1

Integrated Assessment of

Hypoxia in the northern Gulf of Mexico

DRAFT FOR PUBLIC COMMENT
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Acknowledgments

Hundreds of scientists, from a wide array of different fields, contributed over the years to the

extensive knowledge base on which this assessment depends. An intensive collaborative effort

over the past two years has assembled and reviewed available information. Special thanks are

due to all who participated, particularly the many peer reviewers and the editorial board, whose

efforts have sharpened thinking and clarified presentation.

CENR Task Force on Harmful Algal Blooms and Hypoxia

Nancy Foster, NOAA — chair

Brad Campbell, CEQ

Mary Clutter, NSF

Allen Dearry, HHS

Sherwood Hall, FDA

Glenda Humiston, USDA

Anna Johnson-Winegar, Navy

Eric Lindstrom, NASA

Hugh McKinnon, EPA

Michael Rodemeyer, OSTP

Mark Schaefer, DOI

Lois Schiffer, DOJ

Hypoxia Working Group

Don Scavia, NOAA — chair

Kay Austin, EPA

Mary Belefski, EPA

Dale Bucks, USDA

Herb Buxton, USGS

David Correll, Smithsonian

Mike Dowgiallo, NOAA

John Field, NOAA

Penelope Firth, NSF

James Giattina, EPA

Louis Goodmal, Navy

Don Goolsby, USGS

Tom Pullen, USACOE

Donald Pryor, NOAA

Joseph Schubauer-Berigan, EPA

Fred Swader, USDA

Larinda Tervelt-Norton, EPA

Ken Turgeon, MMS

Joan Warren, EPA
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Hypoxia Assessment Reports

As a foundation for the assessment, six interrelated reports that examine various aspects of the

hypoxia issue were developed by six teams with experts from within and outside of government.

The research teams were established not to conduct new research, but rather to analyze existing

data and to apply existing models of the watershed-Gulf system. This integrated assessment

draws heavily from the results in these six reports.

Each of the reports underwent extensive peer review by independent experts guided by an

editorial board. Editorial Board members were Dr. Donald Boesch from the University of

Maryland, Dr. Jerry Hatfield from the U. S. Department of Agriculture, Dr. George Hallberg

from the Cadmus Group, Dr. Fred Bryan from Louisiana State University, Dr. Sandra Batie from

Michigan State University, and Dr. Rodney Foil from Mississippi State University.

# 1. Characterization of hypoxia -- Describes the seasonal, interannual, and long-term variation

of hypoxia in the northern Gulf of Mexico, and its relationship to nutrient loadings. It also

documents the relative roles of natural and human-induced factors in determining the size and

duration of the hypoxic zone.

Nancy N. Rabalais, Louisiana Universities Marine Consortium — Lead

R. Eugene Turner, Louisiana State University

Dubravko Justic, Louisiana State University

Quay Dortch, Louisiana Universities Marine Consortium

William J. Wiseman, Jr., Louisiana State University

# 2. Ecological and economic consequences of hypoxia -- Presents an evaluation of the

ecological and economic consequences of nutrient loading, including impacts on Gulf of Mexico

fisheries and the regional and national economy.

Robert J. Diaz, Virginia Institute of Marine Science — Ecological co-lead

Andrew Solow, Woods Hole Oceanographic Institution -- Economics co-lead

with the assistance of many others

# 3. Flux and sources of nutrients in the Mississippi-Atchafalaya River Basin— Identifies the

sources of nutrients within the Mississippi-Atchafalaya system and within the Gulf of Mexico,

estimating both their location and the relative importance of specific human activities in

contributing to these loads.

Donald A. Goolsby, U.S. Geological Survey -- Lead

William A. Battaglin, U.S. Geological Survey

Gregory B. Lawrence, U.S. Geological Survey

Richard S. Artz, National Oceanographic and Atmospheric Administration

Brent T. Aulenbach, U.S. Geological Survey

Richard P. Hooper, U.S. Geological Survey

Dennis R. Keeney, Leopold Center for Sustainable Agriculture

. Gary J. Stensland, Illinois State Water Survey
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# 4. Effects of reducing nutrient loads to surface waters within the Mississippi River Basin and

Gulf of Mexico — Estimates the effects of nutrient source reductions in the MississippiAtchafalaya Basin on water quality in these waters and on primary productivity and hypoxia in

the Gulf.

Patrick L. Brezonik, University of Minnesota — Upper watershed co-lead

Victor J. Bierman, Jr., Limno-Tech, Inc. — Gulf of Mexico co-lead

Richard Alexander, U.S. Geological Survey

James Anderson, University of Minnesota

John Barko, Waterways Experiment Station, U.S. Army Corps of Engineers

Mark Dortch, Waterways Experiment Station, U.S. Army Corps of Engineers

Lorin Hatch, University of Minnesota

Gary L. Hitchcock, University of Miami

Dennis Keeney, Iowa State University

David Mulla, University of Minnesota

Val Smith, University of Kansas

Clive Walker, Blackland Research Center

Terry Whitledge, University of Alaska

William J. Wiseman, Jr., Louisiana State University

# 5. Reducing nutrient loads, especially nitrate-nitrogen, to surface water, ground water, and the

Gulf of Mexico — Identifies and evaluates methods to reduce nutrient loads to surface water,

ground water, and the Gulf of Mexico.

William J. Mitsch, The Ohio State University -- Lead

John W. Day, Jr., Louisiana State University

J. Wendall Gilliam, North Carolina State University

Peter M. Groffman, Institute of Ecosystem Studies

Donald L. Hey, The Wetlands Initiative

Gyles W. Randall, University of Minnesota

Naiming Wang, The Ohio State University

# 6. Evaluation of economic costs and benefits of methods for reducing nutrient loads to the Gulf

of Mexico -- Evaluates the social and economic costs and benefits of the methods identified in

Topic 5 for reducing nutrient loads, and assesses various incentive programs and any anticipated

fiscal benefits generated for those attempting to reduce sources.

Otto C. Doering, Purdue University — Lead

Francisco Diaz-Hermelo, Purdue University

Crystal Howard, Purdue University

Ralph Heimlich, Economic Research Service, USDA

Fred Hitzhusen, The Ohio State University

Ricahrd Kazmierczak, Louisiana State University

John Lee, Purdue University

Larry Libby, The Ohio State University

Walter Milon, University of Florida

Tony Prato, University of Missouri

Marc Ribaudo, Economic Research Service, USDA
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TABLE OF CONTENTS

Executive Summary

Introduction

1. The Problem

2. Causes of Gulf of Mexico Hypoxia

• Coastal Eutrophication

• Nutrient Over-Enrichment

• Sources, Loads, and Trends of River Nutrients

3. Consequences of Hypoxia in the Northern Gulf of Mexico

• Consequences in the Gulf

• Consequences in the Mississippi-Atchafalaya River Basin

4. Effects of Changing Nutrient Loads

• Effects if Current Activities Remain Unchanged

• Effects if Reduction Measures Are Undertaken

5. Approaches for Reducing Nutrient Loads

• Reducing Inputs

• Restoring and Enhancing Denitrification and Nitrogen Retention Processes

• Cost-Effective Strategies

6. Adaptive Management: Action, Monitoring, and Research

• Monitoring and Research in a Modeling Framework

• Monitoring Needs

• Research Needs

GLOSSARY

REFERENCES
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Executive Summary

Hypoxia is the condition in which dissolved oxygen levels are below those necessary to sustain

most animal life. The largest zone of oxygen-depleted coastal waters in the U.S., and the entire

western Atlantic Ocean, is found in the northern Gulf of Mexico on the Louisiana/Texas

continental shelf. The area affected is about the size of the state of New Jersey. The extent of the

mid-summer, bottom-water hypoxic zone varies but has increased since regular measurements

began in 1985. Since 1993, it has been larger than 10,000 square kilometers (about 4,000 square

miles) and the largest hypoxic zone measured to date was in 1999: 20,000 square kilometers

(about 8,000 square miles).

This assessment summarizes the state of knowledge of the extent, characteristics, causes and

effects of hypoxia in the northern Gulf of Mexico. It also identifies alternatives for reducing

those effects and examines the costs and benefits associated with reduction actions. It is

intended to make scientific information available to non-specialists. The information provides a

basis for an action plan but this assessment is intended to describe options rather than make

recommendations. This assessment also identifies additional research needed to reduce

uncertainties in important aspects of our understanding.

CAUSES AND CONSEQUENCES

Hypoxia results when the rate of oxygen consumption mostly by bacteria-decomposing organic

material in the water column exceeds the rate of oxygen production by photosynthesis and by

replenishment at the air/water interface from the atmosphere. Organic matter is supplied either

from external sources or produced within the system through biological processes stimulated by

nutrient addition. Scientific investigations over the last several decades indicate overwhelmingly

that hypoxia in the northern Gulf of Mexico is caused primarily by excess nutrients delivered to

those waters from the Mississippi-Atchafalaya River Basin (the Basin), in combination with

stratification of Gulf waters. Model analyses also confirm that river nutrient loads are highly

significant in controlling hypoxia and that rates of algal production and oxygen depletion change

significantly when modeled river loads are changed. Nutrients fuel accelerated growth of

phytoplankton. Fecal pellets from grazing zooplankton and dead phytoplankton cells then sink,

increasing the flux of organic material to bottom waters. Decomposition of this organic material

consumes oxygen in the bottom water layers, and stratification of dense bottom water below

warmer and frequently fresher surface water minimizes mixing. At dissolved oxygen levels of

less than 2 milligrams per liter (2 mg/l), the general definition of hypoxia, mobile organisms

leave and those remaining die off at varying rates depending on how low the oxygen level gets

and for how long.

Very low levels of dissolved oxygen occur naturally but evidence from the Gulf of Mexico and

around the world indicates that human activities are intensifying the natural phenomenon.

Sediment cores collected in the hypoxic zone of the Gulf show that algal production and

deposition, as well as oxygen stress, were much lower earlier in the century and that significant

increases occurred in the latter half of the century. Over this period there have been three major

changes in the drainage basin affecting the river flux to the Gulf: (1) channelization of the river
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for flood control and navigation, mostly completed prior to the 1950s (except the Atchafalaya

modifications in the early 1980s); (2) alterations in the landscape (e.g., deforestation and

artificial agricultural drainage) that removed much of the “buffer” for runoff into the Mississippi

tributaries and main stem, with the greatest rates of change in the 50-year period straddling the

turn of the last century and another burst in drainage development during 1945-1960; and (3) a

dramatic increase in fertilizer nitrogen input into the Mississippi River drainage basin between

the 1950s and 1980s.

Since 1980, the Mississippi and Atchafalaya Rivers have discharged, on average, about 1.6

million metric tons of total nitrogen to the Gulf each year. The most significant trend in nutrient

loads has been in nitrate, which has almost tripled from 0.33 million metric tons per year during

1955-70 to 0.95 million metric tons per year during 1980-96. Other forms of nitrogen, as well as

organic carbon and nutrients such as phosphorus, have not increased, and evidence indicates

their fluxes have probably decreased over this century. About 90% of the nitrate comes from

non-point sources, with the remainder coming from point sources. About 56% of the nitrate

transported to the Gulf enters the Mississippi River above the Ohio River. The Ohio basin

subsequently adds another 34% of the nitrate load. The principal sources of nitrate are river

basins that drain agricultural land in southern Minnesota, Iowa, Illinois, Indiana and Ohio.

Fisheries are affected by this large hypoxic zone. Both abundance and biomass of fish and

shrimp are significantly less where bottom water concentrations of oxygen fall below 2 mg/l.

Comparison of the distribution of fishing effort shows that the industry has shifted shrimping

efforts away from hypoxic zones. Yet, brown shrimp catch, the most economically important

commercial fishery in the Gulf, declined from a record high in 1990 to below average during the

years 1992-97, coinciding with years of greatly increased hypoxia. However, economic analysis

of fisheries catch data did not demonstrate statistically significant effects attributable to hypoxia.

Significant changes in the ratios among the nutrients nitrogen, phosphorus and silica may also

lead to important alterations in the types of algae present. Other components of the food chain

and habitat are also affected. For example, copepods, the dominant zooplankton and important

fish prey in the northern Gulf of Mexico, are lower in abundance or absent when dissolved

oxygen is less than 1 mg/l. Bottom communities in the hypoxic zone consist of disturbanceadapted populations, compared to the late-successional stage “equilibrium”-type communities in

less affected areas in Mississippi Sound. During hypoxia, significant amounts of the system’s

energy are shunted from zooplankton and other invertebrates to bacteria, and energy flows

through the ecosystem in pulses, favoring opportunistic species with shorter life cycles that can

take advantage of the shortened time bottom habitats are available. The impact is an overall

reduction in biodiversity, abundance and biomass.

Water quality in the drainage basin is also degraded by excess nutrients. Most states in the Basin

have substantial numbers of river miles impaired by high nutrient conditions or not fully

supporting the resource uses of aquatic life, fish consumption, swimming, and drinking water

supply. Although state numeric nutrient standards are not established, some researchers

considering eutrophication conditions have suggested an approximate boundary of 1.5 mg/l for

total nitrogen and 0.075 mg/l for total phosphorus. About 30-55% of the Ohio, Lower

Mississippi, and Tennessee sub-basins (defined as hydrologic cataloging units) exceed this
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suggested eutrophic criterion for total phosporus; 16-40% exceed the criterion suggested for total

nitrogen. Even higher exceedance frequencies (70-75% for total nitrogen) were found in the

Missouri, Upper Mississippi, and Arkansas-Red sub-basins.

REDUCTION EFFECTS AND APPROACHES

Potential futures if current activities are unchanged -- Nitrogen loading appears to have

reached a plateau at the level of about 1.6 million metric tons/yr, which is the mean loading over

the period 1980-96. If the set of activities which affect nutrient flux continues to prevent

increasing trends, the current range of dimensions and severity of the hypoxic zone on the

Louisiana shelf would most likely be maintained. Hypoxia would vary, depending on other

factors, such as the timing and extent of spring and summer stratification, weather patterns,

temperature, and precipitation in the Gulf and drainage basin. Thus, even if flux did not trend

upward, natural variability could produce new extremes of hypoxia extent and impacts. Nutrient

impairments to water quality in the Basin are unlikely to improve without some changes in

current activities.

