representative DO time series for one site. The horizontal line

represents the CCC of 4.8 mg/L. The portion of the curve below 4.8 mg/L is divided into

four arbitrary intervals (a,b,c,d) to estimate effects on larval recruitment. The DO

minimum and the duration for each interval are determined for each interval.

juvenile survival and larval growth limits depends in part on characterization of the

duration of the hypoxia. To determine whether a given site has a low DO problem,

adequate monitoring data are required. The more frequently DO is measured the better

will be the estimate of biological effects.

Figure 8 is a hypothetical time series for daily average DO. The portion of the data

below the CCC is all that is considered. This area of the graph is first divided into several

intervals. We recommend using no finer than 0.5 mg/L DO intervals because of

limitations on most monitoring programs (see Implementation section). However, larger

intervals may be necessary if monitoring data are not taken frequently enough. The

resulting intervals in our example are (a) below 4.8 mg/L and above 4.3 mg/L, (b) below

4.3 and above 3.8, and so forth for intervals c and d. For each interval, the number of

days is recorded that the DO is between the interval's limits. For example, in interval a,

the DO is below 4.8 mg/L and above 4.3 mg/L from July 13 through 18 and again from

July 23 through 25, for a total of 7 days. This number of days is then expressed as a

fraction of the total number of days that would be allowed for the DO minimum for each

interval. For interval a, the allowed number of days is 15 (using the FRC in Figure 6 at

4.3 mg/L). Table 3 lists the information for all four intervals from this hypothetical time

series. The fractions of allowed days are totaled. If the sum is greater than 1 (as is the

case in our example), then the DO conditions do not meet the desired protective goal for

larval survival. If the sum is less than 1, then the protective goal has been met. 
19

Table 3. Dissolved oxygen and duration data from a hypothetical persistent time series (Figure 8).

Range (mg/L) No. Days

Within Range

No. Days

Allowed

Fraction of

Allowed

Interval Below Above

a 4.8 4.3 7 21 0.35

b 4.3 3.8 3 11 0.30

c 3.8 3.3 1 5 0.20

d 3.3 2.8 1 1 1.00

TOTAL 2.05

The Below and Above columns show the range of DO covered by each interval. Number of Days Within Range refers to the

duration that the observed DO is between the range given. In the last column this duration is expressed as a fraction of the

number of days allowed by the recruitment model (Figure 6) for the DO minimum of the interval. These fractions are totaled

to evaluate whether the larval survival protective goal has been met.

The current recruitment model is a first attempt at providing a method that

incorporates duration of exposure in the derivation of DO criteria. A model that could

integrate gradual change in daily DO concentrations is desirable. However, the current

model may be adequate given the probable inaccuracies in assessments of DO conditions

in coastal waters (Summers et al., 1997). 

Less Than 24 Hr Episodic and Cyclic Exposure to 

Low Dissolved Oxygen

The criteria for continuous exposure to low DO do not cover exposure times less

than 24 hr. This section addresses this topic by describing the available data and how they

were used to evaluate the effect of low DO on exposure durations lasting less than 24 hr. 

These included one-time episodic events, as well as either tidal- or diel-influenced cycles

where the DO concentrations cycle above and below the continuous CCC. The

approaches described for treatment of nonconstant (e.g., cyclic) conditions are intended to

provide protective goals that are equivalent to those established for persistent conditions. 

The data used come from two types of experiments. The first are those that provide

time-to-death (TTD) data and are used to derive TTD curves. The second are

experiments in which there were treatments consisting of a constant exposure to a given

low DO concentration paired with a treatment in which the DO concentration cycled

between that low concentration and a concentration near saturation (or at least well above

concentrations that should cause significant effects). The data from both of these

experiments are discussed below.

Cyclic Juvenile and Adult Survival

The persistent hypoxic criterion for juveniles and adults is 2.3 mg/L. A conservative

estimate of the safe DO concentration for exposures less than 24 hr would be to simply

use 2.3 mg/L. However, TTD data indicate that this would be overprotective. Data are

available for two saltwater juvenile fish (Brevoortia tyrannus and Leiostomus xanthurus),

one freshwater juvenile fish (Salvelinus fontinalis), and three larval saltwater crustaceans

(D. sayi, Palaemonetes vulgaris, and Homarus americanus), providing a total of 33 TTD
20

curves (Appendix G). The curves represent a range of test conditions, including

acclimation to hypoxia with S. fontinalis, and a range of lethal endpoints. Two general

observations were made from these data. First, each curve can be modeled with the same

mathematical expression, a logarithmic regression, of the form:

Y= m(lnX)+b Equation 2

where X=time, Y=DO concentration, m=slope, and b=intercept where the line crosses the

Y-axis at X=1.

Second, the shape of the curve (i.e., the slope and intercept) was governed by the

sensitivity of the endpoint. This is true whether the sensitivity increase was due to

interspecific differences (including saltwater and freshwater species) or the use of different

endpoints (e.g., LC5 is a more sensitive endpoint than LC50).

Figure 9 shows the relationship between sensitivity (i.e., 24 hr LC values) and the

slope (Figure 9A) and the intercept (Figure 9B) for all 33 TTD curves (Appendix G). The

DO value from each TTD curve at 24 hr was used as a measure of sensitivity. Plots using

other time intervals could have been used. The value at 24 hr was chosen in order to

generate a curve for juveniles that meets the constant CMC at its 24 hr value (2.3 mg/L). 

The slope and intercept for a time-to-CMC curve were calculated using Figure 9 equations

and the CMC 24 hr value of 2.3 mg/L. These were then used as the parameters in

Equation 2 to generate a criterion for saltwater juvenile animals for exposures less than 24

hr (Figure 10).

Cyclic Growth Effects

The CCC for continuous exposure was derived based on growth effects data (mostly

from bioassays on larvae, Table 2). The simplest way to determine effects from cyclic

exposure to low DO is to compare growth of organisms under cyclic conditions to those

for the same species under continuous conditions. Growth data are available from cyclic

exposures to low DO for three species of saltwater animals, D. sayi, P. vulgaris, and

Paralichthys dentatus (Coiro et al., 2000). These data are listed in Appendix H and

summarized in Figure 11. Data are from experiments in which a low DO treatment was

paired with a treatment cycling between the same low DO concentration and one that was

above the continuous CCC (usually saturation). All cyclic treatments had 12 hr of low

DO within any one 24 hr period. Most of the cycles consisted of 6 hr at the low

concentration followed by 6 hr at the high concentration. Only two tests (both with 

P. vulgaris) were conducted using a 12hr:12hr cycle. There were a total of 20 paired

treatments spread among the 3 species.

