U.S. Department of the Interior Fact Sheet 2006–3005

U.S. Geological Survey March 2006 

Gulf of Mexico Dead Zone—The Last 150 Years

The Problem 

A dead zone forms seasonally in the northern Gulf of Mexico

when subsurface waters become depleted in dissolved oxygen and

cannot support most life (fig. 1). The zone forms west of the

Mississippi Delta over the continental shelf off Louisiana and

sometimes extends off Texas. The oxygen depletion begins in late

spring, increases in summer, and ends in the fall.

The formation of oxygen-depleted subsurface waters has

been associated with nutrient-rich discharge from the Mississippi

and Atchafalaya Rivers. Bio-available nutrients in the discharge

can stimulate algal blooms, which die and are eaten by bacteria,

depleting the oxygen in the subsurface water.

The oxygen content of surface waters of normal salinity in

the summer is typically more than 8 milligrams per liter (8 mg/L);

when oxygen concentrations are less than 2 mg/L, the water is

defined as hypoxic (CENR, 2000). The hypoxia kills many

organisms that cannot escape, and thus the hypoxic zone is

informally known as the “dead zone.”

Shelf-wide surveys of the oxygen content of subsurface

waters off Louisiana have been conducted annually in midsummer since 1985. These surveys demonstrate that the frequency

and extent of hypoxia have increased since 1985 (fig. 2; Rabalais

and others, 1999). In 2002, the hypoxic zone off Louisiana had a

maximum extent of more than 20,000 square kilometers (fig. 2)

and was about the size of New Jersey.

The hypoxic zone in the northern Gulf of Mexico is in the

center of a productive and valuable fishery. The increased

frequency and expansion of hypoxic zones have become an

important economic and environmental issue to commercial and

recreational users of the fishery.

Figure 1. Location and frequency of hypoxia on the Louisiana continental

shelf mapped on the basis of measurements made between 1985 and 1999

by N.N. Rabalais (CENR, 2000). The hypoxic zone, also called the dead zone,

varies in size and shape and duration; hypoxia is defined as oxygen

concentrations in subsurface water of less than 2 milligrams per liter. The

four colors show areas that were hypoxic for different percentages of the

time. The continental shelf is the area between the shoreline and a water 

depth of 200 meters (m). Black dots indicate collection localities for the four 

cores that yielded the foraminifer data plotted in figure 4.

Low-Oxygen Faunal Proxy 

To identify long-term trends in oxygen content for periods

when systematic measurements were not made, researchers need

an indicator, or proxy. U.S. Geological Survey (USGS) scientists

and colleagues are working to extend the observational record of

oxygen-depleted bottom-water conditions back through time and

space by analyzing sediment cores from the Louisiana shelf

(fig. 1).

Figure 2. Changes in area of the hypoxic zone on the Louisiana continental

shelf from 1985 through 2003, plotted on the basis of measurements made

by N.N. Rabalais (Louisiana Universities Marine Consortium, written

commun., 2005).

Scientists have found that the abundance of three species of

benthic foraminifers, single-celled organisms, in modern and

buried sediment samples can indicate low-oxygen events in the

water overlying the sea floor where they lived. The three species

are Pseudononion atlanticum, Epistominella vitrea, and

Buliminella morgani, and their combined abundance is called the

PEB index (Osterman and others, 2005). The PEB species live on

the surface of the sea floor, are opportunists that prefer nutrientrich environments, and are tolerant of low-oxygen conditions. The

PEB index is much higher in sea-floor samples collected in the

Louisiana hypoxic zone than it is in other areas of the Louisiana

and Texas shelf. By studying sediment cores recovered from the

Louisiana shelf (fig. 3), researchers can reconstruct a history of the

PEB index at different sites through time.

The Last 150 Years 

The PEB index in four sediment cores is shown in figure 4.

The dates for sediment samples are based on a technique using

radioactive decay of lead-210 (

210

Pb). All four cores show a

similar stratigraphic pattern of generally increasing PEB values

from 1950 to the present (fig. 4). The cores were collected from

mean water depths of 24 to 47 meters and encompass an area of

about 2,500 square kilometers. Thus, the PEB data indicate that

low-oxygen bottom-water conditions have increased since 1950

over the sampled part of the shelf. The trend toward higher PEB

values over the last 50 years is consistent with results of previous
studies that have linked the increased occurrence of hypoxia on

the Louisiana shelf with higher nutrient loading associated with

the use of commercial fertilizer in the Mississippi River Basin

(Nelsen and others, 1994).