Under a “no action scenario,” it is important to recognize that other factors may increase nutrient

loads. For example, efforts to reduce loads (e.g., ongoing improvements in farming practices,

better targeting of riparian and wetland restoration, and improved river flow management) may

be offset by increases in population growth, food production, and climate change that will tend to

increase nutrient loss from land. If experiences in other systems are applicable to the Gulf of

Mexico, then increased loads would lead to worsening hypoxic conditions and, at some point

fisheries and other species would decline, perhaps precipitously. Spawning grounds, migratory

pathways, feeding habitats, and fishing grounds of important species are all affected by the

extent and severity of hypoxic waters. Thus, an expanding hypoxic zone could lead to further

declines in productivity at higher levels of the food chain and loss of essential habitats.

Potential futures if loading is reduced -- Experience has demonstrated that large ecosystems do

respond positively to nutrient reductions. Examples include Cheaspeake, Tampa and Sarasota

Bays. There are also many examples of small-scale hypoxia reversals associated with

improvements in treatment of sewage and pulp mill effluents. In the Gulf of Mexico, reductions

in total nitrogen flux of about 40% are necessary to return to loads comparable to those before

the large increases in the 1950-70 period. Model simulations indicate that nutrient load

reductions of 20-30% would result in a 15 to 50% increase in bottom water dissolved oxygen

concentrations and a 5 to 15% decrease in surface chlorophyll concentrations. Such increases in

oxygen are significant in that they represent an overall average for the hypoxic zone and that any

increase above the 2 mg/l threshold will have a significant effect on marine life.

Taking Effective Actions -- The two primary approaches to reduce, mitigate, and control

hypoxia in the Gulf of Mexico are: (1) reducing inputs of nitrogen to streams and rivers in the

Basin and (2) restoring and enhancing natural denitrification processes in the Basin.
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In the first category, the most effective actions include improved management practices to retain

nitrogen on fields, reducing application of nitrogen fertilizer (particularly above recommended

rates), implementing alternative cropping systems, decreasing feedlot runoff, and reducing point

sources. Other measures, such as reducing urban nonpoint sources and atmospheric deposition,

could provide important contributions; but, on a Basin-wide scale, they are not as significant.

Nitrogen trading among all sectors could offer opportunities to obtain least-cost reductions.

Based on economic analysis, the cost of reducing edge-of-field nitrogen losses by 20% under

economically optimum conditions was estimated to be $0.88 per kilogram reduction (about $0.40

per pound). To achieve an equal reduction through limits on fertilizer use would require 45%

fertilizer reduction. However, edge-of-field nitrogen losses could be reduced by about 10%

through a 20% reduction in fertilizer use at estimated costs of $0.69 per kilogram ($0.31 per

pound). Costs of reductions of point sources and atmospheric deposition were estimated to be in

the range of $10-100 per kilogram (about $5-50 per pound).

In the second category, the most effective actions would be increasing the acreage of wetlands

and riparian buffers within the Basin. Nitrogen transformations in wetlands and riparian soils, in

surface water and in ground water involve several microbiological processes, some of which

denitrify – that is, they make nitrogen effectively unavailable as a nutrient for plant uptake by

removing it from the water as nitrogen gas. The most effective use of wetland restoration and

creation would be in watersheds that discharge high amounts of nitrogen. At typical

denitrification rates for flow-through wetlands, 5 million acres (0.7% of the Basin area) of

constructed or restored wetlands would reduce nitrogen load to the Gulf by 20%. Riparian areas

are vegetated areas, mostly forested, next to water resources. They are also effective in

removing nitrates, but typical denitrification rates are less than those of wetlands. An estimated

19 million acres (2.7% of the basin) of additional riparian buffers would be needed to reduce

nitrogen load to the Gulf by 20%. Reintroducing river water to the backwaters, coastal wetlands,

and shallow inshore bodies of water on the delta could augment efforts to reduce nutrient inputs

from the upstream sources and help reduce the rate of coastal land loss. Potential deleterious

effects of diversions such as eutrophication of embayment estuaries receiving the flows should

be avoided as much as possible. Direct costs of creating and restoring 5 million acres of

wetlands were estimated to be $8.90 per kilogram reduction in nitrogen flux (about $4.05 per

pound).

All of the nutrient-reduction approaches will also produce environmental benefits within the

drainage basin. These include the values associated with restored wetlands, reduced soil erosion,

reduced nutrient contamination of drinking water, reduced vulnerability to floods, better wildlife

habitat, and improved water quality for recreational uses. Other potential benefits of nutrient

reduction activities include more efficient use of organic and inorganic fertilizers and the energy

associated with them, lower overall fertilizer costs, decreased health risk from contamination of

public and private drinking-water supplies, and improved aquatic habitat in streams, lakes, rivers,

and estuaries. Good estimates for the economic value of these benefits are only available for a

few categories, such as wetlands and erosion. When these estimated benefits are factored in, the

unit costs of wetlands and farm practices become close. With benefits included, reducing edgeof-field nitrogen loss by 20% was estimated to have a net cost of $0.80 per kilogram ($0.36 per

pound), while equal reductions through restoration and creation of 5 million acres of wetlands

would have net costs of $1.00 per kilogram ($0.45 per pound).
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Based on analysis and modeling of broad aggregates across the Basin, one illustrative approach

to reduce nitrogen loss by 20% would be through a program of 20% reduction in fertilizer use,

and 5 million acres of wetlands construction or restoration. This combination would produce

costs and acreage impacts that would be lower than for other combinations. However broader

combinations of carefully refined, targeted, and flexible variations on the full array of

alternatives, in synergy with other programs, could improve cost-effectiveness.

ADAPTIVE MANAGEMENT: ACTION, MONITORING AND RESEARCH

This assessment is based on assembly and peer review of a massive amount of direct and indirect

evidence collected and reported over many years of scientific inquiry. A comprehensive,

carefully targeted program of monitoring, modeling, and research to facilitate continual

improvement in scientific knowledge and adjustments in management practices should be

coupled to whatever initial nutrient management strategies are chosen. This adaptive

management scheme involves continual feedback between interpretations of new information

and improved management actions.

Monitoring, research, and decisions about taking action to reduce hypoxia should be integrated

using holistic models that simulate our understanding of how the overall system functions and

how actions can best be implemented. A comprehensive monitoring program requires

measurement of the pressures and the environmental response both in the Gulf and in the Basin.

Indicators of programmatic progress should also be established and periodically revisited.

Scientific understanding has been advanced significantly by the work on which this assessment

is based. The conclusions provide a solid foundation on which to build an appropriate response

strategy. There are, however, always uncertainties in scientific analysis. Immediate priorities

for research to address these uncertainties include research on the ecological effects of hypoxia,

watershed nutrient dynamics, and agricultural practices. Focused effort in these areas can provide

near-term information to guide decisions about management actions. Longer-term priorities to

fill critical gaps in understanding and guide future management strategies include research on

nutrient cycling and carbon dynamics, long-term changes in hydrology and climate as well as

economic and social impacts.
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Introduction

The goal of this assessment is to document the state of knowledge of the extent, characteristics,

causes and effects (both ecological and economic) of hypoxia in the northern Gulf of Mexico.

This assessment also aims to identify alternatives to reduce those effects and to examine the

costs and benefits associated with these alternatives.

The National Science and Technology Council’s Committee on Environment and Natural

Resources undertook the task to assess the state of scientific knowledge and understanding of

Gulf of Mexico hypoxia in 1997. An assessment plan was developed in conjunction with the

Mississippi River/Gulf of Mexico Watershed Nutrient Task Force (MR/GM Task Force) and

approved in March 1998. In October 1998, Congress passed the Harmful Algal Bloom and

Hypoxia Research and Control Act, which was signed into law as P.L.105-383 by the President

on November 13, 1998. This law calls for an “integrated assessment of hypoxia in the northern

Gulf of Mexico that examines: the distribution, dynamics and causes; ecological and economic

consequences; sources and loads of nutrients transported by the Mississippi River to the Gulf of

Mexico; effects of reducing nutrient loads; methods for reducing nutrient loads; and the social

and economic benefits of such methods.”

As a foundation for this assessment, six reports that examine various interrelated aspects of the

hypoxia issue were developed by six teams of experts from within and outside of government.

The teams were not established to conduct new research, but rather to review and analyze

existing data and apply existing models of the watershed-Gulf system. This integrated

assessment draws heavily from the results in these six reports. Both the findings and the words

of these reports have been used freely in this assessment. Explicit citations to the reports are

included only when reference is made to specific information that a reader may want to examine

in greater detail.

The six topic reports draw on a rich variety of work that preceeded them, including in-depth

studies of oceanographic, hydrologic, agricultural, economic, and other questions related to the

issue of hypoxia in the Gulf of Mexico. They underwent rigorous peer review with oversight by

an independent editorial board. In addition, the six topic reports were available for a 90-day

public comment period. The reports and the comments received on them are available at

<http//www.nos.noaa.gov/products/pubs_hypox.html>. This draft assessment has been written

with consideration of all that information and will also be available for a period of public

comment before it is finalized.

This assessment is intended to be useful to non-specialists as well as specialists. It has been

written attempting to use a minimum of specialized scientific terminology yet still precisely

describe the state of knowledge. A glossary defining the scientific terms that are used is

included at the back of this document.

In addition to this assessment, P.L. 105-383 calls for the development of a plan of action to

reduce, mitigate and control hypoxia in the northern Gulf of Mexico. The Action Plan will be

developed by the MR/GM Task Force. This integrated assessment is intended to provide

scientific information as a basis for the Action Plan but it is not intended to make
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recommendations for action nor is it the only source of information which the MR/GM Task

Force will consider in developing the Action Plan.
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1. The Problem

Nearly all marine animals depend on oxygen dissolved in the water. Hypoxia is the condition in

which there is so little dissolved oxygen that aquatic life is disrupted.

Hypoxia is generally defined by dissolved oxygen levels below 2 mg/l (or ppm) -- approximately

20% of the dissolved oxygen that northern Gulf of Mexico waters could hold in solution at

typical summer temperatures and salinities. Mobile organisms leave, die, or are eaten as

dissolved oxygen levels decrease below 2 mg/l. Trawlers in hypoxic areas usually do not capture

any shrimp or demersal fish (fish that usually live near the bottom). Burrowing organisms first

emerge from the sediments, then die, if oxygen concentrations remain near 0.5 mg/l for

prolonged periods. In areas where the oxygen concentrations are below 0.2 mg/l, the sediment is

typically black, and sulfur-oxidizing bacteria form mats on the seafloor. Toxic hydrogen sulfide

may be found where bottom waters are completely devoid of oxygen (anoxic).

The largest zone of oxygen-depleted coastal waters in the US, and the entire western Atlantic

Ocean, is found in the Northern Gulf of Mexico on the Louisiana/Texas continental shelf.

FIGURE 1.1 – Map of frequency of occurrence of mid-summer hypoxia

— based on data from Rabalais, Turner and Wiseman from the 60 to 80 station grid

repeatedly sampled from 1985-1999

Hypoxic waters are most prevalent from late spring through late summer. Hypoxia is more

widespread and persistent in some years than in others, depending on river flow, winds, and

other environmental variables. Hypoxic waters are distributed from shallow depths near shore (4

to 5 meters) to as deep as 60 meters but more typically between 5 and 30 meters. Hypoxia

occurs mostly in the lower water column but encompasses as much as the lower half to twothirds of the entire column. Continuous time-series data for the bottom waters in the core of the

hypoxia region show (1) the gradual decline in oxygen in the spring with interruptions due to

wind-mixing events, (2) persistent hypoxia and often anoxia for extended parts of the record

from May through September, (3) occasional summer upwelling of oxygenated water from the

deeper shelf waters, and (4) the seasonal disruption of low oxygen in the fall by tropical storms

or cold fronts.

Systematic study of the hypoxic zone, with regular repeated measurements, began in 1985. As

shown in figure 1.2, the mid-summer bottom areal extent of hypoxic waters in 1985-92 averaged

8,000-9,000 km2 (about 3,000-3,500 square miles) but increased to 16,000-18,000 km2 (about

6,000- 6,500 square miles) in 1993-97. The estimated extent was 12,500 km2 (about 4,800

square miles) in mid-summer of 1998 but increased to 20,000 km2 (almost 8,000 square miles) at

the end of July 1999, the largest ever recorded. The area affected is about the size of the state of

New Jersey.

FIGURE 1.2 – Histogram of estimated areal extent of bottom water hypoxia for mid-summer

cruises 1985-99 --- based on data from N. Rabalais, Louisiana Universities Marine Consortium
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2. Causes of Gulf of Mexico Hypoxia

This chapter examines the scientific evidence concerning the causes of the formation of hypoxic

bottom waters in the northern Gulf of Mexico. While information gaps still exist and several

factors discussed below may contribute to hypoxia, the overwhelming scientific evidence

indicates that inputs of nitrogen from the Mississippi River drainage basin is the primary factor

driving hypoxia. Although hydrologic and climatic factors influence the onset and duration of

hypoxia, it is this nitrogen flux that fuels the annual event. This evidence, described in the

detailed technical reports and the scientific literature upon which this assessment is based, is

summarized in the following section. Analysis of nutrient trends and sources, and the relative

importance of specific human activities in contributing to these trends, are then described,

providing the basis for the chapters that follow.

COASTAL EUTROPHICATION

Development of hypoxia in northern Gulf of Mexico bottom waters, as well as in most ocean and

freshwater systems, is described best by the eutrophication paradigm. Eutrophication is defined

as an increase in the rate of supply of organic matter in an ecosystem (Nixon 1995). This

increase in organic matter can be due to flux from external sources or production within the

system through biological processes stimulated by nutrient addition. Controlling or reversing the

effects of eutrophication necessitates understanding the relative roles of externally supplied and

internally produced organic carbon. Externally supplied organic carbon most often comes from

wetlands or uplands via rivers. Internally produced organic carbon comes from the growth of

algae.

FIGURE 2.1 - The “Eutrophication Process” (modified from Downing et al. 1999) –

Eutrophication occurs when organic matter increases in an ecosystem. Eutrophication

can lead to hypoxia when decaying organic matter on the bottom depletes oxygen and

replenishment is blocked by stratification. The flux of organic matter tothe bottom is

fueled by nutrients carried by riverflow or, possibly, from upwelling that stimulate

growth of phytoplankton algae. This flux consists of dead algal cells together with fecal

pellets from grazing zooplankton. Organic carbon from the river can also contribute to

the flux of organic matter.