As expected, at the end of each test, cyclic exposures generally resulted in more

growth than constant exposures to the minimum DO of the cycle (Figure 11). However, if

the effects of DO on growth were instantaneous (i.e., growth reduction begins as soon as

the DO concentration drops and growth rate returns to normal as soon as DO returns to

above CCC concentrations), then the cyclic exposures in the above experiments would 
21

y = 0.191x - 0.064

R2 = 0.835

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Dissolved Oxygen Concentration

Causing Effect Observed at 24 hr (m g /L)

Slope 

A

y = 0.392x + 0.204

R2 = 0.678

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Dissolved Oxygen Concentration

Causing Effect Observed at 24 hr (m g /L)

Intercepts (

m g /L)

B

Figure 9. Slope (A) and intercept (B) versus low DO effect values at 24 hr from

time-to-death (TTD) curves for two species of saltwater juvenile fish, one species

of juvenile freshwater fish, and three species of saltwater larval crustaceans. Data

used mostly represent LT50 curves, but values for other mortality curves are

included. Species used and their associated TTD curves are presented in

Appendix G. All TTD curves were fit with a logarithmic regression.
22

Figure 10. Criterion for juvenile saltwater animals exposed to low DO for 24 hr or

less. The line represents the same protective limit as the CMC for juveniles for

continuous exposure. The line is a logarithmic expression with a slope and intercept

calculated from the regressions in Figure 9 at the DO concentration of 2.3 mg/L (the

CMC). 

y = 0.778x

R2 = 0.815

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80 90

Constant Exposure

Cyclic Exposure

y = 0.5x

Percentage Reduction in Growth Relative to Control

Figure 11. Plot of test results from growth experiments pairing constant low DO exposure with

exposures to various cycles of low DO and concentrations above the CCC. The dark line is a linear

regression of the data with the line forced through the origin. The lighter weight line is the “expected”

relationship from a slope of 0.5 (see text for explanation). Species used and the experimental

conditions are listed in Appendix H.
10A recent publication of these data (Coiro et al., 2000) clearly demonstrates that the growth

reduction differences between constant and cyclic exposures are more or less constant across all of the DO

concentrations tested. In other words, the ratio between constant and cyclic response should remain

consistent across all concentrations. Thus the slope can be forced through zero.

11The data used to establish the relationship between cyclic and constant exposures (Figure 11)

came from experiments with a total low DO exposure of 12 hr per 24 hr period. We assume that as the

total time of exposure per 24 hr decreases, the discrepancy between expected and observed should also

decrease. Thus the 12 hr data can be considered a worst case for any daily cycle of 12 hr or less exposure

to low DO. There is insufficient information for cycles with greater than 12 hr exposure periods per day. 

We recommend assuming constant exposure conditions for these latter situations.

12Any number of intervals can be chosen, even one. For simplicity, different DO ranges can be

selected for each interval so that each interval has approximately the same total time below the CCC. 

Alternatively, the cycle can be divided by selecting a constant DO range (e.g., 0.5 mg/L), giving each

interval a different time value. Monitoring data, however, must be frequent enough to justify the chosen

interval size.

23

have been expected to cause one-half of the growth reduction observed in the constant

treatment of each pair. (As noted above, the DO cycles had a total of 12 hr of low DO

per day.) If this were true, then the slope of the line in Figure 11 would be 0.5. However,

the slope of the line for the data (forced through the origin10) is 0.778, a factor of 1.56

greater. Thus greater growth impairment occurs from cyclic exposures than expected. 

One hypothesis for this discrepancy is that recovery from the low DO portion of the cycle

is not instantaneous, and the actual low DO effect period is then greater than 12 hr within

each day (by a factor of 1.56).11

Figure 12 shows a dose-response for growth of larval lobster (H. americanus) over a

range of constant DO concentrations. The data are from 10 tests (see Appendix C) with

durations ranging from 4 to 29 days. The percentage growth reduction is relative to a

control response. Growth reduction effects are considered instantaneous; therefore, the

percentage reduction can be applied to any time period. Data for the lobster are

emphasized because it was the most sensitive species tested for which growth was

measured. Its use is consistent with the 1985 Guidelines (Stephan et al., 1985), which

allows a criterion to be established using data for a sensitive economically or ecologically

important species.

To evaluate a cycle for chronic growth effects, the above relationship between cyclic

and constant exposure is needed as well as monitoring data from a representative, or

worst case, cycle of low DO for a given site. Figure 13 provides a hypothetical DO time

series. To estimate the expected growth reduction during this cycle, the curve is divided

into three DO intervals12 for that portion of the cycle that falls below 4.8 mg/L (the CCC). 

The DO mean, and the total duration that the cycle is within the interval's range of DO,

are determined for each interval. Data from this example are presented in Table 4. 

Interval c lasts a total of 5 hours. Interval b lasts a total of 3 hours (b1 before plus b2

after interval c). Similarly, interval a lasts for a total of 41⁄2 hours. Each of these time

intervals is multiplied by 1.56 to adjust for the cyclic effect.
24

y = -23.1x + 138.1

0

10

20

30

40

50

60

70

80

90

100

1.5 2.0 2.5 3.0 3.5 4.0 4.5

% Growth Reduction

Dissolved Oxygen (mg/L)

Figure 12. Plot of dose-response data for growth reduction in American lobster

(Homarus americanus) exposed to various continuous low DO concentrations.

Percentage growth reduction is relative to a control. The dashed line is a linear

regression through the data points. Data are from Appendix C.

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00 12:00 15:00 18:00

Dissolved Oxygen (mg/L)

a2

b1

c

CCC

Time (hr)

b2

a1

Figure 13. A hypothetical representative DO time series for one cycle. The

horizontal line represents the CCC of 4.8 mg/L. The portion of the curve below 4.8

mg/L is divided into three arbitrary intervals (a,b,c) to estimate effects on growth.

The range of DO, the mean DO, and the duration for each interval are listed in 

Table 4.
13See footnote 7.

25

Table 4. Dissolved oxygen and duration data from a hypothetical cyclic time series (Figure 13).

