ypoxia occurs when dissolved oxygen concentrations are below those

necessary to sustain most animal

life. Since 1993, mid-summer bottomwater hypoxia in the northern Gulf of

Mexico has been larger than 4,000 square

miles. In 1999, it was 8,000 square miles,

which is about the size of the state of

New Jersey.

The Harmful Algal Bloom and Hypoxia

Research and Control Act of 1998 calls for

an integrated assessment of causes and

consequences of hypoxia in the Gulf of

Mexico. The Act also calls for a plan of

action to reduce, mitigate, and control

hypoxia. While this integrated assessment

is intended to provide scientific information for that Action Plan, it does not

include recommendations for action, nor

is it the only source of information that

will be used to develop that plan.

Key Findings

Hypoxia in the northern Gulf of

Mexico is caused primarily by excess

nitrogen delivered from the

Mississippi–Atchafalaya River Basin in

combination with stratification of Gulf

waters (Figure E.S. 1). Hypoxia results

when oxygen consumption, primarily

through decomposing organic material,

exceeds oxygen production through

photosynthesis and replenishment from

the atmosphere. Organic matter can be

supplied from external sources, such as

river inflow, or can be produced within

the system through algal growth stimulated by nutrients.
Sediment cores from the hypoxic zone show that

algal production and deposition, as well as oxygen

stress, were much lower earlier in the 1900s and that

significant increases occurred in the latter half of the

twentieth century. During this period, there have been

three major changes in the drainage basin affecting

the river nutrient flux. First, landscape alterations, such

as deforestation and artificial agricultural drainage,

removed most of the river basin’s nutrient buffering

capacity. Landscape alterations were greatest between

1875 and 1925, with a second peak of drainage development activity during 1945–60. Second, river channelization for flood control and navigation was

completed prior to the 1950s, except structures that

have maintained Atchafalaya flows at 30 percent of

the combined flow of the Mississippi and Red Rivers

since the mid-1970s. Third, major increases in fertilizer nitrogen input into the Basin occurred between

the 1950s and 1980s, along with a large increase in

nitrogen removal in harvested crops.

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. Total

nitrogen load has increased since the 1950s, due primarily to an increase in nitrate nitrogen. Nitrate flux

to the Gulf of Mexico has almost tripled between the

periods 1955–70 and 1980–96. Other forms of nitrogen, as well as organic carbon and phosphorus, have

probably decreased over the twentieth century. No

trend in dissolved inorganic nitrogen or total nitrogen flux has been observed since 1980, but these

fluxes have become highly variable, depending on

river discharge. 

About 90 percent of the nitrate load to the Gulf

comes from nonpoint sources. About 56 percent of the

load enters the Mississippi River above the Ohio

River, and the Ohio basin adds 34 percent. Principal

sources are basins draining agricultural lands in Iowa,

Illinois, Indiana, southern Minnesota, and Ohio.

Gulf ecosystems and fisheries are affected by

hypoxia. Mobile organisms leave the hypoxic zone for

healthier waters, and those that cannot leave die at

varying rates, depending on how low the oxygen level

gets and for how long. Fish, shrimp, zooplankton, and

other important fish prey are significantly less abundant in hypoxic bottom waters.

Comparison of the distribution of fishing effort

shows that the industry has shifted shrimping efforts

away from hypoxic zones. 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

reveal statistically significant effects that could be

attributable to hypoxia.

The Mississippi–Atchafalaya River Basin is the largest river basin in North America, draining an area of 3.2 million square

kilometers, or about 41 percent of the conterminous United States. River flux from the Basin to the Gulf of Mexico affects coastal

areas where hypoxic conditions have been observed. This figure shows the extent of the hypoxic zone from a 1999 survey.

FIG. E.S. 1 Mississi p p i –At c h a f a l a y a R i v e r B a si n a n d Gu lf H y p o x i c Z o n e

New Orleans

Source: Donald Goolsby and William Battaglin, U.S. Geological Survey.

