The "Dead Zone" - Hypoxia in the Gulf of Mexico

Jennifer Harr

Known as the "dead zone," a massive area of oxygen-depleted water stretches across the Gulf of Mexico from Louisiana’s Mississippi Delta to the Texas border every summer, beginning in late spring, and disappearing in the fall. The largest dead zone to date occurred in 1999, and measured roughly 8,000 square miles.

The dead zone is caused by "hypoxia," or the condition in which dissolved oxygen levels are below those necessary to sustain most animal life. It occurs when the amount of oxygen consumed by the decomposition of organic materials exceeds the amount produced by photosynthesis and introduced into water from the atmosphere.

The hypoxia in the Gulf is caused by excess nutrients delivered by the Mississippi River, combined with stratification of Gulf waters. The dead zone occurs each year when warmer, nitrogen-rich fresh water from the River moves over Gulf waters, which are cooler and saltier (and therefore heavier). The excess nutrients cause algae to multiply rapidly. As it dies and sinks to the bottom, the algae is eaten by bacteria that use up the limited amount of oxygen available. As levels of oxygen drop below 5 parts per million (ppm), many creatures begin to show signs of stress. Below 2 ppm, the water is considered hypoxic. Any creature that can will leave the area. Slow-moving creatures such as crabs and snails die and remain on the bottom. At 0.5 ppm, worms and other burrowing organisms crawl up from the mud in search of oxygen, and will die if oxygen concentrations remain at this level. The bodies often remain undisturbed for weeks, because there is nothing to eat them.

The National Oceanic and Atmospheric Administration ("NOAA") is currently leading an assessment of hypoxia in the Gulf mandated by the Harmful Algal Bloom and Hypoxia Research and Control Act, Pub. L. 105-383, which Congress passed in 1998. Six interrelated reports were prepared (available at http://www.nos.noaa.gov/products/pubs_hypox.html ) and underwent peer review. NOAA released a draft "Integrated Assessment" based on these reports in late 1999.

The Integrated Assessment reports that there have been three major changes in the River’s drainage basin during the later half of the 20th century: (1) channelization for flood control and navigation, (2) alterations to the landscape (deforestation and artificial agricultural drainage) that removed much of the "buffer" for runoff into the River, and (3) a dramatic increase in fertilizer nitrogen input into the Mississippi River drainage basin. About 90% of the nitrate discharged by the River comes from non-point sources. The principal sources of nitrate are river basins that drain agricultural land in southern Minnesota, Iowa, Illinois, Indiana and Ohio. The Integrated Assessment concludes that the most effective means of reducing nitrogen inputs include improved management practices to retain nitrogen on fields, reducing application of nitrogen fertilizer, implementing alternative cropping systems, decreasing feedlot runoff, and reducing point sources. Nitrogen trading among all sectors could offer opportunities to obtain the greatest reductions for the least cost. The amount of nitrogen reaching the Gulf could also be reduced by increasing the acreage of wetlands and riparian buffers within the basin. For example, five million acres of constructed or restored wetlands would reduce nitrogen load to the Gulf by 20%.

Representative Ron Kind (D-WI) recently introduced the Upper Mississippi River Conservation Act of 2000, H.R. 4013, which proposes to reduce the runoff contributing to the dead zone. The bill would cover Missouri, Illinois, Iowa, Wisconsin, and Minnesota. Kind’s proposal would avoid a "command-and-control" approach, and would instead create a water-quality monitoring system to develop information. It also promises about $200 million in technical help and incentives to take part in existing conservation programs, such as the Department of Agriculture’s Conservation Reserve Program ("CRP"), Wetland Reserve Program ("WRP"), Environmental Quality Incentives Program ("EQIP"), and Wildlife Habitat Incentives Program ("WHIP").

Hypoxia and overloading by nutrients is not limited to the Gulf of Mexico. There are at least 58 known "dead zones" worldwide, with the worst occurring in the Baltic Sea. About one-third of the Baltic – an area covering more than 38,000 square miles – is lifeless.

In the United States, a report released in early April 2000 by the National Academy of Sciences concluded that fish and other marine life are being killed, and marshlands damaged, in more than a third of the nation’s coastal areas from algae blooms caused by the runoff of excess nutrients. Severe problems were identified in 44 of the 139 coastal areas examined, with problem areas occurring off the coasts of Washington, California, Louisiana, Texas, Florida, North Carolina, Maryland, New York and Massachusetts. Excessive nitrogen from agricultural fertilizers or poultry waste has been linked to a decline of fisheries, the death of manatees along the Florida coast, and the loss of coral reefs and sea grasses. Algae blooms have also been linked to the microbe pfisteria that has killed fish in tributaries of the Chesapeake Bay and off the Carolinas. Because rivers often transport chemical nutrients hundreds of miles from inland farmland and urban centers, the most severe problems occur where rivers and bays feed into the ocean. There are currently no federal laws or regulations that limit general runoff from agricultural lands.