of Changing Nutrient Loads of Changing Nutrient Loads
educing nutrient loads, particularly nitrogen from the

Mississippi–Atchafalaya River Basin, will cause improvements

both in the hypoxic conditions in the Gulf and in water

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

presents potential effects over the next 10–20 years, in the Gulf

and the Basin:

■ if there are no major changes in average nitrogen loads,

■ if average nitrogen loads increase, and 

■ if nitrogen loads are reduced.

Effects if Average Loads Are 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. Current activities may be having effects for which data have

not yet been assessed.

Analyses presented previously in this assessment show that

Mississippi–Atchafalaya River Basin nutrient concentrations and

loads, particularly nitrogen, to the northern Gulf of Mexico have

significantly increased over the twentieth century, especially during the last four decades. However, nitrogen loads appear to have

reached a plateau of about 1.6 million metric tons per year, which

is the mean load over the period 1980–96. If nitrogen flux continues at current levels, the present range of dimensions and severity

of the hypoxic zone on the Louisiana shelf may be maintained.

Hypoxia would vary, depending on physical factors in the Gulf

and the Basin, such as spring and summer stratification caused by

fresh river water flowing over marine waters, weather patterns,

and temperature. However, even under these conditions, natural

variability could produce new extremes of the extent and severity

of hypoxia. 

Agricultural nonpoint sources in the MARB constitute about 65

percent of the nitrogen and about 74 percent of the nitrate load to

the Gulf. Nitrogen fertilizer use in the MARB 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 have

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

Chapter 4: The Effects ■ 29
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 bushel per pound of

nitrogen in the late 1970s to approximately 1.0 bushel per pound of nitrogen

today—a 32 percent increase in efficiency. A mass balance of nitrogen inputs

and outputs to the MARB indicates that

they have been approximately equal

and at a steady state since about

1980 and suggests that levels of

nitrogen stored in the Basin

ecosystem are no longer

increasing as they did

between 1955 and 1980

(see Figure 4.1).

If there are no major

changes in average nitrogen loads and the Gulf of

Mexico continues to

experience a large

zone of hypoxia

each summer, the

area affected is

expected to 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

unknown whether the northern Gulf

can sustain this level of impact without

observable reductions in overall fisheries

production. 

Not all nutrients entering tributaries

within the MARB 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. Substantial numbers of river miles in most states in the

Basin suffer use impairment related to

nutrient conditions, meaning that they do

not fully support one or more resource

uses, including aquatic life, fish consumption, and swimming. This situation would

most likely remain the same without some

changes in nutrient loading.

Effects if Average Loads Increase

Average nitrogen loads can increase

because of anticipated growth in population, food production, and fertilizer use.

Changes in climate may also tend to exacerbate hypoxia in the Gulf. Increases in

average nitrogen load might be offset by

changes that tend to mitigate hypoxia,

such as ongoing improvements in farming

practices, better targeting of riparian and

wetland restoration, and improved riverflow management.

Current pressures that drive hypoxic

conditions are likely to intensify. U.S.

population and food demand are projected to continue to grow. Over the past

decade, the U.S. population has increased

by 10 percent (Bureau of the Census

1999); middle series projections predict an

8 percent increase by 2010 and a 17 percent increase by 2020. World population

is expected to grow at 1.2 percent per

year through 2010 according to the

World Bank, 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 percent

higher during 1980–96 than during

1955–70. Part of this increase may be due

to long-term climatic variation, and part

may be driven by shorter-term climatic

cycles. The higher flows in the later half

of the twentieth century are attributed to

increased precipitation throughout the

year and, in particular, to warmer, wetter

springs. During this same period, streamflow has increased 30 percent, while

nitrate flux has increased 300 percent.

Higher precipitation and streamflow as

occurred in the later half of the twentieth

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.

30 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

Reducing

sources of

nutrients from

the MARB may

have as a

primary goal

decreasing the

hypoxia problem

in the Gulf of

Mexico, but

also is expected

to affect water

quality conditions

in the Basin

itself.
Chapter 4: The Effects ■ 31

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 of the world

could be useful in projecting

effects in the Gulf of Mexico.

Experience with other hypoxic

zones around the globe shows that

initially the higher nutrient concentrations lead to increased production of both organic matter and

fisheries (see Figure 4.2). However,

as organic matter production

increases, changes occur in the

food web that lead to both the

ecological and the fisheries effects

becoming progressively more

severe as hypoxia worsens. These

changes have been observed in

marine ecosystems around the

globe, resulting in serious ecological and economic consequences

from hypoxia.

Most notable among these

affected marine systems 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.

In the Gulf of Mexico region,

Louisiana leads in production and

landings of commercial and recreational marine fisheries. The fishery resources of the Gulf are

among the most valuable in the

United States, generating $2.8 billion annually. Although possibly

due to factors other than hypoxia,

catch per unit of 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 nearanoxic conditions, experienced

greater mortality and had fewer

species and lower biomass than

areas with intermittent or less

severe hypoxia. If experiences in

other coastal and marine systems

are applicable to the Gulf of

Mexico, then the potential impact

of worsening hypoxic conditions

could be the decline (perhaps precipitous) of ecologically and commercially important fisheries.

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 and Baltic Seas, 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

Nitrogen inputs to and outputs from the Basin have been approximately equal since about 1980, and the residuals (difference

between inputs and outputs) have been variable. The cumulative residual is at a steady state, indicating that levels of nitrogen

stored in the Basin are no longer increasing as they did between 1955 and 1980.