Interval

DO Range

(mg/L)

DO Mean

(mg/L)

% Daily

Reduction

in Growth

Actual

Duration

(hr)

Cyclic

Adjusted

Duration

(hr)

%

Reduction

for

Duration

a1 – a2 4.8 – 4.0 4.40 36 4.5 7.0 11

b1 – b2 4.0 – 3.5 3.75 51 3 4.7 10

c 3.2 – 3.5 3.35 61 5 7.8 20

These data are used to estimate the growth reduction occurring for the recruitment modeled species during the cycle. Percentage

reductions in growth for constant exposure are calculated with the equation in Figure 12. These in turn are normalized for the cyclic

adjusted duration.

A DO mean concentration for each interval is used with the equation from Figure 12

to estimate a daily growth reduction that is expected for larval crustaceans during constant

exposure to hypoxia. This value is then normalized for the interval's cyclic adjusted

duration. The normalized reductions for all intervals are added (growth effects are

cumulative) for an estimated growth reduction for the cycle. The total percentage

reduction in our example is 44%. This reduction is greater than 25%;13 thus our

hypothetical cyclic hypoxic event does not meet the protective goal for growth.

Cyclic Larval Recruitment Effects

To evaluate cyclic exposures for their potential impact on larval recruitment to the

juvenile life stage, two pieces of information are needed: (1) a set of larval TTD curves 

to estimate the expected daily mortality for a given low DO cyclic exposure and (2) a way

to translate that predicted daily larval mortality into allowable days for the given low DO

cycle using the constant exposure recruitment model output. Creation of the larval TTD

curves is straightforward using the sensitivity information (dose-response curve) from the

FSC in Figure 5 and the sensitivity-dependent relationships for TTD slopes and intercepts

in Figure 9. Creation of a series of larval TTD curves followed the same procedure used

to create the time-to-CMC curve for juveniles (Figure 10). Figure 14 shows the results

for nine calculated curves for mortalities ranging from 5% to 95%.

Estimating the daily mortality expected to occur with the model species also is

straightforward and, as with cyclic growth protection, requires representative or worst

case DO monitoring data. Figure 15 is a hypothetical monitoring data set for a single

cycle. As with growth, the portion of the cycle below the CCC is first divided into several

intervals. The DO minimum is determined for each interval. It should not matter how the

intervals are selected. All that is needed is a set of paired time and DO values. Table 5

lists the data for the intervals in this example. These data were plotted among the family

of larval TTD curves (Figure 16). The greatest effect datum lies between the 15% 
26

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00 12:00 15:00 18:00

Dissolved Oxygen (mg/L)

a

b

c

d

e

CCC

Figure 15. The same hypothetical DO time series as Figure 13. This time the portion of

the curve below 4.8 mg/L is divided into several arbitrary intervals to estimate effects on

mortality. The DO minimum and its duration for each interval are listed in Table 5.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hr)

Dissolved Oxygen (mg/L)

5

10

15

25

50

75

85

95

90

Figure 14. Time-to-death (TTD) curves generated for the Final

Survival Curve “genus.” Data to generate the curves were taken from

Figures 5, 9A, and 9B. The numbers adjacent to each TTD curve are

the percentage mortality that each curve represents. The dashed lines

represent curves created with slopes and intercepts outside the range

of the original data used in Figure 9.
27

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hr)

Dissolved Oxygen (mg/L)

5

10

15

25

50

75

85

95

90

Figure 16. The DO minima and the durations listed in Table 5

superimposed on Figure 14 (solid circles). The expected mortality

from the cyclic exposure is determined by the data point falling

closest to a TTD curve of greatest effect; in this case 25% was

selected.

Table 5. Dissolved oxygen and duration data from the intervals selected from the hypothetical

cyclic time series in Figure 15.

Interval DO Minimum for Interval (mg/L)

Duration of Interval

(hr)

a 4.3 15

b 3.8 11.5

c 3.3 9

d 3.0 6

e 2.8 4

These data are plotted in Figure 16 to estimate the expected mortality occurring for recruitment modeled species during 

the cycle.

and 25% mortality curves. For the purpose of this example, we will select the 25%

mortality curve. Therefore, the hypothetical cycle of DO is expected to cause 25% daily

mortality to the modeled larval crustacean. We are only concerned with the greatest 

effect datum because survival effects are not cumulative (i.e., an individual can die only

once). 

Now all that is needed is to translate the expected 25% mortality into the number of

allowable days for this hypothetical cycle to occur. This is accomplished using the FSC

and FRC curves in Figures 5 and 6, respectively. The information in Figure 5 is for

percentage survival, but it can be converted easily into percentage mortality. Thus the 
28

information shows the expected cohort mortality to occur for a given DO concentration. 

For the example, 25% mortality occurs at a DO concentration of 3.7 mg/L. From the

equation used to fit the data in Figure 6, the 3.7 mg/L is allowed to occur for up to 9 days

without significant impairment to seasonal recruitment. Thus the cycle that resulted in an

estimated 25% daily mortality to larvae can be repeated for up to 9 consecutive days

without exceeding a 5% reduction in seasonal larval recruitment. All of the above can be

simplified by merging the information from the FSC and FRC into one cyclic translator

figure using the DO axis that is common between Figures 5 and 6. This is shown in Figure

17.

Other Laboratory Bioassay Data 

Additional available data on lethal and sublethal effects of hypoxia on saltwater

animals (Appendix J) do not indicate significantly greater sensitivity than indicated

previously. The other data are divided into effects on juveniles and adults, and effects on

larvae. Figure 18 shows all of the juvenile mortality data from Appendix J plotted against

the criteria for juvenile and adult survival (limits for both persistent and cyclic exposures

are included). Most of the other survival data are well below the criteria, with three

notable exceptions. The first is a single datum (LC50 of 1.9 mg/L) for the Atlantic 

Cyclic Translator

0

5

10

15

20

25

30

35

40

45

50

55

60

0 5 10 15 20 25 30 35 40 45 50 55 60

Daily Cohort Mortality (%)

Allowable Number of Days at Given 

Cohort Mortality

UNACCEPTABLE

ACCEPTABLE

Figure 17. A plot that combines the information from Figures 5 (Final Survival Curve)

and 6 (Final Recruitment Curve) into a single cyclic translator to convert expected

daily mortality from cyclic exposures into allowable number of days of those cycles.
29

Figure 18. A plot of the other juvenile/adult mortality data from Appendix J (open symbols) along with

the proposed DO criteria for juvenile/adult survival (solid line).

menhaden, B. tyrannus, at 6 hr (Voyer and Hennekey, 1972). However, several other

LC50 values (Burton et al., 1980) for Atlantic menhaden with durations ranging from 2 to

72 hr were much less (0.70 to 0.96 mg/L). The second is a single datum for the Atlantic

silverside M. menidia at 6 hr (also Voyer and Hennekey, 1972). There are no other data

for juvenile Atlantic silversides, but the unusually high sensitivities reported by Voyer and

Hennekey for the other species suggest that their exposure system might be a confounding

factor. In addition, the authors provided no information on control response for either the

Atlantic menhaden or the Atlantic silversides.

