of Hypoxia and Nutrient Over-Enrichment of Hypoxia and Nutrient Over-Enrichment
he consequences of hypoxia in the Gulf of Mexico are not

fully known. This chapter examines what is known about

hypoxia’s direct effects on fisheries and on 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 Mississippi–

Atchafalaya 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. 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 disease, 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 experiments 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 lower where bottom-water concentrations of oxygen

decline below 2 mg/l. Geographic comparisons 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 (see Figure 3.1). 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.

An economic analysis sought to examine the relationship

between the estimates of the hypoxic area and available fisheries

data, primarily on the two main shrimp species in the Gulf

Chapter 3: The Consequences ■ 25
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

are 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 the Topic 2

report, pp. 7–8, 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, this failure

does not necessarily mean

that hypoxic effects are absent;

it only means 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 the

productivity of benthic (or bottomdwelling) species. In the food web, 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 is less than 1 mg/l.

Comparison of benthic communities in

the area affected by hypoxia with those

unaffected in nearby Mississippi Sound

reveals distinct differences. On the

Louisiana continental shelf, benthic communities consist of disturbance-adapted

populations. In contrast, the Mississippi

Sound contains more fully developed,

late-successional stage, “equilibrium”-type

communities.

In addition to destroying 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, may lead 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 states in the

MARB have substantial numbers of river

miles impaired by high nutrient conditions. This means that those areas are not

fully supporting one or more resource

uses, including aquatic life, fish consumption, and swimming. 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 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

toward 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

26 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

The shallow

continental

shelf area

in the Gulf

shows signs

of hypoxiarelated

stress—low

abundance

of fish and

shrimp and

distinctly

different

benthic

communities. 
Chapter 3: The Consequences ■ 27

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. Under this scheme,

about 30–55 percent of the

hydrologic cataloging units

(HCUs) of the Ohio, Lower

Mississippi, and Tennessee subbasins exceed this eutrophic

threshold boundary for total

phosphorus in flowing waters, and

16–40 percent of the HCUs in

these regions exceed the threshold for total nitrogen in flowing

waters. Higher exceedance frequencies were found in the

Missouri, Upper Mississippi, and

Arkansas–Red sub-basins (~80

percent of the HCUs for total

phosphorus and 70–75 percent for

total nitrogen) (see the Topic 4

report 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 percent of

all shallow ground water beneath

agricultural and urban areas that

has been sampled by the U.S.

Geological Survey 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.

While variable, catch per unit of effort for brown shrimp off Louisiana and Texas has trended down since the late 1970s. In

contrast, white shrimp haven’t shown as great a decline, perhaps because they inhabit near-shore zones away from hypoxic

waters.

Trends in Annual Shrimp Yield: 1979–98

120,000

100,000

80,000

60,000

40,000

20,000

0

1980 1985 1990 1995

Metric Tons/Year

FIG. 3.1

Source: NOAA National Marine Fisheries Service, www.sf.nmfs.gov/st1/commercial/landings/index.html

all shrimp

brown shrimp

3-year moving average

white shrimp