As shown in figure 2.1, nutrients stimulate production of phytoplankton algae. Algae that are not

incorporated into the food web, fecal products, and other debris from zooplankton feeding on

algae, together with externally supplied particulate organic carbon, sink into bottom waters as

“excess” organic material. Decomposition of this organic material by bacteria and other

organisms consumes oxygen, and when the consumption rate is faster than its replenishment,

oxygen concentrations decrease.
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Excess organic carbon and subsequent oxygen consumption alone do not necessarily lead to

hypoxia -- stratification is also necessary for the development of hypoxia in bottom waters. In

most saltwater systems, the water stratifies (creating horizontal layers), with cold and/or saltier

water at the bottom and warmer and/or fresher water at the surface. This layering separates

bottom waters from the atmosphere and prevents re-supply of oxygen from the surface to

replenish deficits created by the decomposing organic matter in the deeper waters. Thus, the

extent of hypoxia is determined by the balance between the rate of delivery and decomposition

of algae and other organic matter and the rate of oxygen re-supply, which is inversely related to

the strength of stratification.

Hypoxia, and other symptoms of eutrophication, such as growth of nusiance or toxic algae and

loss of submerged aquatic vegetation, are major stresses in many coastal ecosystems. Over half

of the nation’s estuaries experience low oxygen and other symptoms of eutrophication (Bricker

et al. 1999). Almost all of these problems are caused or exacerbated by the increased flow of

nutrients from land due to human activities. There is growing evidence around the world that

low oxygen is having pervasive effects on shallow coastal and estuarine areas (Diaz and

Rosenberg 1995). While hypoxia can occur naturally and has existed throughout geologic time,

its occurrence in shallow coastal and estuarine areas appears to be increasing and is most likely

accelerated by human activities (Vitousek et al. 1997; Jickells 1998).

Role of Nutrients

Nitrogen is the most significant nutrient controlling algal growth in coastal waters, while

phosphorus is the most significant nutrient in fresh water (NRC 1993). Silicon also plays an

important role in that it regulates the production of diatoms which are algae that are an important

food for fish and invertebrates. Diatoms, unlike other phytoplankton species, require silicon to

build their cell walls and are the dominant biomass component of many marine and estuarine

phytoplankton communities, particularly in the spring. If silicon supply is limited relative to the

other nutrients, then non-siliceous algae, such as dinoflagellates and cyanobacteria, may become

proportionately more important in the phytoplankton community. Some of these forms are

harmful or toxic. The ratios among dissolved silica, nitrogen, and phosphorus in the lower

Mississippi River now closely approximates the typical ratio of these nutrients in diatoms. Thus,

while nitrogen is the nutrient that controls the quantity of coastal algal production, significant

changes in the relative composition of nutrient supplies may also lead to important alterations in

the type of algae present.

Potentially Contributing Factors

Among the potential factors contributing to Gulf hypoxia, only increased nutrient loads from the

Mississippi/Atchafalaya River system can account for the magnitude of the hypoxic zone and its

increase over time in a non-contradictory manner. While other factors may contribute to the

growth, dynamics, and decay of the hypoxic zone, none of them, alone or in combination, can

explain its overall size and persistence.

Internally produced organic carbon, stimulated by nutrients (from the land, air, or sea), externally

supplied organic carbon, horizontal stratification, ocean circulation, and river hydrology are not

competing hypotheses, but rather interacting factors within the eutrophication paradigm applied
16

to the Gulf of Mexico. These factors have been discussed in the six technical background

papers, in the comments on those papers, in recent reports on this subject sponsored by the

Fertilizer Institute and the Council for Agricultural Science and Technology (Carey et al. 1999

and Downing et al. 1999), as well as the wide range of previous work summarized in volumes

such as the 1995 Gulf of Mexico Hypoxia Management Conference (EPA 1997) and the

December 1994 and June 1996 special issues of Estuaries. The most significant potential

contributing factors for the Gulf of Mexico and its drainage basin are described in the box on the

next page.
17

Factors Potentially Contributing to Hypoxia in the Gulf

Landscape changes in the drainage basin -- Wetlands and riparian zones can improve water quality and

reduce nitrogen flux down the Mississippi River by enhancing denitrification (the conversion of nitrate to

nitrogen gas, with subsequent loss from the aquatic system) and incorporating nitrogen into vegetation.

However, the natural capacity of the river basin to remove nutrients has diminished. Many of the original

freshwater wetlands and riparian zones that were connected to streams and rivers are gone. Ohio, Indiana,

Illinois, and Iowa have had over 80% of their wetlands drained. Indiana, Illinois, Iowa, Minnesota, Missouri,

and Wisconsin collectively have lost the equivalent of 14.1 million ha (35 million acres) of wetlands over the

past 200 years.

Organic loading from the Mississippi River -- Externally supplied organic carbon from the Mississippi River

has been proposed as a cause for the formation of hypoxia in the Gulf. For river-borne organic carbon to be an

important contributor to the increase in hypoxia since the 1950s, significantly increasing quantities would have

to be delivered to the bottom waters of the hypoxic zone and be decomposed by bacteria. Only the particulate

organic carbon (POC) fraction of the total organic carbon load could contribute directly to hypoxia because

only that fraction would be delivered to the bottom waters. The dissolved fraction would remain in the riverfreshened surface waters and would have to be biologically assimilated and deposited as newly formed POC,

with large attendant respiratory losses, to reach hypoxic zones extending 100-200 km from the river discharges.

Suspended sediment in the river has declined by about half since the 1950s, so the POC load that can settle on

the Louisiana shelf has also most likely decreased since then. Also, nutrient cycling affords nitrogen the ability

to stimulate production of organic carbon at rates that far surpass that supplied by the river. Whereas

decomposition of river-supplied organic carbon consumes oxygen only once, river-supplied nitrogen can be

recycled and thus provides a continuous source of comparatively easily decomposed organic carbon (Ryther

and Dunstan 1969).

Channelization of the delta and loss of coastal wetlands -- Although in some years over-bank flow has been

significant, flooding through coastal wetlands has been reduced since the 1927 flood, but geochemical and

biological indicators show that hypoxia has intensified only since the 1950s. Coastal wetland loss rates

exceeded 100 km2 (about 40 square miles) per year between 1950 and 1980. Recent wetlands protection and

restoration programs helped reduce losses in the 1990s to 65-90 km2 (about 25-35 square miles) per year. Both

the loss of wetland filtering capacity and the direct contribution of organic matter eroded from wetlands and

carried to the Gulf may contribute to the problems of hypoxia; however, the total contribution is relatively small

compared with the nitrogen-related factors.

Intrusion of deeper offshore waters -- Some have suggested that flow of low-oxygen water from water layers

in the deeper Gulf on to the continental shelf was the source of shelf hypoxia. However, these features are

always physically distinct, with the shelf hypoxic zone at depths of less than 60 meters, and the deep lowoxygen layer at depths of 400 to 700 meters. Further, oxygen concentrations and consumption rates, salinity,

and temperature of the deep low-oxygen layer differ considerably from the waters of the hypoxic zone. Flow of

nitrate from deeper waters may be important at the shelf edge (at depths of approximately 100 meters);

however, all data indicate that the Mississippi and Atchafalaya Rivers contribute substantially more nutrients to

the inner shelf and hypoxic region.

Short- or long-term climate changes -- River discharge and nitrate concentrations, and sediment core data,

provide almost 100 years of record for this system. On that time scale, there is no indication that climate

factors override the impacts of human activities in the basin. Average annual flow in the Mississippi River

increased 30% between 1955-70 and 1980-96, compared to the 300% increase in nitrate flux over this period.

Episodic events, such as the 1993 flood, can nearly double the nitrate flux to the Gulf in a given year as a result

of both higher-volume flow and increased leaching of nitrate from the drainage basin. There are indications

that the future climate for this basin may be wetter and may include more extreme events, leading potentially to

increased water and nitrate fluxes (Kunkel et al. 1999).
18

NUTRIENT OVER-ENRICHMENT

The evidence of nutrient over-enriched production in the northern Gulf of Mexico and its linkage

with oxygen depletion in the lower water column is consistent with the eutrophication paradigm,

with data and experiences world wide, and with Gulf- and basin-specific information on a variety

of scales.

Scientific investigations over the last several decades indicate overwhelmingly that

oxygen stress in the northern Gulf of Mexico is caused primarily by excess nutrients

delivered to Gulf waters from the Mississippi-Atchafalaya River drainage basin, in

combination with the stratification of Gulf waters.

The scientific evidence supporting river nutrient loads as the primary factor fueling hypoxia in

the Gulf of Mexico through over-enrichment is outlined in this section. It is supported by

analysis of the sources and loads, as described in the next section.

Stratification

If Louisiana shelf waters did not stratify, hypoxia would be unlikely. However the river and

Gulf oceanography create a strongly stratified system each year, characterized by relatively

warm and fresher water that forms a high-production lens over the deeper salty, colder waters.

The greater the density difference (related to temperature and salinity) between layers, the more

stable the stratification. Very strongly stratified systems require a lot of wind energy –

sometimes a tropical storm – in order to mix. If storms do not mix the waters, they will remain

in layers, isolating bottom waters from aeration until fall brings cooler surface temperatures and

the density of the surface water approaches that of the bottom water.

Trends in Indicators of Production, Low-Oxygen Stress, and River Nutrient Flux

Analysis of river discharge data and sediment cores from the Louisiana shelf in the Mississippi

River delta bight indicate that eutrophication and hypoxia in the northern Gulf increased

coincidently with increases in nutrient loads from the Mississippi River. The data exhibit both

long-term trends and short-term variations. One particularly important factor is that peak river

flows have increased over the past 150 years due to flood control and navigation projects in the

river and climate (precipitation) changes.

--- Long-term records – Gulf ecosystem changes. Since there are no comprehensive, direct

measurements of oxygen in the northern Gulf of Mexico prior to 1985, evidence for long-term

changes in hypoxia is based on indirect analysis from sediment records. While it is possible to

explore temporal changes in water-column production, oxygen-relevant geochemistry, and

biology because such properties are preserved sequentially down a sediment core, it is important

to recognize that bottom mixing and sediment reworking limits the resolution of temporal change

to scales approaching decades, as opposed to years.
19

FIGURE 2.2 – Long-term records of Gulf ecosystem change -- These are profiles from cores

taken in the Mississippi River bight in an area of persistent seasonal bottom water

hypoxic conditions. The top profile shows a decrease in the Shannon-Wiener Diversity

Index (SWDI) of ostracods, a family of minute shelled organisms. The next profile

shows increasing glauconite grain abundance, indicating low oxygen conditions. The

third profile shows increasing average concentration of biologically-bound silica,

indicating changes in the algal community composition. The bottom profile shows

increasing organic carbon accumulation rates.

The data shown in figure 2.2 are from cores collected within one area of the current hypoxic

zone, and therefore do not necessarily represent the timing of the transitions for the entire

contemporary hypoxic zone. They do, however, clearly show that increased algal production and

deposition, as well as oxygen stress, were much lower earlier in the century and that significant

changes have occurred in the latter half of the century.

The sediment record clearly shows significant increase in algal production beginning near the

1950s. Accumulation of organic carbon and biogenic silica (a measure of diatom abundance)

both increased significantly after that time period. Because there have been no significant

increasing trends in either the organic carbon or silica river loads, it is reasonable to infer that

these increases in the sediment record since the 1950s are due to production of marine algae.

The sediment geochemistry record indicates that while there was likely some oxygen stress on

the Louisiana shelf as early as the turn of the century, there was a striking increase in that stress

starting roughly during the 1940s and 1950s. Because the mineral glauconite forms only under

reducing conditions, its presence in the sediment is an indication of low oxygen conditions.

Sediment cores show the history of glauconite formation from sediments as they were deposited

on the bottom. As shown in the core profile in figure 2.2, glauconite abundance was relatively

low in the early 1900s, but increased dramatically in the 1940s-1950s, indicating increased low

oxygen stress.

Another indirect indicator of a significant change in the relative magnitude of hypoxia during

this same transition period is the biodiversity of benthic foraminifera and ostracods preserved in

the sediments. These single-celled organisms and metazoans, respectively, produce carbonate

shells which remain intact in buried sediments. After a transition period in the 1940s-1950s, the

diversity of these organisms decreased significantly, which is typical of stressed environments.

Also, the types of organisms that remained after the transition period were those that are more

tolerant of low oxygen conditions. 

-- Long-term records – drainage basin changes. Three key historical trends are important to

consider in relationship to the above long-term trends in hypoxia. First, most of the significant

flood-control and navigation channelization occurred prior to the 1950s. Second, significant

alterations in the landscape (e.g., deforestation and expansion of artificial agricultural drainage)

that removed much of the “buffer” for runoff into the Mississippi tributaries and main stem
20

occurred with the greatest rates of change in the 50-year period straddling the turn of the last

century and another burst in drainage development during 1945 to 1960. Third, there was a

dramatic increase in nitrogen input into the Mississippi River drainage basin, primarily from

fertilizer applications, between the 1950s and 1980s. This increase in human activity in the

drainage basin has led to significant increases in the flux of nitrogen from the Mississippi River

system to the Gulf.

FIGURE 2.3 – Long-term records of drainage basin changes – annual amount of fertilizer

application and area artificially drained (see #5 figure 1.2 and #3 page 44 for details)

-- Shorter-term records. Extensive monitoring of the hypoxic zone did not begin until 1985, and

areal estimates (see figure 1.2) since then have been primarily from a single shelf-wide survey

taken during mid-summer each year. Thus, for any given year, the maximum extent of hypoxia

could have been significantly greater. Despite this limitation, three significant extreme events

illustrate the nature of the variation in this record. First, during the summer of 1988, following a

severe regional drought and subsequent low nutrient load (see Figure 2.4), the hypoxic zone

covered only 40 square kilometers, compared to 5,000-20,000 square kilometers in other years

(15 square miles, compared to 2,000-8,000 square miles). The hypoxic zone had developed in the

spring but was not maintained into the summer because of reduced flow of freshwater and

resulting lack of stratification. Second, the size of the hypoxic zone nearly doubled during the

summer following the 100-year flood of 1993. While it is not clear why this larger area has

persisted in years following 1993, nutrient recycling and resuspension of recently deposited,

algal-produced organic material are likely important factors. The record large extent observed in

1999 may be related to abnormally wet conditions producing high nitrate concentrations in rivers

draining the upper Basin.