Executive Summary ■ 3
Water quality in the drainage basin has been

degraded by excess nutrients. Most states in the

Mississippi–Atchafalaya River Basin have significant

river miles impaired by high nutrient concentrations,

meaning that they are not fully supporting one or

more resource uses, including aquatic life, fish consumption, and swimming. In some areas ground-water

supplies are threatened by excess nitrate.

Potential Futures for 

Different Load Scenarios

If nutrient loads do not increase, the current size

and severity of Gulf hypoxia and Basin water quality

impairments would most likely remain the same.

Hypoxia would vary annually, depending on the timing and extent of spring and summer stratification,

weather patterns, temperature, and precipitation in the

Gulf and drainage basin. This variability could alter

the extent and severity of hypoxia, including creating

new extreme increases or decreases.

Efforts to reduce loads may be offset by increases

in population and food production and by climate

change. If these other factors increase nutrient loads,

hypoxia may expand. Because spawning grounds,

migratory pathways, feeding habitats, and fishing

grounds of important species are affected by the

extent and duration of hypoxia, expanded hypoxia

could lead to declines in productivity at higher levels

of the food web and additional loss of essential habitat. At some point, fisheries and other species would

be expected to decline, perhaps precipitously.

A 40 percent reduction in total nitrogen flux to the

Gulf is necessary to return to loads comparable to

those during 1955–70. Model simulations indicate that

nutrient load reductions of about 20–30 percent would

result in a 5–15 percent decrease in surface chlorophyll concentrations and a 15–50 percent increase in

bottom-water dissolved oxygen concentrations. Such

increases in oxygen concentrations are significant

because they represent an overall average for the

hypoxic zone, and any increase above the 2 mg/l

threshold will have significant positive effects on

marine life. Reduced nutrient loads to surface waters in

the Basin would also be expected to decrease nutrient

concentrations in its rivers and streams. These changes

should induce positive changes in trophic conditions

and result in Basin-wide improvements in surfacewater quality.

Considerations for Taking Action

In many areas, significant efforts to reduce nutrient

flux to surface waters are already underway. This

assessment is based primarily on conditions observed

through the mid-1990s; thus, current activities may be

having effects that have not yet been documented.

Based on these basin-scale analyses, the primary

approaches to reduce hypoxia in the Gulf of Mexico

appear to be to: (1) reduce nitrogen loads to streams

and rivers in the Basin and (2) restore and enhance

denitrification and nitrogen retention within the Basin.

Another potential approach might be to divert water

from the Mississippi and Atchafalaya Rivers directly to

areas of the Gulf not currently experiencing hypoxia.

However, such an approach would have multiple consequences, none of which have been analyzed.

While this assessment suggests that changes in

agricultural practices could provide many elements of

a solution at least cost to society overall, analyses contributing to this assessment identified several possible

approaches to sharing the burden of nutrient load

reductions among all sectors in the Mississippi

drainage. There are no single solutions to managing

hypoxia in the Gulf. An optimal approach would take

advantage of the full range of possible actions to

reduce nutrient loads and increase nutrient retention

and denitrification within a framework that encourages adaptive management. Such an approach could

be initiated within the existing array of state and federal laws and programs. 

In this assessment, a national model of the agriculture sector was used to examine many of the economic

effects and the changes in nitrogen loading under various scenarios. While specific actions at local levels

will most likely require analyses at higher spatial resolution, the following findings should be considered

when developing a plan of action for improving Basin

and Gulf water quality and habitat.

Management practices that retain more nitrogen on

fields—including applying nitrogen fertilizer at not

more than recommended rates, implementing alternative cropping systems, and improving manure management—as well as reducing nitrogen flux from point

sources will reduce nitrogen loads to rivers and

streams. Reducing nitrogen loss at the edge of the

field by 20 percent through a combination of economically optimal improvements in farming practices

would be expected to cost producers and consumers

4 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO
(net cost) about $0.40 per pound of nitrogen reduction. For comparison, reduction in fertilizer use alone

(without other changes in farming practices) was modeled at several levels. A 45 percent reduction in fertilizer use would be required to generate a comparable

(20 percent) reduction of edge-of-field loss at a cost of

about $1.30 per pound, while a 20 percent reduction

in fertilizer use would be required to achieve a 10 percent edge-of-field loss at about $0.31 per pound.