The third set of data above the criteria is a series of values at 0.5 hr for the copepod

Eurytemora affinis. Some are below the criteria, but many are above it (Vargo and

Sastry, 1978). However, the authors did not give any details on their experimental

methods, including the number of replicates and the number of animals in each replicate,

or on the response in the control. Thus, it is difficult to adequately assess the significance

of these results. However, in the absence of data to the contrary, it is worth noting that

the DO limit for juveniles and adults may not be protective of copepods. Alternatively,

one could consider that short-lived species with high reproductive outputs (such as

copepods) may be more appropriately protected in a manner similar to larval recruitment. 
30

In this case, all of the E. affinis LC50 values would fall below the criterion provided by

the larval recruitment (see explanation for Figure 19A below).

Figures 19A and 19B present all of the lethality data from Appendix J for tests using

larval life stages. All of these data are from tests for effects on individuals, and the

criterion for larval survival acknowledges that some larval mortality is acceptable. Most

of the data for larvae are LC50 values for exposure durations other than 24 or 96 hr (these

two durations are used elsewhere in the document). The LC50 data are plotted in Figure

19A. The most appropriate protective limit with which to compare these values is the

TTD curve for 50% mortality from Figure 14. There are two series of data points for

LC50 values for larval rock crab (Cancer irroratus) for exposure durations of 2 and 4

hours; each has some values above the 50% TTD curve (Vargo and Sastry, 1977). The

more sensitive values in these sets are for tests run at 25°C; thus the animals were likely

exposed to multiple stressors (temperature and low DO). 

The rest of the other lethality data for larvae are plotted in Figure 19B. These data

are separated into three categories, LC5 to LC35, LC40 to LC65, and LC90 to LC100. 

As with the LC50 values in Figure 19B, these values are plotted along with TTD curves

(10%, 50%, and 90% mortality) from Figure 14. All of the LC5 to LC35 values are close

to or below the 10% TTD curve. All of the LC40 to LC65 values are well below the 50%

TTD curve. Finally, all but one of the LC90 to LC100 values are below the 90% TTD

curve. This one value is for 100% mortality of striped bass larvae (M. saxatilis) that

occurred after a 2 hr exposure to 1.90 mg/L DO. However, there are two other striped

bass tests where 100% mortality of the larvae did not occur until 24 hr of exposure to

similar low DO.

There are fewer other data on sublethal effects than on lethality effects (Appendix J). 

The sublethal effects included reduced feeding, growth, locomotion, and bivalve

settlement, as well as delays in hatching and molting. However, none of these values

indicate that the CCC would not be protective against these effects.

Laboratory Observed Behavioral Effects of Hypoxia 

A number of laboratory studies report behavioral alterations following exposure to

hypoxia. The effects include low DO avoidance, changes in locomotion, burrowing and

feeding activity; and altered predator-prey behaviors. Because most of the effects

observed occurred <2.3 mg/L, the 24 hr acute limit CMC would be protective. The most

hypoxia-sensitive behavioral effect occurs in red hake (Urophycis chuss). In red hake, age

0+ fish leave their preferred bottom habitat and begin to swim continuously as DO

concentrations fall below 4.2 mg/L (Bejda et al., 1987). Food search time is also reduced

as a consequence. Below 1.0 mg/L, most locomotor and other behavioral activity ceases,

and at 0.4 mg/L there is loss of equilibrium. Older red hake (age 1+ and 2-3+) did not

exhibit these responses with low DO, except for loss of equilibrium at 0.6 mg/L. 
31

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.01 0.10 1.00 10.00 100.00

Time to Effect (days)

Effect Conc. (D.O. 

m g/L)

C. irroratus , LC50s

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.01 0.1 1 10 100

M. saxitalis

100% 

mortality

F. heteroclitus

10% embryo

mortality

C. bosquianus

5 % mortality

10% TTD

50% TTD

90% TTD

Time to Effect (days)

Effect Conc. (D.O. 

m g/L)

Figure 19. A plot of the other larval survival data from Appendix J. Figure 19A

presents the available LC50 data (open circles) along with the 50% TTD curve from

Figure 14. Figure 19B presents mortality data for other than 50%. Open circles

represent 5% to 35% mortality, open squares 40% to 65% mortality, and closed

circles 90% to 100% mortality. Figure 19B also includes the 10%, 50%, and 90%

TTD curves from Figure 14.
32

The following effects are reported at less than the 2.3 mg/L protective limit. In the

red morph of green crabs (C. maenas) the low DO avoidance EC25 was <2.3 mg/L and

the EC50 was 1.8 (Reid and Aldrich, 1989). The green morph was less sensitive. In

naked goby (Gobiosoma bosc) larvae, avoidance at 2.0 mg/L occurred with $1 hr

exposure (Breitburg, 1994). No avoidance was observed at 3.0 mg/L. This same author

reported 100% avoidance in larval bay anchovy (Anchoa mitchilli) at 0.75 mg/L following

a 1 hr exposure. Reduced locomotor activity occurred in daggerblade grass shrimp (P.

pugio) at 1.8 mg/L (Hutcheson et al., 1985). Burrowing in the northern quahog (M.

mercenaria) was reduced 1.4- to 2-fold when exposed to 1.8 to 0.8 mg/L and slowed 4-

fold in Atlantic surfclam (Spisula solidissima) at 1.4 mg/L (Savage, 1976). The

polychaete, Nereis virens, EC25 for emergence from the sediment was 0.9 mg/L

(Vismann, 1990). The shelter guarding and nest guarding behavior by adult male naked

goby (G. bosc) was not altered at 0.7 mg/L, but they abandoned shelters at 0.38 mg/L and

nests at 0.3 mg/L. Death occurred in these animals at 0.26 to 0.24 mg/L (Breitburg,

1992).

