of Hypoxia in the Northern Gulf of Mexico of Hypoxia in the Northern Gulf of Mexico
early all marine animals depend on oxygen dissolved in the

water. 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. Growth of marine organisms is inhibited

when dissolved oxygen is less than about 5 mg/l. Mobile organisms leave as dissolved oxygen levels decrease below 2 mg/l, and

trawlers in these areas produce little or no catch. Burrowing

organisms first emerge from the sediment, and 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).

Hypoxia is the condition in which dissolved oxygen is below

the level necessary to sustain most animal life—generally defined

by dissolved oxygen levels below 2 mg/l (or ppm). The largest

hypoxic zone in U.S. coastal waters—and in the entire western

Atlantic Ocean—is found in the northern Gulf of Mexico on the

Louisiana/Texas continental shelf (Figure 1.1). The area affected,

which is about the size of the state of New Jersey, has increased

since regular measurements began in 1985 (Figure 1.2).

The fishery resources of the Gulf are among the most valuable

in the United States, generating $2.8 billion annually. Although

economic analyses have not shown a statistically significant correlation with the extent of 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. 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 species.

In October 1998, Congress passed the Harmful Algal Bloom

and Hypoxia Research and Control Act, which the President

signed into law as P.L.105-383 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 nutriChapter 1: The Problem ■ 7
ents 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.” P.L.105-383 also

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 Mississippi River/Gulf

of Mexico (MR/GM) Watershed

Nutrient Task Force. This integrated assessment is intended to

provide scientific information to

both specialists and nonspecialists as

a basis for the Action Plan. This

assessment does not make specific

recommendations for action,

nor is it the only source of

information that the

MR/GM Task Force

will consider in developing the Action Plan.

The National Science

and Technology Council’s

Committee on Environment and Natural

Resources had already taken steps in 1997

to assess the state of scientific knowledge

and understanding of Gulf of Mexico

hypoxia, and by March 1998 it had

developed and approved an assessment

plan in conjunction with the MR/GM

Task Force. The plan called for six teams

of experts from inside and outside of government to develop six reports that examine various interrelated aspects of

hypoxia. The teams were established to

review and analyze existing data and

apply existing models of the watershed–

Gulf system. The six peer-reviewed

reports are based on a massive amount of

direct and indirect evidence collected and

reported over many years of scientific

inquiry, including in-depth studies of

oceanographic, hydrologic, agricultural,

economic, and other questions related to

the issue of hypoxia in the Gulf.1

This integrated assessment draws heavily from the six reports, freely using both

their findings and their words, and explicitly cites them only when it refers to specific information a reader may want to

examine in greater detail. The assessment

summarizes the state of knowledge of the

extent, characteristics, causes, and effects

8 ■ HYPOXIA IN THE NORTHERN GULF OF MEXICO

The frequency of occurrence of hypoxia has been mapped from mid-summer “snapshots” obtained by sampling a 60- to 80-

station grid in the Gulf annually from 1985 through 1999.

1 The six topic reports underwent a 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 assessment has been written with consideration of all that information. The assessment was available, in draft

form, for a 60-day public comment period. Those comments are also posted on this web site and were carefully considered in producing the

assessment in this final version. 

FIG. 1.1 Frequency of Occurrence of Hypoxia in the Gulf of Mexico: 1985–99

Source: Based on data from Rabalais, Turner, and Wiseman.

30.0

29.5

29.0

28.5

93.5

S abine Lake

92.5

Longitude

Latitude

91.5 90.5 89.5

Lake Calcasieu Atchafalaya River

Terrebonne Bay

Mississippi River

>75%

>50%

>25%

<25%

50 km

The potential

impact of

worsening 

hypoxic

conditions 

could be the

decline 

(perhaps 

precipitous) 

of ecologically

and commercially 

important species.
Chapter 1: The Problem ■ 9

of hypoxia in the northern Gulf of

Mexico. It outlines a range of

approaches for reducing those

effects and examines the costs and

benefits associated with those

approaches. It also describes additional research and monitoring

needed to reduce uncertainties, to

track progress following any

efforts developed in the Action

Plan, and to identify potential

future adjustments to any initial

actions that may be taken to

reduce hypoxia and improve

water quality. 

Annual mid-summer cruises have been conducted systematically over the past 15 years (with the exception of 1989). Hypoxia

in bottom waters covered an average of 8,000–9,000 km2 in 1985–92 but increased to 16,000–20,000 km2 in 1993–99.

FIG. 1.2

Source: Nancy Rabalais, Louisiana Universities Marine Consortium. NO D

ATA

Areal Extent of Mid-summer Hypoxia: 1985–99

20,000

15,000

10,000

5,000

0

1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

Square Kilometers