FIG. 4.1

Source: Figure 6.4B of the Topic 3 Report for the Integrated Assessment.

Cumulative Residual from 

Nitrogen Mass Balance: 1955–97

100

90

80

70

60

50

40

30

20

10

0

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Millions of Metric Tons

cumulative residual,

excluding nitrogen ou tpu t

in Mississippi River discharge

cumulative residual,

including nitrogen ou tpu t

in Mississippi River discharge
32 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

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 spend 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 web

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. Pseudo-nitzschia

spp., a group of diatoms, including some toxic forms, that require

silica, have increased in abundance in spite of silica decreases,

most likely in response to increasing nitrogen inputs from the

Mississippi River.

Although higher nutrient concentrations initially increase the productivity of fisheries, ecological systems worldwide show

negative effects as nutrient loading increases and hypoxic or anoxic conditions develop. Each generic curve in the lower half

of the figure represents the reaction of a species guild to increasing nutrient supplies. The top half of the figure illustrates

trends in various marine systems around the world. Reversals show that trends toward over-enrichment have been turned

around in several areas.

Comparative Evaluation of Fishery Response to Nu trients

Production and Fishery Yield

Nu trients Loading

SE North Sea

N. Gulf of Mexico

N. Adriatic Sea

Great Lakes

Kattegat

Baltic Sea

Seto Inland Sea

Yellow Sea

NW Black Sea

Chesapeake Bay

Meso Mesotrophic Eu trophic Dystrophic trophic

(meduim productivity)

Eu trophic

(high productivity)

Dystrophic

(no productivity)

Oligotrophic

(low productivity)

Other zooplankton feeders (e.g. medusae)

Zooplanktontivorous fish

Benthos-feeding fish

Benthos

Seasonal Permanent bottom anoxia

FIG. 4.2

Source: Redrawn from Caddy 1993.
NOAA NATIONAL MARINE FISHERIES SERVICE

Chapter 4: The Effects ■ 33

Effects if Average 

Loads Are Reduced

Reducing sources of nutrients

from the MARB may have as a

primary goal decreasing the

hypoxia problem in the Gulf of

Mexico but also is expected to

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 Basin,

response variables of interest

include various nutrient forms,

chlorophyll, dissolved oxygen,

water clarity, planktonic and benthic biota, and higher organisms.

However, the response variable of

primary interest for hypoxia is dissolved oxygen in the bottom

waters of the Gulf of Mexico.

Depending on the suite of

techniques used to reduce loads

(i.e., the mix of source reductions,

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).

In addition to reducing risks of

precipitous decline in Gulf fisheries from hypoxic/anoxic conditions, these costs would be offset

by a number of benefits within

the Basin, including improved

water and habitat quality, reduced

soil erosion, reduced contamination of drinking-water sources by

nitrates, improved water quality

for recreational use, improved

recreational fisheries and wildlife,

cost-effective flood damage reduction, and restoration of critical

coastal wetlands in Louisiana.

Changes in the Gulf of Mexico

Simulations with a quantitative

water quality model (Bierman et al.

1994) indicate that dissolved oxygen and chlorophyll concentrations on the Louisiana continental

shelf are likely to be responsive to

reductions in nutrient loads from

the MARB. However, there are

large uncertainties in the magnitudes of these responses for a given

load reduction. For nutrient load

reductions of 20–30 percent,

bottom-water dissolved oxygen

concentrations were estimated to

increase by 15–50 percent, and

surface chlorophyll concentrations

were estimated to decrease by

5–10 percent. 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.

Although both nitrogen and phosphorus load reductions produced

responses in the model, the

response, particularly for dissolved

oxygen, was somewhat greater for

nitrogen reductions.

There has been no statistically

significant long-term trend in

phosphorus flux to the Gulf for

Thousands of small shrimp boats trawl the northern Gulf of Mexico. Important commercial species in the Gulf, such as

shrimp, avoid hypoxic areas, where they would otherwise spend part of their life cycle. Benthic organisms that provide

food for other sought-after species are also disrupted in the hypoxic zone. 
KEVIN F. DENNEHY, U.S. GEOLOGICAL SURVEY

34 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

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 load

reductions of 30–50 percent

(returning to the levels that

existed in 1955–70), the model

estimates that dissolved oxygen

concentrations would increase by

20–75 percent.

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 ecosystems can

respond positively to nutrient

reductions. Examples include

Chesapeake, Tampa, and Sarasota

Bays. Reductions in loads to

Tampa Bay are attributed to

improvements in wastewater treatment and in fertilizer loading

operations in the Port of Tampa.

In Sarasota Bay, reductions

occurred as a result of improvements in wastewater treatment

and agricultural re-use, and the

use of deep-injection wells for

wet-weather effluent disposal

(Johansson and Greening 1999).

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 is 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 to 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, submerged aquatic

macrophyte distribution would be

expected to 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

Public water supplies are treated, if necessary, to ensure that nitrate levels

are below the drinking-water standard of 10 mg/l. People drinking ground

water from shallow wells in vulnerable geologic settings (sand, gravel, or

karst) in rural agricultural areas, however, are exposed to risks that could be

reduced through reductions in nitrogen loading.
Chapter 4: The Effects ■ 35

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 be expected to

strongly affect sport fisheries

because the rivers and streams in

the Basin would most likely continue to be highly productive systems. Any decline that might

occur in total biomass production

would likely be more than offset

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 nitrogen 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. 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.