The following low DO effects on feeding are reported in a bivalve and four

polychaetes. In eastern oyster (Crassostrea virginica) early postsettlement stage (436 :m

mean shell height), exposure to 1.9 mg/L for 6 hr resulted in 54% to 61% reduction in

feeding rate; at <0.4 mg/L for the same period, 86% to 99% reduction occurred (Baker

and Mann, 1994b). In older postsettlement animals (651 :m mean shell height), feeding

rate was not altered with 1.9 mg/L exposure for 6 hr, but at <0.4 mg/L it was reduced

97% to 99%. In the polychaetes, feeding stopped in Nereis diversicolor at 1.2 mg/L and

in N. virens at 0.9 mg/L (Vismann, 1990). In adult Loimia medusa, feeding stopped at 1.0

mg/L during <20 hr exposure, then resumed in 42 to 113 hr in 42% of the animals (Llansó

and Diaz, 1994). At 0.5 mg/L, there was no resumption of feeding after initially ceasing

during the same initial exposure period. Following exposure in Streblospio benedicti

adults, the initial response to 1.0 mg/L was cessation of feeding, but it resumed in 3.5

days; with 0.5 mg/L exposure, the initial response was the same, with feeding resuming in

4.5 days (Llansó, 1991).

Changes were observed in predator–prey activities in two fishes in low DO. In

naked goby (G. bosc) larvae, avoidance of the sea nettle (Chyrsaora quinquecirrha)

predator was reduced 60% following 3 hr exposure to 2.0 mg/L. In striped bass (M.

saxatilis) juveniles, predation on naked goby larvae was reduced 50% following 1 hr 35

min exposure to 2.0 mg/L (Breitburg et al., 1994).

Observed Field Effects

Field reports of the biological consequences of hypoxia could be used to derive DO

criteria if they include information to describe the exposure conditions. Yet sufficient data

are rarely available. In most cases, DO conditions prior to observed effects are unknown,

making it difficult to predict an exposure threshold for the observed effect. A field report

of hypoxic effects must, at a minimum, provide a description of the concurrent DO

exposure conditions if it is to be useful in deriving criteria. Ten studies in the Virginian
33

Province have provided concurrent DO measurements. The DO observations often are

only point measurements, not continuous records, and they rarely provide information on

DO conditions prior to the observed effects. The biological effects reported include

alterations in the following: presence of fish and crustaceans, diel vertical migration of

copepods, recruitment and population density of an oyster reef fish (naked goby),

recruitment and growth of eastern oyster spat, and macrobenthic community parameters. 

Effects were usually not observed above 2 mg/L. Exceptions are the Long Island Sound

trawl studies, where effects were reported in the 2.0 to 3.7 mg/L range.

The relationship between low DO and presence of fish and shellfish in Long Island

Sound was examined in two trawl studies. Howell and Simpson (1994) reported marked

declines in abundance and diversity in 15 of 18 study species when DO was below 2 mg/L. 

When DO was between 2 and 3 mg/L, there were significantly reduced abundances of

three species: winter flounder, windowpane flounder, and butterfish. In a subsequent 3-

year study, the aggregate data for 23 species of demersal finfish showed a decline for two

community indices, total biomass and species richness, with declining DO (Simpson et al.,

1995). The DO concentration that corresponded with a 5% decline below a response

asymptote was 3.7 mg/L for total biomass and 3.5 mg/L for species richness. DO declines

below these concentrations resulted in further exclusion of these animals, which has

implications for the secondary productivity of these waters. Reduced species number

implies reduction of community resilience, should this condition persist. The

consequences of habitat crowding on animals occurring in adjacent waters are unknown.

Hypoxia-induced changes in the distribution of fish and crustaceans have also been

reported in the lower York River, located in the Virginian portion of Chesapeake Bay

(Pihl et al., 1991). Subpycnocline DO <2 mg/L developed during neap tide periods, and

the study species (spot, croaker, hogchoker, blue crab, and mantis shrimp) migrated to

shallower and better oxygenated habitats. The degree and order of vertical movement was

believed to be a function of the water column DO concentration and species sensitivity to

hypoxia; that is, croaker > spot = blue crab > hogchoker . mantis shrimp. Water column

destratification and reaeration occurred with spring tide or strong winds, and all species

except the burrowing mantis shrimp returned to the deeper strata, indicating a preference

for the deeper habitats.

Diel vertical migration of copepods Acartia tonsa and Oithona colcarva was

disrupted by hypoxia (Roman et al., 1993). In mid-Chesapeake Bay during the summer,

these copepods typically occurred near the bottom during the day and migrated to the

surface waters at night. However, when DO concentrations fell below 1 mg/L in

subpycnocline waters, the copepods were displaced to the pycnocline, where the highest

numbers were found both day and night. When mixing occurred during the summer, the

bottom waters were reaerated, and the copepods once again were found at depth during

the day. Vertical migration is believed adaptive in that it places the copepods in the

chlorophyll maximum at night to maximize food intake, yet it provides daytime avoidance

of the surface waters, protecting the copepods from visual feeding bay anchovy.
34

The consequences of hypoxia on recruitment were examined for two species at a

mid-Chesapeake Bay site: the naked goby (G. bosc), a benthic oyster reef fish (Breitburg,

1992), and eastern oyster (C. virginica) (Osman and Abbe, 1994). In the naked goby

study, low DO episodes were short-lived, but extreme (<0.5 mg/L), the result of

movement of deep, oxygen-depleted bottom water into the near-shore reef habitat. 

Following each severe intrusion, the naked goby population density fell dramatically at the

deeper stations, which experienced the lowest DO (0.4 mg/L). Small, newly recruited

juveniles were absent, presumably due to extremely high mortality. There is evidence,

based on observed densities, that older juveniles and adults survived these events by

temporarily moving to inshore portions of the reef where DO was not as low, then

returning during the weeks following the event. Embryonic development was also

affected. Males abandoned egg-containing tubes placed at deeper sites, and the majority

to all of the embryos were dead. In addition, the youngest embryos collected from the

shallower, less hypoxia-stressed site developed abnormalities following laboratory

incubation. The severe intrusions occurred during peak periods of recruitment, with the

lowest DO occurring on portions of the reef where recruitment was expected to be

highest. These adverse effects were not observed at sites having low DO $0.7 mg/L. 