Model Simulations

Model analyses provide further evidence connecting the extent of the hypoxic zone to river

nutrient flux. Computer models have been constructed to simulate algal dynamics, nutrient

cycles, sedimentation, organic carbon production and decay, and oxygen dynamics as functions

of river nutrient loads and other environmental factors, such as variations in sunlight and ocean

currents (Bierman et al. 1994; Justic et al. 1996; Justic et al. 1997). These models were designed

to examine the relative roles of various factors on hypoxia, such as ocean circulation, vertical

stratification, light variation, nutrient loads, and climate change. These analyses show that river

nutrient load is a highly significant factor in controlling hypoxia and that rates of algal

production and oxygen depletion change significantly when modeled river loads were changed.
21

SOURCES, LOADS, AND TRENDS OF RIVER NUTRIENTS

Nutrient Trends

River-borne nutrients and water-column stratification are the major factors controlling hypoxia

in the northern Gulf of Mexico. The key nutrients in this process are nitrogen, phosphorus, and

silica. Of these, nitrogen is the most important nutrient leading to the production of excess algae

and subsequent hypoxia in the Gulf and elsewhere (NRC 1993). Nitrogen is also the only

nutrient that has increased significantly in concentration and loads in the Mississippi River in

recent decades. Phosphorus loads have not changed significantly since the early 1970s when

records began. Silica loads decreased between the 1950s and 1970s, and have not changed

significantly since.

Nitrogen is present primarily in three forms in the Mississippi River and its tributaries -- nitrate,

ammonium, and organic nitrogen. On average, 61% of the nitrogen load is nitrate; 24% is

dissolved organic nitrogen; 13% is particulate organic nitrogen associated with suspended

material; and the remaining 2% is ammonium. There has been no significant trend in the organic

nitrogen load since 1973 when it was first measured. However, indirect evidence suggests that

the particulate organic nitrogen load in the suspended sediment may have been as much as 50%

larger before completion of the Missouri River reservoirs in 1950s and 1960s. These reservoirs

now trap about half the sediment that the Mississippi River once discharged to the Gulf of

Mexico (Meade 1995).

The most significant nutrient trend has been nitrate loads, which have almost tripled from 0.33

million metric tons per year during 1955-70 to 0.95 million metric tons per year during 1980-96.

Most of this increase has occurred north of the convergence of the Mississippi and Ohio Rivers.

FIGURE 2.4 – Annual loads of nitrate, organic nitrogen and annual streamflow from the

Mississippi River Basin to the Gulf of Mexico 1955-98 

Longer-term records indicate that nitrate concentrations increased even more over the last 100

years. For example, concentrations in the Lower Illinois River averaged 1.01 mg/l in 1906-07,

but averaged 4.12 mg/l between 1980 and 1996. Average concentrations increased from 0.7

mg/l in 1906-07 to 4.67 mg/l in 1980-96 in the Cedar River, IA; from 0.75 to 4.12 mg/l in the

Des Moines River, from 0.32 to 4.19 mg/l in the Minnesota River; from 0.36 to 1.43 mg/l in the

Muskingum River, OH and from 0.14 to 1.45 mg/l in the Lower Mississippi River, LA over the

same time period (see #3 for additional comparisons).

Figure 2.5 shows trends in annual nitrogen inputs to and outputs from the basin. The most

significant changes in both inputs and outputs occurred between the 1950s and 1980s, when

fertilizer input more than tripled and crop harvest and pasture production increased about 2.4

times. Total nitrogen inputs increased from about 13 to nearly 22 million metric tons per year,

an increase of about 9 million metric tons per year. Outputs increased by about 11 million metric

tons per year.
22

FIGURE 2.5A – Annual Nitrogen Inputs to the Mississippi/Atchafalaya River Basin

FIGURE 2.5B – Annual Nitrogen Outputs from the Mississippi/Atchafalaya River Basin

The difference between these inputs and outputs (the residuals) represents nitrogen available for

leaching to the river system (see figure 2.6). The rapid decline in residuals corresponds to the

period of rapid change in fertilizer application and crop harvests and may represent increased

efficiency in the use of nitrogen in crop production. Residuals since around 1980 have exhibited

no trend, although they have become highly variable from year to year. The leveling off of

nitrogen fertilizer, crop outputs of nitrogen, and the nitrogen residuals may indicate that a new

steady-state condition was established about 1980.

FIGURE 2.6 – Annual Nitrogen Inputs, Outputs and Residuals (inputs minus outputs) from

Nitrogen Mass Balance for 1951-96

Seasonal Variability

Nitrogen concentrations and loads vary seasonally. Loads and freshwater discharge are usually

highest during the late winter, spring, and early summer when runoff is highest. The seasonal and

annual cycles of nitrate transport are illustrated in figure 2.7.

FIGURE 2.7 – Flux of Nitrate from the Mississippi River Basin to the Gulf of Mexico 1980 to

August-1999

Precipitation leaches the highly soluble nitrate from the soil into streams via overland runoff,

ground-water discharge, and agricultural drains. Nitrate is subsequently transported into the

Mississippi River and eventually discharges to the Gulf of Mexico.

Nitrogen Sources

The principal sources of nitrate are river basins that drain agricultural land in southern

Minnesota, Iowa, Illinois, Indiana, and Ohio. This is an area of intensive corn and soybean

agriculture. Large amounts of nitrogen from fertilizer and manure are applied to soils in this

region each year. Legumes and atmospheric deposition add nitrogen. Also, soils in this region

contain large amounts of nitrogen, some of which is converted to soluble nitrate each year. The

nitrate accumulated from all sources and not utilized by crops or removed by biochemical

processes is subject to being leached to streams and ground water by precipitation. Extensive use

of tile drains in this region can intercept water with high levels of nitrate and accelerate its

transport directly to ditches and streams. In addition, in some basins sewage treatment plants and

industrial sources add nitrogen directly to streams. All of these sources contribute to the nitrogen
23

load transported by the Mississippi River to the Gulf of Mexico. About 56% of the nitrate

transported to the Gulf enters the Mississippi River above the Ohio River. The Ohio basin

subsequently adds another 34% of the nitrate load.

FIGURE 2.8 -- Average annual nitrate yields (1980-96) for 42 basins within the MississippiAtchafalaya River Basin

Since 1980, the Mississippi and Atchafalaya Rivers have discharged, on average, about 1.6

million metric tons of total nitrogen to the Gulf each year; 0.95 million metric tons of that is in

the form of nitrate. About 90% of the nitrogen comes from non-point sources, with the remainder

coming from point sources. Table 2.1 presents the estimated contributions of nitrate and total

nitrogen from the predominant sources in the basin.

TABLE 2.1 – Estimated Contributions of Nitrogen Input Sources to the Total Nitrogen and

Nitrate Nitrogen Yield of the Mississippi/Atchafalaya River Basin and Flux to the Gulf of

Mexico

 Source of Nitrogen

Transported to the Gulf Percent of Nitrate Percent of Total Nitrogen

 NON-POINT SOURCES

Fertilizer and mineralized 58 50

soil nitrogen

Animal manure 16 15

Atmospheric deposition and 16 24

unmeasured inputs

 POINT SOURCES

Municipal and industrial 9 11

point sources
24

3. Consequences of Hypoxia in the Northern Gulf of Mexico

The consequences of hypoxia in the Gulf of Mexico are not fully known. This chapter

examines what is known about the direct effect on fisheries and the structure of the marine

ecosystem in the Gulf. It also describes the consequences of excess nutrients, the probable

primary cause of hypoxia, for water quality and ecosystem functioning within the MississippiAtchafalaya River Basin (MARB).

CONSEQUENCES IN THE GULF

The shallow continental shelf area in the Gulf of Mexico that is affected by hypoxia shows signs

of hypoxia-related stress -- low abundance of fish and shrimp and distinctly different benthic

communities in hypoxic areas. While current ecological conditions are a response to a variety

of stressors, the most obvious effects of hypoxia are that many bottom-dwelling, or benthic,

organisms die; larger, long-lived species are eliminated; and productivity is shifted to

nonhypoxic periods (energy pulsing).

The effects of hypoxia on fishery resources could include direct mortality of both fish and their

food base, as well as such indirect effects as altered migration patterns, reduction in suitable

habitats, increased susceptibility to predation, and disruption of spawning and recruitment.

Trawl data from the fishery-independent SEAMAP database showed a very consistent pattern

that whenever dissolved oxygen approached 1-3 mg/l, catch of shrimp and fish rapidly declined

to zero. Laboratory experiements have shown that both white and brown shrimp are able to

detect and attempt to avoid hypoxic waters. Both abundance and biomass of fish and shrimp

are significantly less where bottom-water concentrations of oxygen decline below 2 mg/l.

Geographic comparison of the distribution of fishing effort around the Gulf show that the

industry has shifted shrimping efforts away from hypoxic zones (Downing et al. 1999)

Overall, fisheries landings statistics for at least the last few decades have been relatively

constant. However, the brown shrimp catch, the most important commercial fishery (by dollar

value) in the Gulf, declined from a record high in 1990 to below average during 1992-97,

coinciding with years of greatly increased hypoxia. Catch per unit effort for brown shrimp,

while variable, has trended down since the late 1970s. Near-shore zones, away from hypoxic

waters, are the usual habitat for white shrimp, which have not shown as great a decline.

FIGURE 3.1 -- Trends in shrimp yield recorded by the National Marine Fisheries

Service for Louisiana and Texas, 1979-98 -- The lines indicate 3-year moving averages.

The data are available at <www.st.nmfs.gov/st1/commercial/landings/index.html>
25

An economic analysis sought to examine the relationship between the estimates of the hypoxic

zone area and available fisheries data, primarily on the two main shrimp species in the Gulf

because they are part of the benthic community and are commercially important. Since data on

the area of the hypoxic zone are only single annual estimates and not available before 1985, the

time series was judged too short to establish a credible relationship and the analysis resorted to

an extrapolation back to 1960 (see #2, pp. 19-20, for details). Fisheries variables examined

included catch per unit effort, depth of landings, and shrimp size. This economic assessment

based on fisheries data failed to detect effects attributable to hypoxia (i.e. correlations between

the extrapolated time series and fisheries data were below levels usually considered statistically

significant). However, the failure to identify hypoxic effects does not necessarily mean that

they are absent, only that the data available for analysis were inadequate to identify the reasons

for variability.

Fisheries data are highly variable and affected by many factors. Fisheries productivity,

particularly in pelagic or near-surface species, may increase as a result of nutrient enrichment,

but enrichment can fuel bottom hypoxia, thereby decreasing productivity of bottom-dwelling (or

benthic) species. In the food chain, the documented responses of zooplankton to hypoxia

include direct mortality, avoidance behavior in adults, interference with vertical migration, and

changes in species composition toward smaller species that carry their eggs. Copepods, the

dominant zooplankton in the northern Gulf of Mexico, are lower in abundance or absent when

dissolved oxygen was less than 1 mg/l.

Comparison of benthic communities between the area affected by hypoxia with those unaffected

in nearby Mississippi Sound shows distinct differences. On the Louisiana shelf, benthic

communities consist of disturbance-adapted populations, whereas the Mississippi Sound

contains more fully developed, late-successional stage “equilibrium”-type communities.

In addition to the loss of bottom and near-bottom habitat, hypoxia alters energy flows in the

ecosystem. During hypoxia, significant amounts of the system’s energy are diverted from

invertebrates to microbial decomposition. Energy flows through the ecosystem in pulses,

favoring opportunistic species with shorter life cycles that can take advantage of the abbreviated

time bottom habitats are available. A reduction in overall biodiversity, abundance and biomass

of the ecosystem is associated with pulsed energy systems, since longer-lived species tend to be

eliminated.

CONSEQUENCES IN THE MISSISSIPPI-ATCHAFALAYA RIVER BASIN

Nutrient concentrations affect many aspects of ecosystem and water quality in the Basin,

including plankton composition and production, nuisance algal blooms, macrophyte

communities and fish communities, as well as the suitability of waters for swimming and

drinking and, ultimately, to violations of water quality standards.

Review of state assessments submitted to the U.S. Environmental Protection Agency under

section 305b of the Clean Water Act indicates that most MARB states have substantial numbers

of river miles that do not fully support three resource uses (aquatic life support, fish

consumption, and swimming) because of nutrient conditions. Elevated nutrient concentrations,

primarily nitrogen and phosphorus, can result in excessive growth of algae and other nuisance

aquatic plants, disrupting the ecological balance, clogging pipes, and interfering with
26

recreational activities. Subsequent decay of algae can result in foul odors, bad taste, and further

ecological disruption through oxygen depletion.

Legally binding numeric standards have not been established in any state for nutrients in

flowing- water systems or lakes, but efforts have been directed at the development of such

standards for many years. At present, many states have a non-numeric (narrative) standard that

in essence says nutrients must not be added to a water body to the extent that they cause an

imbalance in the natural flora and fauna. Although science-based numerical criteria for nutrient

concentrations to classify lake ecosystems according to trophic state have been available for

many years, few classification methods exist in the literature to evaluate the trophic status of

stream ecosystems in quantitative terms. One simple classification scheme developed by

researchers (Dodds et al. 1998) for streams would use nutrient concentrations at the boundary

approximately defining mesotrophic and eutrophic conditions, which is 1.5 mg/l for total

nitrogen and 0.075 mg/l for total phosphorus. Using this scheme, about 30-55% of the

hydrologic cataloging units (HCUs) of the Ohio, Lower Mississippi, and Tennessee sub-basins

exceed this proposed eutrophic criterion for total phosphorus in flowing waters, and 16-40% of

the HCUs in these regions exceed the proposed flowing-water criterion for total nitrogen.

Higher exceedance frequencies were found in the Missouri, Upper Mississippi, and ArkansasRed sub basins (~80% of the HCUs for total phosphorus and 70-75% for total nitrogen) (see #4

for details).