(Note that these estimates of the impacts of changes

in agricultural practices are for reductions at the “edge

of the field,” the location where sediment and nutrients leave the farm. Estimated edge-of-field source

reductions do not translate to equivalent reductions in

nitrogen loading to the Gulf, as only a portion of

nitrogen sources in the Basin reaches the Gulf.) 

Other measures to reduce nitrogen loads to rivers

and streams, such as reducing urban point and nonpoint sources and atmospheric deposition, could provide important contributions in some instances.

Average costs of reducing point sources and atmospheric deposition are about $5–50 per pound. In addition, nitrogen trading among all sectors could offer

opportunities to obtain least-cost reductions.

Increasing the acreage of wetlands and vegetated

riparian buffers within the Basin would enhance denitrification (a process that removes nitrogen from the

system) increase nitrogen retention, and decrease the

amount of nitrogen entering streams and rivers. Model

analyses demonstrate that the most effective use of

restored and created wetlands would be in watersheds

that discharge high amounts of nitrogen. At typical

denitrification rates for flow-through wetlands, 5 million acres of wetlands would reduce nitrogen load to

the Gulf by 20 percent and would cost $4.05 per

pound of nitrogen denitrified. An estimated 19 million

acres of additional riparian buffers would be needed to

reduce nitrogen load to the Gulf by the same amount.

Reintroducing river water to the backwaters,

coastal wetlands, and shallow inshore bodies of water

on the Louisiana delta could augment efforts to reduce

nutrient inputs from the upstream sources and could

also help to reduce the rate of coastal land loss.

However, diversions have potential deleterious effects,

such as eutrophication of embayment estuaries receiving the flows, that must also be considered.

All of these nutrient-reduction approaches are

expected to produce other important economic and

environmental benefits within the drainage basin.

These include those associated with restored wetlands,

reduced soil erosion, reduced nutrient contamination

of drinking water, reduced vulnerability to floods,

improved water quality for recreational uses, and

improved fish and wildlife habitat in streams, lakes,

rivers, and estuaries. Other potential benefits include

more efficient use of fertilizers and the energy associated with them, and lower overall fertilizer costs.

However, reliable estimates of the economic value of

these benefits are only available for a few categories,

such as wetlands and erosion. When only these benefits are included in the analysis, the net unit costs of

wetlands creation and improved farm practices are

comparable. Accounting for some benefits, reducing

edge-of-field nitrogen loss by 20 percent through

improved farm practices has an approximate net cost of

$0.36 per pound. Reducing nitrogen flux from the river

to the Gulf by 20 percent through wetland restoration

and creation has an approximate net cost of $0.45 per

pound when those benefits are considered.

Adaptive Management: 

Coupling Monitoring, Research, and Action

The complex nature of nutrient cycling and transport within the Basin and the Gulf of Mexico requires

an adaptive management framework. Also, the potentially lengthy period of time required to observe

changes resulting from management action calls for an

adaptive management scheme. Such an approach provides a comprehensive, carefully targeted program of

monitoring, modeling, and research to facilitate continual improvement in scientific knowledge and gradual adaptation of management approaches. This

adaptive management scheme will require a long-term

commitment to monitoring, research, and assessment

and continual feedback between interpretations of

new information and management actions. A comprehensive monitoring program is needed to measure

environmental pressures and responses, and programmatic progress in the Gulf and in the Basin.

The work synthesized in this assessment significantly advances our understanding; however, specific

uncertainties remain. Immediate priorities for reducing

these uncertainties include research on the ecological

effects of hypoxia; watershed nutrient dynamics, particularly between the edge of the field and stream; and

the effects of different agricultural practices on nutrient losses from land. Longer-term priorities include

research on nutrient cycling and carbon dynamics,

long-term changes in hydrology and climate, as well

as economic and social impacts.

Executive Summary ■ 5