In the study with the eastern oyster (C. virginica) (Osman and Abbe, 1994),

mortality was observed in newly set (2 to 4 days old) animals during periods of prolonged

intrusions of low DO water (<1 mg/L 40% of the time in bottom water during the first 2

weeks of two experiments). Mortality was proportional with depth, which corresponded

to severity of hypoxia. Growth rate of surviving spat decreased after 1, 2, and 4 weeks

following deployment, with a greater effect also occurring at the deeper stations. Survival

and growth of juvenile oysters were unaffected following simultaneous deployment at the

same stations, indicating greater tolerance of the older animals. The authors concluded

hypoxia to be a plausible causative factor, acting directly or indirectly, although other

causative factors also are possible.

Responses of the macrobenthic community to DO <2 mg/L are reported for the

lower Chesapeake Bay and tributaries (Dauer and Ranasinghe, 1992; Diaz et al., 1992;

Llansó, 1992; Pihl et al., 1991, 1992). Two community effects are reduced species

number and abundance, with these effects increasing spatially and temporally with

increasing severity and duration of hypoxia. There also is a shift with hypoxia from

dominance of longer-lived, deeper burrowing species of a mature community to

short-lived, shallow burrowing opportunistic species. The response of benthic species,

and their subsequent recoveries following hypoxia, depends on species tolerance, the

timing of the hypoxic event relative to larval availability and settlement, and life history

strategy. Some infaunal organisms migrate toward the sediment surface with hypoxia,

beginning around 2 mg/L (Diaz et al., 1992). Animals that migrate to the surface are

exposed to predation by hypoxia-tolerant fish and crustaceans (Pihl et al., 1992). 

Defaunation may only occur below 1 mg/L. These studies support 2 mg/L as the hypoxic

effect threshold for the macrobenthos, which is consistent with the global literature (Diaz

and Rosenberg, 1995).
35

To summarize, demersal finfish community biomass has been observed to diminish at

DO <3.7 mg/L, and species richness to diminish at <3.5. These effects become

increasingly pronounced with further DO decline. Below 2.0 mg/L, migration of the

infaunal species to the sediment surface and movement of epifaunal species to better

aerated water were observed. All effects reported at <1 mg/L DO concern

hypoxia-tolerant species and life stages (i.e., disruption of diel vertical migration in

copepods, reduced growth and survival of newly settled oysters, and lethality in larval

goby) as demonstrated in parallel laboratory studies (Breitburg, 1992; Roman et al., 1993)

or by other workers (Baker and Mann, 1992, 1994a). 

Data Not Used

Data from a variety of published literature were not used. The literature on effects of

anoxia was not used, as it provides negligible information on threshold requirements of

aerobic animals. Information on anoxic effects may be found in a recent symposium

(Tyson and Pearson, 1991) and a review (Diaz and Rosenberg, 1995) on this subject. 

Results of hypoxia effects studies were not cited for species that do not commonly occur

in coastal and estuarine waters between southern Cape Cod, MA, and Cape Hatteras, NC,

during the spring to autumn period that brackets the occurrence of hypoxia. Reports for

occasional visitor species that occur in these waters during a favorably warm or cold

summer were excluded. 

Data were not cited if the test temperature was outside the temperature range of

Virginian Province waters during the hypoxic season; for example, American lobsters

tested at 5°C (McLeese, 1956). Data were not used if they are probably not reliable. 

Examples include indications that the test animals may have been stressed, for example,

American lobster tested at 25°C that were not fed during an 8- to 10-week acclimation

period (McLeese, 1956); excessive control mortality (>10% for juveniles or adults and

>20% for early life stages); uncertain DO exposure concentration, whether due to

questionable DO measurements or failure to directly measure test chamber DO conditions

(e.g., Reish, 1966); or if test animals were removed and handled during the test to make

other measurements, for example, for an energetics study (Das and Stickle, 1993). 

Literature on physiological responses of animals to hypoxia was reviewed but was not

found useful to determine low DO effect thresholds. See Herreid (1980) for a discussion

of difficulties in using oxygen consumption results to describe DO requirements of

invertebrates. Rombough (1988b) has developed an approach to identify the DO

requirements for fish embryos and larvae, but this approach has not been employed with

species applicable to Virginian Province saltwaters.

Some data are not used for juvenile blue crabs, C. sapidus (Stickle, 1988; Stickle et

al., 1989). Effect concentrations for this species from this laboratory are an order of

magnitude higher than values from an earlier study using adult C. sapidus (Carpenter and

Cargo, 1957). In addition, these effect concentrations for juvenile blue crabs are almost

all higher than values for larvae of all tested species. Another study (DeFur et al., 1990)

showed that adult C. sapidus make respiratory adjustments that allow them to tolerate
36

long-term (25 days at 22°C) exposure to 2.6 to 2.8 mg DO/L. These data for juvenile

blue crabs are considered outliers until further testing shows otherwise.

Just prior to completion of this document, a paper appeared (Secor and Gunderson,

1998) describing the effects of hypoxia and temperature on juvenile Atlantic sturgeon,

Acipenser oxyrinchus. There was 22% mortality at 19°C and an average within-tank DO

concentration of 2.7 mg/L (within-tank data provided by author). This sensitivity is not

that different from that of striped bass. However, a combination of low DO (ca. 3.5

mg/L) and high temperature (26°C) resulted in 100% mortality of A. oxyrhincus within

approximately 24 hr. Because the greatest sensitivity was associated with the high

temperature, the data were not included in this document. In addition, the salinity during

the experiments only ranged between 1 and 3 ppt; therefore, it is likely that these data are

more appropriately associated with freshwater criteria, which are higher than those for

saltwater (see Implementation section).

Virginian Province Criteria

The recommended criteria for ambient DO for the protection of saltwater aquatic life

in the Virginian Province: Cape Cod to Cape Hatteras are summarized in Table 6. These

criteria are briefly described below:

(1) Protection of Juvenile and Adult Survival from Persistent Exposure

This limit is derived following the Guidelines procedures and is analogous to the

CMC, except that a protective DO concentration limit is expressed as a minimum as

opposed to a maximum, as would be the case for a toxicant. This limit represents the

floor below which DO conditions (for periods of >24 hours) must not occur. Shorter

durations of acceptable exposure to conditions less than the CMC have been derived from

laboratory studies, as described in (4) below. Refer to Table 1 and Figure 2 for an

explanation of the derivation of this limit.