Excessive nitrate in drinking water can result in “blue baby syndrome” (methemoglobinemia),

which causes oxygen levels in the blood of infants to be low, sometimes fatally. About 15% of

all shallow ground water beneath agricultural and urban areas that has been sampled by the

USGS exceeded the 10 mg/l drinking-water standard for nitrate (USGS 1999). Contamination

of shallow ground water may be a warning to alert populations to potential future risks from

consumption of water from deeper wells in these aquifers.
27

4. Effects of Changing Nutrient Loads

Evidence described in the previous chapter indicates that high nutrient loads, particularly

nitrogen, from the Mississippi-Atchafalaya River Basin is a primary cause of hypoxia in the

Gulf of Mexico. Actions to reduce these nutrient loads will cause changes in both the hypoxic

conditions in the Gulf and water quality and land-use conditions within the Basin. This chapter

presents projected effects over the next 10-20 years both in the Gulf and in the watershed:

 • if there are no major changes in current activities or practices related to nitrogen loads, and

 • if additional efforts are undertaken to reduce, mitigate, and control hypoxia in the Gulf.

EFFECTS IF CURRENT ACTIVITIES REMAIN UNCHANGED

Analyses supporting this assessment were based on data that were generally complete through

the mid-1990s. Thus, the assessment of "current" activities is up-to-date only to about 1995 or

1996.

“No Loading Change” Scenario

Analyses presented earlier in this assessment show that Mississippi River nutrient concentrations

and loadings, particularly nitrogen, to the northern Gulf of Mexico have changed dramatically

this century, with an acceleration of these changes in the last four decades. However, nitrogen

loading appears to have reached a plateau at the level of about 1.6 million metric tons/yr, which

is the mean loading over the period 1980-96. If the set of activities that affect nutrient flux

continues to prevent increasing trends, the current range of dimensions and severity of the

hypoxic zone on the Louisiana shelf may be maintained. Hypoxia would vary depending on

other factors, such as spring and summer stratification caused by fresh river water flowing over

marine waters, weather patterns, temperature, and other physical factors in the Gulf and the

Basin. However, even under these conditions natural variability could produce new extremes of

hypoxia extent and impacts.

Nitrogen fertilizer use (together with mineralized soil nitrogen) constitutes about 58% of the

input of nitrate to the MARB. Nitrogen fertilizer use in the Basin appears to have reached a

plateau beginning in the 1980s, while phosphorus fertilizer use stabilized in 1980 and then

dropped slightly. At the same time, both the rise in river nitrate concentration and the fall in

silicate concentration ceased.

Agricultural policies and practices have changed over the last two decades so that both crop

production and efficiency has increased. Average fertilizer use per acre of corn in the U.S. has

been essentially constant since the late 1970s; however, corn yields have continued to climb,

resulting in an increase in the bushels of corn produced per unit of fertilizer applied. According

to comments from the Fertilizer Institute, farmers have also increased the amount of grain

produced from 0.76 bushels per pound of nitrogen in the late 1970s to approximately 1.0 bushel

per pound of nitrogen today -- a 32% increase in efficiency. A mass balance of nitrogen inputs

and outputs to the Basin indicates that these have been approximately equal and at a steady state

since about 1980 and may indicate that levels of nitrogen stored in the Basin ecosystem are no

longer increasing as they did between 1955 and 1980 (see figure 4.1). However, even if

pressures driving hypoxia do not increase, it is not known if significant changes in species
28

abundance and composition or ecosystem function will result as a consequence of prolonged

maintenance of the existing levels of hypoxia in the Gulf of Mexico.

FIGURE 4.1 – Cumulative residual (nitrogen inputs minus outputs) from nitrogen mass

balance for the MARB

Under the “no loading change” scenario, if the Gulf of Mexico continues to experience a large

zone of hypoxia each summer, then the area affected will continue to suffer from annual losses

of biodiversity, abundance and biomass. The loss of harvest from the hypoxic region may, or

may not, be offset by harvests from nonhypoxic waters. It is not known whether the northern

Gulf can sustain this level of impact without observable reductions in overall fisheries

production.

Not all nutrients entering tributaries within the Basin are transported to the Gulf. Substantial

processing of nutrients occurs in the rivers and streams. Excess nutrients there can overstimulate primary production, which commonly leads to impaired water quality. Most states in

the Basin have substantial numbers of river miles that suffer use-impairment related to nutrient

conditions or do not fully support the resource uses of aquatic life, fish consumption, swimming,

and drinking-water supply. This situation is unlikely to improve without some changes in

nutrient loading.

Increased Loading Scenario

Any “no loading change” scenario must assume that the anticipated increases in population

growth, food production and fertilizer use, and changes in climate that will tend to exacerbate

Gulf hypoxia will be offset by changes that tend to mitigate hypoxia (e.g., ongoing

improvements in farming practices, better targeting of riparian and wetland restoration, and

improved river flow management).

Pressures that drive hypoxic conditions are likely to increase. U.S. population and food demand

are projected to continue to increase. Although yields may increase, it may be difficult to meet

food demand without increased use of nitrogen fertilizer. Over the past decade the U.S.

population has increased by 10% (Statistical Abstract 1998); middle series projections predict an

8% increase by 2010; and 17% by 2020. World population is expected to grow at 1.2% per year

through 2010 according to the World Bank (World Development Indicators 1998), and the U.S.

has provided a large percentage of the world's food, particularly grain.

The northern Gulf of Mexico is a coastal area that may experience increased freshwater runoff or

greater extremes of influx as a result of global climate change. Streamflow was approximately

30% higher during 1980-96 than during 1955-70. Part of this increase may be due to long-term

climatic variation, and some may be driven by shorter-term climatic cycles. The higher flows in

the later half of the century are attributed to increased precipitation throughout the year and, in

particular, to warmer, wetter springs. However the 30% increase in streamflow is small

compared to the 300% increase in nitrate flux over this period.
29

The higher precipitation and streamflow in the later half of this century could influence nitrate

flux in several ways. More nitrate would be transported with the larger volume of flow unless

concentrations decreased more readily than flow volume increased. The higher precipitation

could leach more accumulated nitrate from soils in the basin into tile drains and ditches, resulting

in higher nitrate concentrations in streams. Higher streamflow would decrease the residence

time of water in the river, reducing the rates of denitrification, burial, and uptake by aquatic

plants and riparian vegetation. The resulting higher runoff in summer would most likely affect

water column stability, surface productivity and oxygen cycling, leading perhaps to an expanded

extent of the hypoxic zone.

Understanding the effects of hypoxia in other areas can aid in projecting effects in the Gulf of

Mexico. Experience with other hypoxic zones around the globe shows that both the ecological

and fisheries effects become progressively more severe as hypoxia worsens. Several large

systems around the globe have suffered serious ecological and economic consequences from

seasonal hypoxia. Most notable are the Kattegat, the Baltic and Adriatic Seas, and the

northwestern shelf of the Black Sea. The consequences range from localized loss of target fish

catch and recruitment failure (low numbers joining a population, generally due to low

reproduction rates) to complete system-wide loss of fishery species. Where oxygen depletion is

severe, the food web that supports bottom feeders such as shrimp and drum is disrupted, as well

as the natural processing of organic matter, nutrients, and pollutants.

At first, the higher nutrient concentrations lead to increased production of both organic matter

and fisheries. However, as organic matter production increases, changes occur in the food web

that lead to different endpoints. These changes have followed the same path in many marine

ecosystems.

FIGURE 4.2 -- Comparative evaluation of fishery response to nutrients based on data

from around the world (redrawn from Caddy, 1993) -- Each curve represents a general

guild of species and their reaction to increasing nutrient supplies. The top part of the

figure lists recent trends for various systems around the world. Vertical dashed lines

separate general categories of organic production that result from different levels of

nutrients.

In the Gulf of Mexico region, Louisiana leads in production and landings of commercial and

recreational marine fisheries which are dependent on conditions in the Gulf. The fishery

resources of the Gulf are among the most valuable in the United States, generating $2.8 billion

annually. Catch per unit effort for brown shrimp, one of the most commercially valuable species

in the Gulf, has trended down since the late 1970s.

Other areas that have experienced severe hypoxia, with near-anoxic conditions, experienced

greater mortality and had fewer species and lower biomass than areas with intermittent or less

severe hypoxia. If experiences in other systems are applicable to the Gulf of Mexico, projections

are that, in the face of worsening hypoxic conditions, at some point fisheries and other species

will decline, perhaps precipitously.
30

The degree of ecological and economic effects related to hypoxia varies from system to system.

For example, both ecological and economic effects of the combined problems of eutrophication

and hypoxia have been seen in the Black Sea and Baltic Sea, where demersal trawl fisheries have

been either eliminated or severely stressed. Initially in the Kattegat, hypoxia caused mass

mortality of commercial and noncommercial species. Now large-scale migrations and/or

mortality among demersal fish and the Norway lobster (Nephrops) continue, resulting in a

changed species composition and reduced growth and biomass. Hypoxia in the Kattegat is

believed to be partly responsible for the overall decline in stock size, recruitment, and landings

of commercial fish over the last two decades.

Louisiana’s commercial and recreational fisheries depend on species that have part of their life

cycles within shallow continental shelf waters that often overlap the hypoxic zone. This is true

not only for shrimp but also for the benthic organisms that form the forage base for valuable

commercial and recreational species. Because shrimp must move from inshore wetland nurseries

to offshore feeding and spawning grounds, hypoxia can block their migration. Spawning

grounds, migratory pathways, feeding habitats, and fishing grounds of important species are

affected by the extent and severity of hypoxic waters. Expansion of the hypoxic zone could lead

to further decreases in productivity at higher levels of the food chain and loss of essential

habitats.

Increasing nitrogen loads have decreased the silica- to- nitrogen ratio. Decreased silica

availability in relation to increasing availability of other nutrients could stimulate blooms of

harmful algae, such as dinoflagellates. On the other hand, Pseudo-nitzschia spp., a group of

toxic diatoms that require silica, have increased in abundance in spite of silica decreases, likely

in response to increasing nitrogen inputs from the Mississippi River.

EFFECTS IF REDUCTION MEASURES ARE UNDERTAKEN

Reducing sources of nutrients from the MARB may have as a primary goal decreasing the

hypoxia problem in the Gulf of Mexico, but also will affect water quality conditions in the Basin

itself. Although there are distinct differences between the nature of the responses to such

reductions in the two systems, not surprisingly, there are also substantial similarities. In both the

Gulf and the freshwater Basin area response variables of interest include various nutrient forms,

chlorophyll, dissolved oxygen, water clarity, and planktonic and benthic biota, and higher

organisms. However, the response variable of primary interest for hypoxia is dissolved oxygen

in the bottom waters in the Gulf of Mexico.

Depending on the suite of techniques used to reduce loads (i.e., the mix of source controls,

wetland restoration, and river management), a number of costs would be incurred, including

those associated with higher material, equipment, and management costs of improved farming

practices; loss of productive farmland; and increased treatment costs for municipalities and

industry (air and wastewater controls). These costs would be offset by a number of benefits,

including: improved water and habitat quality within the basin; reduced soil erosion; reduced

contamination of drinking water by nitrates; improved water quality for recreational use;

improved recreational fisheries and wildlife; cost-effective flood control improvements;

restoration of critical coastal wetlands in Louisiana; and reduced risk to Gulf fisheries from

catastrophic hypoxic/anoxic conditions.
31

Changes in the Gulf of Mexico

Simulations with a quantitative water quality model indicate that dissolved oxygen and

chlorophyll concentrations on the Louisiana Inner Shelf do appear to be responsive to reductions

in nutrient loadings from the MARB. However, there are large uncertainties in the magnitudes

of these responses for a given nutrient loading reduction. For nutrient loading reductions of 20-

30%, bottom-water dissolved oxygen concentrations were estimated to increase by 15-50% and

surface chlorophyll concentrations were estimated to decrease by 5-10%. The ranges correspond

to different assumptions for sediment responses and large-scale Gulf of Mexico water quality,

and to different hydrometeorological conditions among different years. Both nitrogen and

phosphorus loading reductions produced responses although, particularly for dissolved oxygen,

the response was somewhat greater for nitrogen reductions.

There has been no statistically significant long-term trend in phosphorus flux to the Gulf for the

period 1972-96. Over the past half century, phosphorus flux, a large portion of which is

associated with sediments, has probably decreased. Nitrogen is the only component of nutrient

flux that has increased. For nitrogen loading reductions of 30-50% (returning to the levels that

existed in 1955-70), the model estimates that dissolved oxygen concentrations would increase by

20-75%.

There appears to be no evidence that the ecological changes related to hypoxia in the northern

Gulf of Mexico at this point are irreversible. Experience has demonstrated that large ecosystems

respond positively to nutrient reductions. Examples include Cheaspeake, Tampa, and Sarasota

Bays. There are many examples of small-scale hypoxia reversals associated with improvements

in treatment of sewage and pulp mill effluents. In the U.S., the improved water quality in Lake

Erie is a good example of positive response to nutrient reductions but also a demonstration that

the time interval for achieving noticeable improvements may be long, on the order of five to ten

years after nutrients are reduced. Substantial nutrient reduction targets are being proposed for the

drainage area of the Baltic Sea in order to control eutrophication (Jansson and Dahlberg 1999).

Changes in the Watershed

The most direct effect of actions to reduce nutrient loading to surface waters in the MARB will

be to decrease concentrations and shift the composition of nutrients in its rivers and streams.

The changes in nutrient concentrations and composition will induce other changes in trophic

conditions and should result in Basin-wide improvements in surface-water quality. The extent of

reductions achieved (and subsequently the effects) will depend on site-specific characteristics

(climate, soils, cropping history), the types of improvements in management, and the baseline

conditions to which the management improvements are being compared.

Aquatic macrophytes (such as duckweed, water lilies, and sedges) have important effects on

water quality in shallow waters throughout the Basin. If reductions in nitrogen and phosphorus

levels cause an increase in underwater light, submersed aquatic macrophyte distribution will

expand in the Upper Mississippi River. The effects on water quality will be beneficial, both

locally and system-wide. Increased macrophyte abundance may enhance nutrient retention

significantly, leading to lower delivery rates of nutrients to the Gulf of Mexico than would

otherwise be predicted for direct effects of source reductions.

Reductions in nutrient concentrations would not strongly affect sport fisheries because the rivers

and streams in the Basin would most likely continue to be highly productive systems. Any
32

decline that might occur in total biomass production would likely be more than compensated for

by habitat and other improvements that would promote game fish over rough fish populations.