Several brief excursions to higher values of the PEB index at

about 1880, 1900, and 1920 indicate that low-oxygen bottomwater events or intervals occurred in the late 1800s and early

1900s (fig. 4, peaks C–E). The maximum values of the pre-1950

excursions in the PEB index are less than the maximum post-1950

PEB values observed near the top of each core, but several of the

excursions are present in more than one core, which indicates that

they are reliable events.

Fluctuations in the PEB index prior to 1950 have been

attributed to natural variations in Mississippi River discharge that

enhanced transport of nutrients and freshwater to nearshore waters

(Osterman and others, 2005). A combination of natural processes,

as well as possible human activities such as land clearing, may

have resulted in the development of widespread algal blooms and

associated low-oxygen bottom water on the Louisiana shelf prior

to 1950 (Turner and Rabalais, 2003). The results from the four

cores studied (fig. 4) indicate that the low-oxygen events of the

last few decades were more extreme than any that occurred in the

previous 150 years.

Figure 3. USGS scientists describe and sample a sediment core that

had been recovered from the Louisiana shelf. The benthic foraminifers from

the core are studied for indications of low-oxygen events in the water 

overlying the sea floor when they were alive. The age of the samples in the

core is determined by measuring the decay of radioactive lead-210 (

210

Pb)

in the sediments. Photograph by Marci Marot, U.S. Geological Survey, 2005.

Future Research 

The existing record indicates that the development of lowoxygen bottom-water conditions over the last several centuries

was a complicated process. Research is underway to understand

better how hypoxic zones form and to extend the record of lowoxygen events on the Louisiana shelf in time and space.

Figure 4. Plots of the PEB index (% PEB) in four sediment cores from the

Louisiana shelf (scales vary depending on the maximum percentage of PEB

foraminifers). Higher values of the PEB index indicate lower dissolved

oxygen (O2) contents in bottom waters. Years from 1850 to 2003 were

derived from measuring the decay of radioactive lead-210 in the sediments.

Letters B, C, D, and E label peaks in the PEB index before 1950. All cores

show an increasing trend of low-oxygen bottom waters during the last 50

years, and the low-oxygen events of the last few decades are more

extreme than any of the pre-1950 events. Locations where cores were

collected are shown in figure 1.

By L.E. Osterman, P.W. Swarzenski, and R.Z. Poore

References Cited 

CENR, 2000, Integrated assessment of hypoxia in the northern Gulf of 

Mexico: Washington D.C., National Science and Technology

Council Committee on Environment and Natural Resources (CENR),

58 p. (Also available online at

http://www.nos.noaa.gov/products/pubs_hypox.html#fia)

Nelsen, T.A., Blackwelder, Pat, Hood, Terri, McKee, Brent, Romer, Nidia,

Alvarez-Zarikian, Carlos, and Metz, Simone, 1994, Time-based

correlation of the biogenic, lithogenic and authigenic sediment

components with anthropogenic inputs in the Gulf of Mexico

NECOP study area: Estuaries, v. 17, p. 873–885.

Osterman, L.E., Poore, R.Z., Swarzenski, P.W., and Turner, R.E., 2005,

Reconstructing a 180 yr record of natural and anthropogenic induced

low-oxygen conditions from Louisiana continental shelf sediments:

Geology, v. 33, p. 329–332.

Rabalais, N.N., Turner, R.E., Justiç', Dubravko, Dortch, Quay, and

Wiseman, W.J., Jr., 1999, Characterization of hypoxia; Topic 1 report

for the integrated assessment on hypoxia in the Gulf of Mexico:

NOAA (National Oceanic and Atmospheric Administration) Coastal

Ocean Program Decision Analysis Series, no. 15, 167 p. (Also

available online at

http://www.nos.noaa.gov/products/pubs_hypox.html.)

Turner, R.E., and Rabalais, N.N., 2003, Linking landscape and water 

quality in the Mississippi River Basin for 200 years: BioScience, v.

53, p. 563–571.

For more information, please contact:

Lisa E. Osterman

U.S. Geological Survey

600 4th Street South

St. Petersburg, FL 33701

Telephone: 727–803–8747, ext. 3084

E-mail: osterman@usgs.gov