(2) Protection of Growth Effects from Persistent Exposure

This limit is derived following the Guidelines procedures and is analogous to the

CCC for a toxicant. This limit represents the ceiling above which DO conditions should

support both survival and growth of most aquatic species from Cape Cod to Cape

Hatteras. Refer to Table 2 and Figure 3 for an explanation of the derivation of this limit. 

This limit may be replaced with a limit derived in (3) as described below, when exposure

data are adequate to derive an allowable number of days of persistent exposure. 
37

t * . *Gred %i i

n

156

24

25

1

∑ <

t actual

t allowed

i

i

( )

( )

∑ < 1.0

DO

e

i

tsubi

=

+

−

130

2 80 184 0 10

.

( . . )

.

 Table 6. Summary of Virginian Province saltwater dissolved oxygen criteria.

Endpoint Persistent Exposure (24 h or greater continuous low DO

conditions)

Episodic and Cyclic Exposure (less than 24 h

duration of low DO conditions)

Juvenile and Adult

Survival 

(minimum allowable

conditions)

(1) a limit for continuous exposure

DO = 2.3 mg/L

(criterion minimum concentration, CMC)

(4) a limit based on the hourly duration 

of exposure

DO = 0.370*ln(t) + 1.095

where:

DO = allowable concentration (mg/L)

t = exposure duration (hours)

Growth Effects

(maximum conditions

required)

(2) a limit for continuous exposure

DO = 4.8 mg/L

(criterion continuous concentration, CCC)

(5) a limit based on the intensity and hourly 

 duration of exposure

Cumulative cyclic adjusted percent daily 

reduction in growth must not exceed 25%

and

Gredi = −23.1* DOi + 138.1

where:

Gredi = growth reduction (%)

DOi = allowable concentration (mg/L)

t

i

 = exposure interval duration (hours)

i = exposure interval

Larval Recruitment

Effectsa

(specific allowable

conditions)

(3) a limit based on the number of days a continuous

exposure can occur

Cumulative fraction of allowable days above a given

daily mean DO must not exceed 1.0

and

where:

DOi = allowable concentration (mg/L)

t

i

 = exposure interval duration (days)

i = exposure interval

(6) a limit based on the number of days an

intensity and hourly duration pattern of

exposure can occur

Maximum daily cohort mortality for any hourly

duration interval of a DO minimum must not

exceed a corresponding allowable days of

occurrence

where:

Allowable number of days is a function of

maximum daily cohort mortality (%)

Maximum daily cohort mortality (%) is a

function of DO minimum for any exposure

interval (mg/L) and the duration of the interval

(hours)

a

 Model integrating survival effects to maintain minimally impaired larval populations.
38

(3) Protection of Larval Recruitment Effects from Persistent Exposure

This limit is derived from a generic larval recruitment model. The limit represents

allowable DO conditions below the CCC, provided the exposure duration does not exceed

a corresponding allowable number of days that ensure adequate recruitment during the

larval recruitment season. The cumulative effects of all exposure interval durations at a

given DO below the CCC can be accounted for by totaling the fractions of the actual (or

projected) exposure duration (in days) divided by the allowable exposure duration for each

interval of a specific DO concentration. Refer to Table 3 and Figure 6 of this document

for an explanation of the derivation of this limit.

(4) Protection of Juvenile and Adult Survival from Episodic or Cyclic Exposure

This time-dependent limit was derived to represent the responses of the most

sensitive juveniles tested in the laboratory. It provides a degree of protection equivalent

to the CMC, but for shorter exposure durations than a day. It is assumed that adults are

no more sensitive than juveniles. This limit represents the minimum DO conditions that

must be maintained on an hourly basis (e.g., 1-hour minimum, 2-hour minimum). The

limit applies to conditions occurring on a single given day; even if this limit is met,

recurring exposure patterns still must be checked for agreement with the larval recruitment

limit described in (6) below. Refer to Figure 10 of this document for an explanation of the

derivation of this limit.

(5) Protection of Growth Effects from Episodic or Cyclic Exposure

This limit is derived from the dose-response relationship for DO vs. growth reduction

for the American lobster, and comparisons of the effects of cyclic exposure versus

constant exposure on growth for a variety of species. It provides a degree of protection

equivalent to the CCC, but for exposure durations shorter than a day. The limit represents

the DO conditions that maintains a daily percent growth reduction not greater than 25%. 

The cumulative effects of all exposure interval durations at a given DO below the CCC are

accounted for by summing the percent reductions for time intervals at representative DO

concentrations. An adjustment factor of 1.56 was derived to estimate time-variable effects

from intermittent exposure tests that indicated residual, or delayed, recovery effects from

various growth-inhibiting conditions. The limit applies to DO conditions that may occur

as a recurring pattern throughout the year without adverse growth effects at the CCC level

of protection. However, a recurring pattern of exposure may be limited for a certain

number of days based on the larval recruitment limit (6). Recurring patterns of DO

conditions that do not meet the growth limit may be allowed for a limited number of days

in a recruitment season, provided the larval recruitment limit is met according to (6). 

Refer to Table 4 and Figure 12 of this document for an explanation of the derivation of

this growth limit. The larval recruitment limit can be substituted in whole for the growth

limit.
39

(6) Protection of Larval Recruitment Effects from Episodic or Cyclic Exposure

This limit is derived from the modeled relationships between daily cohort mortality

and the allowable number of days at a given maximum daily larval cohort mortality that

protects against greater than 5% cumulative impairment of recruitment over a recruitment

season. It provides a degree of protection equivalent to the limits described in (3) above,

but for recurring patterns of low DO as opposed to continuous low DO conditions. 

Figure 16 of this document illustrates how to determine the maximum daily cohort

mortality from duration intervals of DO minima. Figure 17 of this document illustrates

how to determine the allowable number of days of cyclic exposure for a given maximum

daily cohort mortality. This limit provides additional information that should be used in

conjunction with the limits described in (4) and (5) above. The limit determines the

number of days that recurring episodic or cyclic conditions may occur, including whether

the pattern may occur for an unlimited number of days. For example, a cyclic pattern that

includes a DO minimum of 3.0 mg/L for 6 hours results in a daily cohort mortality of

almost 25% (see Figure 16). Assuming this represents the maximum daily cohort

mortality for the cyclic pattern, the allowable number of days for the cyclic exposure is 9

(see Figure 17). Refer to pages 31-34 of this document for a detailed explanation of the

derivation of this limit.