All the nutrient-reduction approaches considered, including steps to reduce nutrient losses from

farmlands, reduce nutrient discharges from point sources, and increase natural processes to

remove excess nutrients from surface waters, are expected to produce environmental benefits

within the Basin. These include the values associated with restored wetlands, reduced soil

erosion, reduced nutrient contamination of drinking water, reduced vulnerability to floods, better

wildlife habitat, and improved recreational water quality. Other potential benefits of nutrientreduction activities include more efficient use of organic and inorganic fertilizers and the energy

associated with them, lower overall fertilizer costs, decreased health risk from contamination of

public and private drinking-water supplies, and improved aquatic habitat in streams, lakes, rivers,

and estuaries. All the approaches considered will also have associated costs, including costs for

changes in land use; introduction of new technologies, such as precision agriculture and

biological nitrogen removal; and implementation of improved management practices. The

estimated economic costs of the various approaches to reduce nutrients are described in the next

chapter.
33

5. Approaches for Reducing Nutrient Loads

This chapter deals with methods for reducing nutrient loads and their social and economic costs

and benefits. Analysis of the causes of hypoxia in the Gulf of Mexico, as reported in chapter 3,

indicates that nitrogen is the principal nutrient fueling excess organic matter sedimentation

which, when stratified conditions exist, results in hypoxia. Thus, the primary approaches to

reduce, mitigate, and control hypoxia in the Gulf of Mexico are:

• reducing inputs of nitrogen from farmlands, point sources, atmospheric deposition and

other sources to streams and rivers in the Basin, and

• restoring and enhancing natural denitrification and nitrogen-retention processes to

reduce nitrogen loads from the Mississippi and Atchafalaya Rivers to the Gulf.

Analyses of these options are organized around two levels of potential impact:

• reduction of total nitrogen loads to the Gulf of Mexico by 40% (approximately 625,000

metric tons/yr), to the average level that existed in the period 1955-70, and

• reduction of nutrient loads by approximately 20% as a middle option between reduction

to historical levels and no reduction.

Precise estimates are difficult due to the complexity of the issues compounded by natural

variability. For many aspects, available models offer only rough approximations.

REDUCING INPUTS

After a period of marked increase in nitrogen flux to the Gulf between 1970 and 1983, loadings

have remained variable but without statistically significant trends, despite increasing agricultural

production, increasing population, and other pressures in the Mississippi River Basin. Improved

agricultural practices are probably the major contributors to keeping nitrogen loads from

significantly increasing since the 1980s. Innovations by researchers, industry, and individual

farmers have reduced nitrogen losses, in part to reduce fertilizer expenditures. Agricultural

fertilizer’s share of the nitrogen loading to the Gulf is estimated to represent a loss of $410

million annually (Downing et al. 1999).

Some reduction in nitrogen loads from municipalities could be achieved by the elimination of

combined sewer overflows and the installation of biological nutrient control processes in new

sewage treatment plants. While such sources are not the major contributor to the total nitrogen

load from the Basin, they discharge directly to major streams, with little opportunity for natural

treatment.

Reduction in discharges of nitrogen from farms to streams and rivers could be achieved by

implementing a wide variety of changes in management practices, such as:

• limiting the application of nitrogen fertilizer and manure to agronomically

recommended rates;

• switching from fall to spring application of fertilizer;

• better management of livestock manures, whether stored or applied to the land;

• changing from row-cropping to perennial-cropping systems;

• planting cover crops for fall and winter nutrient absorption;

• switching from conventional to ridge-tilling or other reduced tillage practices;
34

• increasing the lateral spacing of subsurface tile drainage to 20 meters, rather than the

usual 5-10 meters;

• controlling water tables to promote denitrification within the soil column; and

• routing soil drainage effluent through wetlands, grass buffer strips, or riparian forest

buffers.

Nitrification inhibitors for fertilizers, amino acid feed supplements, and other measures have also

been suggested. For some of the above improved management practices, reduction in nitrogen

losses to streams are on the order of 10-20% of baseline conditions, but others have been shown

to reduce losses by as much as 90% in specific field studies (see #4 and #5 for details).

Of these measures, the three with greatest estimated potential to reduce nitrogen sources to

streams and rivers are:

• Reduced application of nitrogen fertilizer -- This is estimated to offer the greatest

potential reduction of nitrogen loading. By reducing “insurance” rates of nitrogen

fertilizer1, improving management of manure, applying appropriate credits for

previous crops and manure, and using improved soil nitrogen testing methods,

nitrogen losses at the edge of fields could be reduced by 10-15%, or about 0.9 to 1.0

million metric tons per year2.

• Alternative cropping systems — If 10% of the corn-soybean farms in the Basin were

changed to include alfalfa or alfalfa-grass mixes as crops, edge-of-field nitrogen

losses could be decreased by an estimated 0.5 million metric tons per year.

• Improved management of feedlot runoff — A 20% decrease in feedlot runoff could

decrease edge-of-field nitrogen losses by 0.5 million metric tons/year.

Measures to reduce nutrient loads would generally have net costs. Assuming constant levels of

efficiency, large decreases in nutrient use below agronomic rates would lead to reduced

agricultural productivity in the Mississippi Basin and reduced total production as marginally

fertile land is retired. The agriculture sector’s response to these alternative management

strategies was simulated with the U.S. Mathematical Programming Model for Agriculture (see

box). The model predicts both the economic effects and the changes in nitrogen loading under

the various scenarios examined.

 

1 Data on “insurance” application of nitrogen fertilizer, application at rates above

agronomic recommendations, are sparse. “Insurance” application is an economic wager — in a

good year, on a per acre basis, an extra 10-20 pounds of nitrogen fertilizer may result in 10-15

more bushels, worth $20-30. In practice, the Potash and Phosphate Institute reported nitrogen

use on corn in Iowa in 1996 was within the range of agronomic recommendations when manure

was not used but exceeded agronomic recommendations by more than 30%, or about 33 kg of

nitrogen/ha (30 lb/acre) when manure was used.

2 Note that these estimates of the impacts of changes in agricultural practices are for

reductions at the “edge of field”; reduction of loadings to the Gulf of Mexico could be less,

depending on the denitrification between the field edge and major rivers.
35

This model predicts that edge-of-field nitrogen losses can be reduced by 20% (941,000 metric

tons/yr) at a total societal cost under economically optimum conditions of $831 million/yr, or

$0.88 per kg reduction in edge-of-field nitrogen flux. Reduction in fertilizer use is less than

optimum economically but less difficult, in concept, to implement. Achieving a 20% reduction

in edge-of-field nitrogen loss would require a 45% reduction in fertilizer use, resulting in costs of

$2.85 per kg of nitrogen reduction.

However, smaller reductions would be more

attractive on a per kg basis: 20% reduction

in fertilizer would reduce edge-of-field

losses by 503,000 metric tons/yr (about

10%) at a total societal cost of $347 million

or $0.69 per kg. Fertilizer taxes were

predicted to be economically inefficient,

with a 500% tax required to reduce edge-offield losses by 20%.

Nonagricultural, nonpoint sources of

nitrogen also contribute to the total nitrogen

load to the Gulf. Nitrate concentration in

urban nonpoint sources are generally not

high compared to urban point sources or

cornbelt cropland. Urban areas constitute

only 0.6% of the Basin, but improved

management practices could provide costeffective reductions in nitrogen load.

Biological nitrogen removal from urban

point sources typically costs $40/kg of

nitrogen removed.

Reductions of atmospheric nitrogen emissions beyond those now being implemented are

probably not warranted from the perspective of hypoxia in the Gulf of Mexico. Atmospheric

deposition of nitrogen is estimated to be less than 1% of the nitrogen input to the Gulf from the

Mississippi-Atchafalaya River Basin. The costs of reducing atmospheric deposition of nitrogen

are estimated to be $20-100/kg of nitrogen.

Nitrogen trading among all sectors could offer opportunities to obtain least-cost reductions.

US Mathematical Programming Model for

Agriculture (USMP)

Both the economic and environmental effects

of agricultural changes were estimated using

the USMP, a model developed by the USDA

Economic Research Service for analysis of

government commodity programs. The

model calculates commodity prices and

quantities, net returns to producers, net social

benefits, and environmental emissions

recognizing spatial variation and explicitly

representing various management practices

used by farmers. Adjustments to crop

production can be estimated in response to

restrictions placed on nitrogen or land use by

altering any or all of the following: acreage

planted, crop mix, rotations used, tillage

practices, and fertilizer application rates (see

#4, p. 27ff for details).
36

TABLE 5.1 – Potential Approaches to Reducing Nitrogen Inputs

Potential Nitrogen Reduction

Approach (Thousands of metric tons/yr)

1. Changing Farm Practices

Nitrogen management 900 - 1,4003

(reduction in “insurance” rates of N fertilizer application, improved manure

management, crediting of nonfertilizer nitrogen)

Alternative cropping systems 5004

(Perennial crops in lieu of corn and soybeans on 10% of acreage)

Decrease feedlot runoff by 20% 5005

2. Reduction in Point Sources

Tertiary treatment of domestic wastewater 20

RESTORING AND ENHANCING DENITRIFICATION AND NITROGEN RETENTION

PROCESSES

A second general approach to reducing nitrogen flux to the Gulf of Mexico is to place or restore

ecosystems that are effective nitrogen sinks along the paths of nitrogen flow from sources to the

Gulf. Nitrogen transformations in wetlands and riparian soils, surface water, and groundwater

involve several microbiological processes, some of which denitrify, that is, make the nutrients

effectively unavailable for plant uptake. Wetlands also serve as traps for phosphorus, which

limits primary production within the Basin and, under some conditions, on the shelf in the Gulf.

High rates of denitrification are possible but wetlands and riparian zones can also act as sources

of nitrogen under some conditions. Loading rates (flow times concentration of inflowing water)

dictate the effectiveness of wetlands in reducing nitrogen. The most effective use of wetland

restoration and creation would be in watersheds that discharge high amounts of nitrogen.

Retention time is also important. Storm events, if significant enough, can cause nitrogen to shoot

through the system, bypassing effective retention.

Extensive experience with flow-through wetlands suggests a narrow range centered around 10-

25 g N/m2/yr as a reasonable target for denitrification by wetlands. Assuming denitrification at

15 g N/m2/yr, 2.1 million hectares (5 million acres, or 0.7% of the Mississippi River Basin area)

of constructed or restored wetlands would reduce nitrogen load to the Gulf of Mexico by 20%.

The current national goal is to restore wetlands at a rate of 100,000 acres/yr (about 40,000 ha/yr).

The total direct costs for permanent easements and restoration of wetlands to reduce nitrogen

load by 20% when selected at least cost was estimated to be $4.7 billion. Annual costs,

 

3 Note that these estimates of the impacts of changes in agricultural practices are for

reductions at the “edge of field”; reductions of loadings to the Gulf of Mexico could be less

depending on the denitrification between the field edge and major rivers.
37

including changes to consumer and producer surpluses plus annualized wetlands restoration

costs, were estimated to be $3.1 billion, or about $8.90 per kg reduction in nitrogen flux per year.

There would be ecological and wildlife benefits in addition to those that might accrue in the Gulf

from using this approach.

Riparian areas are vegetated areas, mostly forested, next to water resources. They have been

shown to be effective in removing nitrates from ground water as it passes beneath the vegetated

buffer. Typically denitrification rates are 2-6 g N/m2/yr. Assuming denitrification at 4 g

N/m2/yr, 7.8 million hectares (19 million acres, or 2.7% of the basin) of additional riparian

buffers would be needed to reduce nitrogen load to the Gulf by 20%. A current national goal is

to establish two million miles of conservation buffers on agricultural lands. Total direct costs for

permanent easements and restoration of riparian buffers to reduce nitrogen load by 20% were

estimated to be $46.3 billion equating to $18.0 billion annually in changes to consumer and

producer surpluses plus annualized restoration costs, or about $26 per kg reduction in nitrogen

flux per year.

FIGURE 5.1 – Role of forested riparian buffers in trapping nutrients before entering

streams

Separation of the Mississippi River from its deltaic plain may be an important factor in supplying

nutrients to the Gulf. The river once spread out over the delta during flood periods. Today,

however, with the exception of such locations as the Bonne Carre Spillway and the Atchafalaya

Basin, the river is now mostly shunted directly to the sea. Through enhanced water management

strategies, it may be possible to increase the amount of water reaching these coastal wetlands

and thereby increase nitrate removal from the water and reduce coastal land loss. Using a

denitrification rate of 10 g N/m2/yr based on studies at Caernarvon, Louisiana, and other

locations, diverting 13% of the total river flow over 500,000 hectares would remove 50,000

metric tons of nitrate per year. However, without upstream controls, the deltaic system might

become nitrogen-saturated, or it might release nitrogen in a different form, in a different season,

or in a different location. A recent study (Turner 1999) compared nutrient concentrations

entering and leaving the Atchafalaya River basin, a major floodway used to divert floodwaters

from the Mississippi and Red Rivers to the Gulf during the annual high-water season. This study

found that nitrate concentrations remained essentially unchanged from upstream to downstream

monitoring stations. Other studies have shown that harmful algal blooms may result when large

quantities of Mississippi River water are diverted into coastal areas.

The construction and restoration of wetlands and riparian zones in the Basin to reduce nutrients

would contribute to several important national goals, including those for drinking-water

protection, adding to the nation’s disappearing wetland habitat, improving river ecosystems,

enhancing terrestrial wildlife in river corridors, and mitigating the effects of floods. This

restoration of wetlands is in keeping with recommendations by the National Research Council’s

Committee on the Restoration of Aquatic Ecosystems (NRC 1992), which called for a national

program of wetland restoration that would contribute to an overall gain of 10 million acres by the

year 2010. Well-placed wetlands and riparian buffers generally support larger populations of
38

wildlife because of the diverse habitats they provide. The restoration of riparian vegetation

improves the ecological condition of streams and rivers and protects the aquatic communities

that depend on them. Roots of riparian vegetation stabilize the stream bank and prevent both

bank erosion and downstream sedimentation. The National Research Council also called for

restoration of about 640,000 km (400,000 miles) of streams, rivers, and floodplains across the

nation (NRC 1992).

FIGURE 5.2 – The role of riparian wetlands in capturing nutrients during flood events

TABLE 5.2 – Potential Approaches to Increasing Denitrification

Potential Nitrogen Reduction

Approach (Thousands of metric tons/yr)

1. Creating and Restoring Wetlands 300 - 800

(Create and restore 5 to 13 million acres of new wetlands)

2. Creating and Restoring Riparian Buffers 300 - 800

(Restore 19 to 48 million acres of riparian bottomland hardwood forest)

3. Diverting Rivers in Coastal Louisiana 50 - 100

COST- EFFECTIVE STRATEGIES

Various potential components of a program to reduce nitrogen loadings to the Gulf via the

MARB system were analyzed to develop cost-effective strategies to achieve various levels of

reduction, considering impacts upon the economy, farmers and consumers, and the reductions in

loadings.

Direct costs, including changes in consumer and producer surpluses and easement and

restoration costs for wetlands, were estimated using the USMP. All of the nutrient-reduction

approaches will also produce environmental benefits within the drainage basin. These include

the values associated with restored wetlands, reduced soil erosion, reduced nutrient

contamination of drinking water, reduced vulnerability to floods, better wildlife habitat and

improved recreational water quality. Other potential benefits of nutrient-reduction activities

include more efficient use of organic and inorganic fertilizers and the energy associated with

them, lower overall fertilizer costs, decreased health risk from contamination of public and

private drinking-water supplies, and improved aquatic habitat in streams, lakes, rivers, and

estuaries. Good estimates for the economic value of these benefits are only available for a few

categories, such as wetlands and erosion. When these estimated benefits are factored in, the net

costs of wetlands and farm practices become close. 20% edge-of-field nitrogen-loss reduction

was estimated to have a net cost of $0.80 per kilogram ($0.36 per pound), while 5 million acres

of wetlands would have a net cost of $1.00 per kilogram ($0.45 per pound).
39

TABLE 5.3 – Summary of Economic Costs of N-loss Reduction Actions

N-loss Reduction Unit Cost Net Cost

Scenario (Thousand metric tons/yr) ($/kg N-loss) ($/kg N-loss)

edge-of-field N-loss reductions

20% 941 0.88 0.80

30% 1,412 1.90 1.80

40% 1,882 3.37 3.25

50% 2,352 5.20 5.08

60% 2,822 7.48 7.37

fertilizer reductions: 20% 503 0.69 0.67

45% 1,027 2.85 2.81

500% fertilizer tax 1,027 14.54 14.50

wetlands 1M acres 67 6.06 - 2.19

5M acres 350 8.90 1.00

10M acres 713 10.57 2.81

18M acres 1,300 11.93 4.27

riparian buffers (19M acres) 692 26.03

river diversions to coastal wetlands 75 ~6

tertiary treatment of waste water 20 ~40

An optimal strategy would take appropriate advantage of the full range of possible measures to

deal with hypoxia in the Gulf, including modification of farm practices, creation and restoration

of wetlands and riparian ecosystems, implementation of nitrogen control on wastewater plants,

and diversion of floodwaters to backwaters and marshlands of the Mississippi River delta and

adjacent coastal wetlands.

Based on analysis and modeling of broad aggregates across the Basin, one illustrative approach

to reduce nitrogen loss by 20% would be through a program of:

• 20% fertilizer use reduction, and

• 5 million acres of wetlands construction or restoration

This combination would produce costs and acreage impacts that would be relatively low,

requiring smaller adjustments both inside and outside the MARB. However, broader

combinations of carefully refined, targeted, and flexible variations on the full array of

alternatives, in synergy with other programs, could improve cost-effectiveness.

The benefits of a program to reduce nitrogen loadings to the Gulf are difficult to calculate.

Economic analysis failed to show direct effects on Gulf fisheries based on past data. The
40

information is not available to estimate the benefit value for such actions as restoring the

ecological communities in the Gulf or improving water quality in the Basin.

An effective program will require adaptive management. Hypoxia is a complex phenomenon,

incompletely understood with numerous interacting variables, many of which are not

controllable. Increases in population, with associated increases in food requirements, and

domestic, commercial and industrial waste production changes would necessitate continued

adjustments in programs.

Substantial time lags may also complicate management measures. Such time lags have two

policy implications. First, observed ambient water quality conditions may be the result of past

management practices or of sources that are no longer discharging. Second, the results of a

program may not be immediately apparent, making it difficult to assess its actual effectiveness,

and potentially difficult to maintain a consensus to sustain the program. Many people may suffer

modest inconvenience or cost to try to improve the situation in the Gulf and may not see clear

improvements for several years or longer.
41

6. Adaptive Management: Action, Monitoring and Research

This assessment of the causes and consequences of Gulf of Mexico hypoxia, and its analysis of

options for dealing with it, are drawn from the assembly and peer review of a massive amount of

direct and indirect evidence collected and reported over many years of scientific inquiry. The

conclusions presented are thus well founded and grounded in those research and monitoring

results. There are, however, always uncertainties in scientific analysis. This section identifies

areas of further monitoring and research that are needed to reduce those uncertainties in future

assessments and to aid decision making in an adaptive management framework

The complex nature of nutrient cycling and transport within the MARB and Gulf of Mexico

make it difficult to predict specific improvements in water quality that will occur for a given

reduction in nutrient inputs. Nutrient cycling is affected by atmospheric, watershed, riverine, and

marine processes. Many of these processes, such as nitrogen transformations in river reaches, are

not fully understood at the local scale at which they occur. Large-scale, multidisciplinary

interpretations that integrate knowledge across these hydrologic compartments are difficult.

Further, it is clear that environmental responses to management actions in the MARB likely will

be slow, possibly requiring decades of data to demonstrate statistically that remedial actions have

helped the recovery of oxygen concentrations in the Gulf and have improved water quality in the

Basin. For example, the nitrogen balance in the soil zone and ground water will adjust relatively

slowly to changes in nitrogen inputs and outputs, slowing any change in the flux of nitrogen to

the Gulf. At the same time, the flux of nutrients to the Gulf most likely will respond quickly and

dramatically to large variations in precipitation and runoff -- further complicating measurement

of reductions in nutrient flux.

A comprehensive program of monitoring, interpretation, modeling, and research to facilitate

continual improvement in scientific knowledge and adjustments in management practices should

be coupled to whatever initial nutrient management strategies are chosen. This adaptive

management scheme involves continual feedback between interpretation of new information and

improved management actions.

FIGURE 6.1 -- Adaptive Management Feedback Loop -- connecting the following 4 steps in

a continuous process:

1. Monitor System Response;

2. Data Interpretation, Model Analysis and Improvement;

3. Model Prediction and Management Plan Improvement;

4. Implement Management Strategy)
42

MONITORING AND RESEARCH IN A MODELING FRAMEWORK

The adaptive management framework includes monitoring programs that use integrated models

of the hydrologic and ecological systems for interpretation of system change. Whole-system

monitoring will enable comprehensive interpretation of processes and linkages that affect

nutrient concentrations and transport within the MARB and development of hypoxia in the Gulf.

These coordinated monitoring efforts must be able to:

• Detect environmental trends to evaluate the effectiveness of management actions,

to enable effective adaptation of strategies over time;

• Observe physical, chemical, and biological processes and their roles in the causeand-effect relationships between nutrient inputs and resulting environmental

quality; and

• Differentiate among trends caused by changes in climate, streamflow, nutrient and

landscape management measures, and other concurrent factors.

An effective research strategy is also integral to the adaptive management framework.

Coordinated research efforts improve monitoring designs, support the interpretation of

monitoring output, and increase the predictive power of models and other assessment tools used

in the management process.

For a system as large and complex as the Mississippi-Atchafalaya River drainage basin and the

northern Gulf of Mexico, monitoring and research should be integrated using holistic models that

simulate our understanding of how the overall system functions and how management practices

can best be implemented. Such holistic models include a suite of conceptual, functional, and

numerical formulations; integrate research findings; and are tied to monitoring programs

designed to both provide input variables and verify model outputs. An effective modeling

framework would include models that simulate:

• Transport and transformation of nutrients (nitrogen, phosphorus and silica) from

natural, urban, and agricultural landscapes to ground water and surface waters;

• Inputs and outputs of nutrient flow throughout the landscape to improve estimates

of nutrient mass balances;

• Biogeochemical cycling and water quality effects of those nutrients on river

ecosystems within the drainage basin;

• Oceanographic and climate influences on those nutrients and their impacts on

Gulf productivity as they leave the Mississippi and Atchafalaya River system; and

• Impact of increased nutrient flux on productivity in the northern Gulf of Mexico

ecosystems, including commercially and recreationally important fisheries.
43

• Three dimensional coupling of biological and physical processes in the Gulf

ecosystem influenced by the Mississippi River discharge.

Within this larger modeling framework, research and monitoring needs have emerged from this

assessment. Monitoring gaps are found in environmental programs of the MARB and Gulf, as

well as programs to monitor management measures. Research needs include improving the

quantitative understanding of the biogeochemical cycling of nutrients within the watershed and

the Gulf and its relationship to the dynamics of organic carbon flux in the Gulf; the roles of longterm change in climate, hydrology, and population in year-to-year and long-term trends in

nutrient loads and hypoxia; and the social and economic impacts of various management and

policy alternative strategies.

MONITORING NEEDS

A comprehensive monitoring program requires both measurement of environmental response in

the MARB and the Gulf of Mexico and tracking indicators of programmatic progress toward

mitigating excessive nutrients.

Environmental Responses in the Gulf - Essential components of an environmental monitoring

program in the Gulf of Mexico include efforts to:

• Document the temporal and spatial extent of shelf hypoxia, and to collect basic

hydrographic, chemical, and biological data related to the development and

maintenance of hypoxia over seasonal cycles. A triad of mid-summer shelf-wide

hypoxia surveys, monthly transects off Terrebone Bay, and instrumented arrays at

stations in the core of the hypoxic zone would provide an optimal combination of

spatial and temporal scales of measurement and be consistent with the existing

long-term data. Establishing multiple vertical and horizontal instrument arrays

oriented cross-shelf and along-shelf will better define processes that control the

temporal and spatial development of hypoxia.

• Improve the collection of ecological, production, and economic information

related to fishery and nonfishery species; and

• Facilitate synthesis and interpretation of these data through an integrated

database.

Environmental Responses in the MARB - Essential components of an environmental

monitoring program in the MARB include efforts to:

• Document the flux of nutrients, carbon, and selected other water quality

constituents from the MARB to the Gulf of Mexico systematically on at least a

monthly basis and more frequently at high flows -- at least 25-30 times annually.

Additional monitoring sites in the main channel of the Mississippi River are

needed to evaluate the extent of nutrient retention/loss within the lock-and-dam

system, and to clarify the extent of nutrient retention in the Lower Mississippi.
44

There is a need to re-establish monitoring of nutrients, carbon, and selected other

water quality constituents in the major sub-basins (the 42 interior basins)

throughout the MARB and to establish monitoring in selected small basins within

some of the 42 interior basins where the effects of changes in land management

practices on nutrient concentrations and yields will be easiest to detect and

quantify.

• Monitor nutrients from atmospheric wet deposition in the MARB, and expand the

current limited monitoring of nutrients in atmospheric dry deposition. This

information is needed to determine if nutrient management strategies affect

precipitation chemistry.

• Establish a periodic inventory of effluent reporting conducted through the

National Pollution Discharge Elimination System (NPDES) to systematically

improve current estimates of nitrogen and phosphorus loads discharged to streams

from municipal and industrial point sources.

• Improve measurements for soil nitrogen and nitrogen loss.

Programmatic Measures - Ongoing programs are taking action to improve water quality

conditions within the MARB and Gulf of Mexico. The following describes major measures of

progress of those programs. Coordination of these and future programmatic actions to improve

water and ecological conditions can increase their overall effectiveness in achieving goals.

• Inventory changing patterns in other nutrient inputs to the Basin, such as fertilizer

use and manure application and disposal.

• Inventory acres of land leased through the Conservation Reserve Program and the

Conservation Reserve Enhancement Program annually.

• Inventory acres of restored wetlands implemented through the Wetlands Reserve

Program and the Emergency Wetlands Reserve Program.

• Inventory acres of riparian buffers implemented through the Conservation Buffer

Initiative.

• Inventory acres of created or restored wetlands implemented through the various

environmental restoration and related authorities, such as the Coastal Wetlands

Planning, Protection and Restoration Act and the Partners for Fish and Wildlife

Program.

• Inventory farmers’ actions to reduce nutrient runoff stimulated by the

Environmental Quality Incentive Program.
45

RESEARCH NEEDS

The research needs outlined below fall into two categories: (1) immediate priorities that are

essential for designing near-term management actions, and (2) longer-term priorities that fill

critical gaps in understanding as well as guide efforts to mitigate and control the effects of

hypoxia and excess nutrients. These are outlined below and provided in the accompanying

boxes.

Immediate Priorities

Ecological Effects of Hypoxia - A better definition of past, current, and potential impacts from

hypoxia on both commercially and ecologically important species and ecosystems is needed.

New retrospective analyses over longer temporal and spatial scales, based on data from marine

sediment cores should improve the historical perspective.

Watershed Nutrient Dynamics - There is a need to better understand the dynamics and timing of

movement of nitrogen and other nutrients from the “edge of field” in agricultural landscapes to

small streams and tributaries. Additional information is also needed on the geographic

distribution and design criteria for targeting wetlands creation and restoration efforts and to

determine if other strategies (e.g., riparian buffers) and mixtures provide the best nitrate

reduction for the least cost.

Agricultural Practices - While improvements in agricultural practices have been achieved in

recent years and the efficiency of nitrogen use has increased substantially, there is a need to

better quantify the effects of on-farm practices and methods that intercept agricultural nutrients

between the field and ground water and adjacent streams. There is also a need to better quantify

and understand the economics associated with current and proposed policies (e.g., nutrient

trading, fertilizer-use insurance) that increase incentives to reduce nitrogen loss.

Longer-term Priorities

Nutrient Cycling and Carbon Dynamics - Research is needed to better understand

mineralization and immobilization processes, to develop better means for measuring the amount

and forms of nitrogen in the soil reservoir, and to develop strategies to minimize leaching of

nitrate from the soils to the streams. There is a need to better quantify denitrification and

phosphorus retention rates in small and large streams and in Gulf sediments, and to compare

these rates to those achieved in riparian buffers and wetlands. Further refinements of the

relationships among nutrient fluxes, nutrient ratios, nutrient cycling, and organic carbon on the

continental shelf in the Gulf are necessary.