In summary, limits (1) and (4) establish 1-day and hourly minimum conditions that

should be maintained for persistent and cyclic exposures, respectively; limits (3) and (6)

establish conditions that may occur for a limited number of days for persistent and cyclic

exposures, respectively; and limits (2) and (5) establish long-term conditions that should

be maintained for the remaining number of days for persistent and cyclic exposures,

respectively.

Implementation

Dissolved oxygen criteria should be implemented differently from those of toxicants,

but not for reasons associated with biological effects or exposure. Uncertainties

associated with aquatic effects of DO, such as behavior, synergistic relationships with

temperature, salinity, or toxics, apply to toxics as well. Dissolved oxygen also does not

differ from toxics for reasons associated with exposure. Dissolved oxygen can vary

greatly in the environment, but so can toxics. Effluents and their receiving waters can vary

daily, even hourly, in their toxicity to aquatic life. Toxicity of saltwater-receiving waters

also can vary with the tide and the depth of water (Thursby et al., 2000). It may be

mistakenly perceived that DO varies more in concentration simply because it can be

measured easily and nearly continuously.

From the standpoint of environmental management, DO differs from toxic

compounds primarily because it is not regulated directly. Hypoxia is a symptom of a

problem, not a direct problem. Dissolved oxygen is regulated primarily by controlling

discharges of nutrients (in the marine environment, most commonly nitrogen). Dissolved

oxygen also differs from most toxic compounds because hypoxia can have a large natural
40

component. Therefore, criteria for hypoxia should not automatically be applied in the

same way as limits for toxicants are. 

This document provides the information necessary for environmental planners and

regulators in the Virginian Province to address the question of whether DO at a given site

is sufficient to protect coastal or estuarine aquatic life. The document does not address

how compensatory mechanisms such as avoidance can influence the response of local

populations to seemingly adverse DO conditions. The document also does not address the

issue of spatial extent of a DO problem. In other words, even if the DO at a site is low

enough to significantly affect aquatic life, the environmental manager will have to judge

whether the hypoxia is widespread enough for concern. Finally, as with all criteria, this

document does not address changes in sensitivity to low DO that accompany other

stresses such as high temperature, extremes of salinity, or toxicants. Chief among these

concerns would be high temperature because high temperature and low DO often appear

together. Low DO will be more lethal at water temperatures approaching the upper

thermal limit for species. This effect has been seen for freshwater species (U.S. EPA,

1986; Secor and Gunderson, 1998), and saltwater species (e.g., C. irroratus and E.

affinis). The limits provided here should be sufficient under most conditions where

aquatic organisms are not otherwise unduly stressed.

Many programs that monitor coastal DO with electronic equipment cannot measure

DO to better than 0.5 mg/L due to limitations of instrument accuracy and resolution (e.g.,

Strobel et al., 1995; Strobel and Heltshe, 1999) or sampling design (Summers et al.,

1997). Attempts to refine the limits presented here or to apply these limits in assessing

field DO conditions should take this into account. Criteria for DO can be appropriately

used in a risk assessment framework. The approach outlined in this document can be

easily used to compare DO conditions among areas, and determine if the DO conditions

are adequate to support aquatic life. Environmental managers can determine which sites

need the most attention, and evaluate the spatial and temporal extent of hypoxic problems

from one year to the next for sites of concern.

Environmental managers who wish to use the protective approach presented here

will have to decide several questions about how the limits will be used, five of which are

described below.

1. Accuracy of monitoring data—The most important decision is to determine

how accurate the monitoring data are—the better that hypoxia is characterized,

the more reliably one can decide whether it meets the criteria. Data from

existing monitoring programs may not always be accurate enough to take full

advantage of the approach provided here. For example, a recent assessment of

conventional sampling procedures along the Atlantic and Gulf coasts has

suggested that hypoxia in their estuarine waters is substantially more

widespread than previously believed (Summers et al., 1997). Deciding what

data can adequately characterize hypoxia is a matter of risk management. 

Cyclic conditions may require measurements every 30 min for several days,

whereas persistent hypoxia may need only several measurement a week.
41

Decisions also have to be made about the number and locations of sampling

sites to properly represent a given area. 

2. Biological effects—Potential biological effects are most difficult to predict

when DO lies between the limits for juvenile and adult survival and larval

growth. Deciding whether concentrations between these limits are acceptable

will depend in part on several biological parameters related to the recruitment

model. How to best represent these issues is a risk-management decision. The

5% impairment level for seasonal larval recruitment was selected to be

consistent with the protection provided to juvenile and adult life stages, but a

different percentage (higher or lower) may be valid for a site-specific DO

criteria. The biological effects data represent the expected range of sensitivity

to hypoxia for the Virginian Province. In certain site-specific situations, data on

additional species more representative of the site may be desirable. Deletion of

data from the current data set, however, should be done with caution. The fact

that a species (e.g., American lobster) may not be present at a more southern

site does not mean that it does not represent sensitive species in the community

that could not be tested. In addition, the lengths of recruitment season and

larval development period may be adjusted to be consistent with conditions

expected at a site.

3. Spatial extent—After environmental managers have found a hypoxic area, they

must decide whether it is small enough relative to nearby unaffected areas to

allow the coastal region as a whole to meet the criteria.

4. Freshwater versus saltwater—It is not trivial to decide whether the DO in

certain parts of estuaries should be judged by freshwater criteria or saltwater

criteria, particularly where the tides vary the salinity between near fresh and a

few parts per thousand. This decision is important because the criteria for

freshwater are greater than the saltwater limits developed here, depending on

water temperature and the life stage being protected (U.S. EPA, 1986). A

reasonable way to start is by considering their biological communities. If they

are more like freshwater organisms, freshwater criteria should be applied. If

they are more like saltwater, then saltwater criteria apply. 

5. Threatened and endangered species—In cases where a threatened or

endangered species occurs at a site, and sufficient data exist to suggest that it is

more sensitive at concentrations above the criteria, it is appropriate to consider

development of site-specific criteria based on this species.
43

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