Long-term Changes in Hydrology, Climate, and Population - The relationship between

large-scale climate patterns and their potential impacts on river flows, nutrient flux, and flow

dynamics on the continental shelf need further evaluation. Studies are needed to evaluate the

potential role of flood prevention and control methods that seek to distribute and retain more

floodwater within the basin and thus increase nitrogen retention, while protecting against flood

damage. Future changes in management activities within the deltaic plain, such as the diversion

of floodwaters to delta backwaters and coastal wetlands restoration, should be understood in the

context of potential changes (both decreases and increases) in nutrient flux to the shelf.
46

Economics and Social Impacts - There is a growing need for an aggregated analysis of both

direct (e.g., drinking-water protection) and indirect (increased recreation) improvements in water

quality, for the basin as a whole. Additional work needs to be done to explore a broader range of

ecological impacts, including potential impacts to biodiversity and to nonmarket- valued

ecosystem goods and services. Better estimates of cost savings to agricultural producers from

reduced fertilizer nutrient inputs are needed.
47

Immediate Priorities

Ecological Effects of Hypoxia

Contemporary effects - A better definition of past, current, and potential impacts from hypoxia on both

commercially and ecologically important species and ecosystems is needed. Additional data sources have yet to

be examined exhaustively, most notably the SEAMAP database, which includes long-term fishery-independent

data on nektonic species’ composition in the northern Gulf of Mexico ecosystem. Model analyses of trophic

structure and ecosystem dynamics -- which will help identify affected fishery resources, and assess potential

future impacts -- are also needed.

Historical perspective - The northern Gulf of Mexico ecosystem may have already undergone significant

ecological change prior to initiation of the first in-depth scientific investigation in the mid-1980s. Thus, the

system is likely in a transitional state as nutrient loading approaches new plateaus. Further research and

assessment of these longer-term trends are needed. New retrospective analysis over longer temporal and spatial

scales, based on data from marine sediment cores should improve the historical perspective.

Watershed Nutrient Dynamics

From “edge of field” to streams - There is a need to better understand the dynamics and timing of movement

of nitrogen and other nutrients from the “edge of field” in agricultural landscapes to small streams and

tributaries. This is especially true as it relates to tile drains and other practices that move nitrogen and other

plant nutrients through the soil drainage system.

Wetlands creation - Additional information is needed on the geographic distribution and design criteria for

targeting wetlands creation and restoration efforts. There is also a need to determine which other strategies

(e.g., riparian buffers) and mixtures provide the best nitrate reduction for the least cost. It is important to

understand and quantify the potential for changes in the production of the greenhouse gas N2O that could occur

from wetland creation and restoration efforts.

Agricultural Practices

While dramatic improvements in agricultural practices have been achieved in recent years and the efficiency of

nitrogen use has increased substantially, there is still room for continued improvement in these systems.

Watershed/farm-scale studies - There is a critical need to scale up from experimental plots to watershed/farmscale studies falling into two classes. The first class includes studies to better quantify and demonstrate the

effects of on-farm practices, such as “precision farming”, altered lateral spacing of drainage tiles, controlled

water table levels, use of fall and winter cover crops, altered timing of fertilizer application, and exploring

alternatives to traditional crop rotations. The second class includes studies on better means to intercept

agricultural nutrients between the field and ground water and adjacent streams through riparian buffers,

wetlands, and other means.

Experimental policies and practices - Measuring and quantifying the effectiveness of recent policies and

voluntary actions to reduce nutrient inputs should be coordinated on a basin scale. There is a need to better

quantify and understand the impacts of current and proposed policies (e.g., nutrient trading, fertilizer-use

insurance) that increase incentives to reduce nitrogen loss. Additionally, there is a need to evaluate how future

policies or practices might best be implemented and administered under various institutional frameworks.
48

Longer-term Priorities

Nutrient Cycling and Carbon Dynamics

Soil organic nitrogen - Scientific investigations indicate that the soil zone is a huge storage reservoir of

nitrogen. Both inputs to and outputs from this reservoir have increased dramatically in recent decades.

Research is needed to better understand mineralization and immobilization processes, to develop better means

to measure the amount and forms of nitrogen in the soil reservoir, and to develop strategies to minimize

leaching of nitrate from the soils to the streams.

In-stream, in-river, and Gulf sediment denitrification - There is a need to better quantify denitrification and

phosphorus retention rates in small and large streams and in Gulf sediments, and to compare these rates to those

achieved in riparian buffers and wetlands.

Nutrient cycling in the northern Gulf of Mexico - Further refinements of the relationships among nutrient

fluxes, nutrient ratios, and nutrient cycling on the continental shelf in the Gulf are necessary. Such refinements

would improve simulations of subsequent effects on primary productivity, species composition, development of

hypoxia, and higher trophic-level productivity in the Gulf ecosystem.

Organic carbon - There is a need to improve upon the characterization and potential role of both particulate

and dissolved organic carbon, and to more fully quantify the relationship between this carbon and trends in

river discharge which drive carbon transport to the bottom waters of the northern Gulf of Mexico. The

subsequent comparison of this information with carbon from marine production (stimulated by river nutrients)

will clarify the relative roles of river-borne nutrients and organic matter in the formulation of hypoxia.

Atmospheric Deposition - Additional research on atmospheric deposition of nitrogen in the Gulf is also needed

to examine hypotheses that suggest this may be a larger input factor than presently estimated.

Long-term Changes in Hydrology, Climate, and Population

Large-scale climate effects - The relationship between large-scale climate patterns (e.g., long-term trends and

changes in variability of precipitation) and their potential impacts on river flows, nutrient flux, and flow

dynamics on the continental shelf need further evaluation. Changes in precipitation, temperature, and flow

patterns may have significant long-term influence on the rate and pattern of nitrate flux within the basin, and on

the physical constraints of nitrate assimilation in the northern Gulf. The potential effects of future global

climate change on such large-scale climate patterns should also be taken into consideration.

Flood events - Episodic events, such as the Great Mississippi River Flood of 1993 have not only caused

significant damage to life and property, but have also transported abnormally high quantities of nitrogen and

phosphorus to the Gulf. Studies are needed to evaluate the potential role of flood prevention and control

methods that seek to distribute and retain more floodwater within the basin and thus increase nitrogen retention,

while protecting against flood damage.

Mississippi-Atchafalaya River delta management and restoration - Studies are needed to improve

understanding of nutrient cycling in the deltaic plain in order to guide possible changes in management

activities, such as the diversion of floodwaters to delta backwaters and coastal wetlands restoration.

Point-source and urban nonpoint source controls - The cost of nitrogen reduction from point sources and

from urban nonpoint sources has been analyzed on the basis of existing technologies and human population

densities. The potential for additional population and landscape changes to offset reductions achieved in

nitrogen loading from the basin should be carefully evaluated.
49

Longer-term Priorities

(continued)

Economics and Social Impacts

Economic values of river and lake water-quality improvements - The benefits of reducing nutrient loads in

the freshwater system are considered significant. A great deal of research into these benefits has been

conducted. Most studies, however, have been site-specific and performed for selected watersheds or water

uses. There is a growing need for an aggregated analysis of both direct (e.g., drinking-water protection) and

indirect (increased recreation) improvements in water quality, for the basin as a whole.

Economic values of Gulf water quality improvements - To date only the potential direct economic effects of

Gulf hypoxia on commercial fish catch have been attempted. Additional work needs to be done to explore a

broader range of ecological impacts, including potential impacts to biodiversity and to nonmarket-valued

ecosystem goods and services.

Economic Trade-offs in Agricultural Systems - Better estimates of cost savings to agricultural producers

from reduced fertilizer nutrient inputs are needed. Similarly, better estimates of the social costs that could result

from nitrogen management or reduction strategies (e.g., from dislocation in land use, agribusiness

infrastructure, and farm communities) are needed.
50

Glossary

algae: a group of chiefly aquatic plants (e.g., seaweed, pond scum, stonewort, phytoplankton)

that contain chlorophyll and may passively drift, weakly swim, grow on a substrate or take root

in a water body.

anoxia: the absence of dissolved oxygen.

benthic organisms: organisms living in or on the bottom of aquatic environments (e.g.,

polychaetes, clams, snails).

chlorophyll: pigments found in plant cells that are active in harnessing energy during

photosynthesis.

copepod: zooplankton whose bodies are covered with a hard shell or crust.

cyanobacteria: formerly known as blue-green algae.

demersal organisms: organisms associated with the bottom of aquatic environments, but capable

of moving away from it (e.g., blue crabs, shrimp, red drum).

denitrification: nitrogen transformations in water and soil that make nitrogen nutrients effectively

unavailable for plant uptake, usually returning it to the atmosphere as nitrogen gas.

diatom: a major phytoplankton group characterized by cells enclosed in silicon frustules, or

shells.

eutrophic: waters or soils that are high in nutrients; in aquatic systems, associated with wide

swings in dissolved oxygen concentrations and frequent algal blooms.

eutrophication: an increase in the rate of supply of organic matter to an ecosystem.

hydrogen sulfide: a toxic chemical that diffuses into the water as the oxygen levels above the

seabed sediments become zero.

hypoxia: very low dissolved oxygen concentrations, generally less than 2 milligrams per liter.

mesotrophic: intermediate between oligotrophic (low nutrient) and eutrophic (high nutrient)

systems.

nitrate: inorganic form of nitrogen; chemically NO3

nonpoint: a diffuse source of chemical and/or nutrient inputs not attributable to any single

discharge (e.g., agricultural runoff, urban runoff, atmospheric deposition).

nutrients: inorganic chemicals (particularly nitrogen, phosphorus and silicon) required for the

growth of plants, including crops and phytoplankton.
51

oligotrophic: waters or soils that are poor in nutrients and have low primary productivity.

pelagic: living or growing near the surface of the ocean.

phytoplankton: minute plant life (e.g., algae), usually containing chlorophyll, that passively drifts

or weakly swims in a water body.

productivity: the conversion of light energy and carbon dioxide into living organic material

pycnocline: the region of the water column characterized by the strongest vertical gradient in

density, attributable to temperature, salinity or both.

recruitment: the influx of new members into a population by reproduction or immigration.

respiration: the consumption of oxygen during energy utilization by cells and organisms.

riparian areas: vegetated areas, mostly forested, next to water resources.

senescence: the aging process in mature individuals; in plants, the process that occurs before the

shedding of leaves

stratification: a multilayered water column, delineated by pycnoclines.

zooplankton: minute animal life that passively drifts or weakly swims in a water body, often

feeding on phytoplankton.
52

References

Bierman, Victor J. et al 1994, A Preliminary Mass Balance Model of Primary Productivity and

Dissolved Oxygen in the Mississippi River Plume/Inner Gulf Shelf Region, Estuaries 17(4)

886-899

Bricker, S. B. et al 1999, National Eutrophication Assessment: Effects of Nutrient Enrichment in

the Nation’s Estuaries, NOAA National Ocean Service, Silver Spring, MD, 71 pp.

Caddy, J. F. 1993, Toward a Comparative Evaluation of Human Impacts on Fishery Ecosystems

of Enclosed and Semi-Enclosed Seas, Reviews in Fisheries Science 1(1) 57-95

Carey, Anne E. et al 1999, The Role of The Mississippi River in Gulf of Mexico Hypoxia,

Report Number 70, Environmental Institute, University of Alabama, Tuscaloosa, AL

Diaz, R. J., and R. Rosenberg 1995, Marine benthic hypoxia: a review of its ecological effects

and the behavioural responses of benthic macrofauna, Oceanography and Marine Biology: An

Annual Review. Vol. 33:245-303.

Dodds, W. K. et al 1998, Suggested Classification of Stream Trophic State: Distribution of

Temperate Stream Types by Chlorophyll, Total Nitrogen, and Phosphorus, Water Research

32(5) 1455-1462

Downing, John A et al 1999, Gulf of Mexico Hypoxia: Land and Sea Interaction, Task Force

Report No. 134, Council for Agricultural Science and Technology, Ames IA

EPA 1997, Proceedings of the First Gulf of Mexico Hypoxia Management Conference,

December 1995, Gulf of Mexico Program Office, Stennis Space Center, MS

Jansson, Bengt-Owe and Kristina Dahlberg 1999, The environmental status of the Baltic Sea in

the 1940s, today, and in the future, Ambio 28(4) 312-319

Jickells, T.D. 1998, Nutrient Biogeochemistry of the Coastal Zone, Science 281, 217-221

Justic, Dubravko et al. 1996, Effects of Climate Change on Hypoxia in Coastal Waters: A

Doubled CO2 Scenario for the Northern Gulf of Mexico, Limnology and Oceanography 41(5)

992- 1003

Justic, Dubravko et al. 1997, Impacts of Climate Change on Net Productivity of Coastal Waters:

Implications for Carbon Budgets and Hypoxia, Climate Research 8, 225-237

Kunkel, Kenneth E. et al 1999, Long Term Trends in Extreme Precipitation Events over the

Conterminous United States and Canada, Journal of Climate 12, 2515-2527

Meade, R. H. (ed) 1995, Contaminants in the Mississippi River, US Geological Survey Circular

1133, 140 pp
53

National Research Council 1992, Restoration of Aquatic Ecosystems, National Academy Press,

Washington DC, 552 pp.

National Research Council 1993, Managing Wastewater in Coastal Urban Areas, National

Academy Press, Washington DC

Nixon, Scott W. 1995, Coastal Marine Eutrophication: A Definition, Social Causes, and Future

Concerns, Ophelia 41, 199-219

Ryther, J. H. and W. M. Dunstan 1969, Nitrogen, Phosphorus and Eutrophication in the Coastal

Marine Environment, Science 171, 375-380

Statistical Abstract 1998

Turner, R. Eugene 1999, A Comparative Mass Balance Budget (C, N, P and suspended solids)

for a Natural Swamp and Overland Flow Systems, pp. 61-71 in J. Vymazal (ed), Nutrient

Cycling and Retention in Natural and Constructed Wetlands

USGS 1999, The Quality of Our Nation’s Waters: Nutrients and Pesticides, U. S. Geological

Survey Circular 1225, 82 p.

Vitousek, Peter M. et al 1997, Human Alteration of the Global Nitrogen Cycle: Sources and

Consequences, Ecological Applications 7(3) 737-750

World Development Indicators 1998, World Bank, Washington, DC