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The Gulf of Mexico 'Dead Zone'

Substantial increases in synthetic nitrogen fertilizer production and fossil fuel combustion, and land-use changes involving clearing and conversion, crop cultivation, and drainage of wetlands, among other factors, have significantly altered the global nitrogen cycle. The increased rate of nitrogen input is affecting the quality of the atmosphere and, in many regions, soil, groundwater, lakes and streams, and estuarine and nearshore marine environments.

While nitrogen is essential to the high productivity of estuaries and other coastal regions, an excessive supply can set into motion a cascade of detrimental effects. Microscopic algae (phytoplankton) are stimulated into high production and, if not incorporated into the food web, can form dense growths or blooms. Dead and dying cells sink and are decomposed by bacteria and other microbes. The decomposition process requires, and can remove, substantial amounts of oxygen from the surrounding water and, in conjunction with stratification (where bottom waters are isolated from the atmosphere and hence oxygen resupply), may lead to 'hypoxia' - the condition in which dissolved oxygen is below the level necessary to sustain most animal life.

The Problem

  • The largest oxygen-depleted area - or 'dead zone' - in the entire western Atlantic Ocean occurs in the northern Gulf of Mexico on the Louisiana/Texas continental shelf. Stretching westward from the mouth of the Mississippi River in water up to 200 feet deep, it encompassed approximately 7900 square miles in 2007, an area almost as large as the state of New Jersey, and the third-largest ever recorded. The dead zone is a seasonal occurrence triggered by the high influx of nutrient-laden freshwater washing down from the Mississippi with the onset of melting snow and spring rains; it typically forms in May - though sometimes as early as February - and remains until September or October.
  • The dead zone forms in one of the most productive fisheries areas of the United States, and has led to increasing concern that catches are being adversely affected - or will be affected if hypoxic conditions continue or worsen.
  • While the causes of eutrophication and hypoxic water conditions have been well-characterized, still relatively little is known of their overall impacts in the Gulf of Mexico. In general terms, however, effects of hypoxia include the direct mortality of organisms, including fish and their food base, high losses of benthic or bottom-dwelling plants and animals, reductions in numbers of species, and disruption of fish spawning, recruitment, and migration.

The Causes

  • The primary human-related factor in the formation of the dead zone is agriculture. It is estimated that about 1.6 million metric tons of nitrogen - primarily from the highly fertilized agricultural regions of southern Minnesota, Iowa, Illinois, Indiana, and Ohio - enters the Gulf annually from the Mississippi Basin. Close to one million metric tons of this is nitrate (the most important form of nitrogen involved in the creation of the dead zone), an amount some three times more than what was discharged in 1970.
  • Agriculture (fertilizer and livestock manure) accounts for 65% of the nitrogen flux to the Gulf; soil erosion, ground-water discharge and atmospheric deposition are estimated at 24% while municipal (mainly wastewater treatment systems) and industrial point sources contribute some 11%.
  • High losses of wetlands and riparian areas throughout the U.S. - in large part due to agricultural expansion - have exacerbated the problems associated with nutrient pollution. Wetlands and riparian vegetation, such as forests, are important natural 'sinks' in that they can retain nitrogen or convert nitrate to nitrogen gas.

The Context

  • The northern Gulf of Mexico has undergone significant change over the last decades due to human activity. Along with nutrient pollution and eutrophication, impacts have accrued from: fisheries, especially the extreme levels of bycatch and the damage to seabed habitats associated with shrimp trawling; destruction of wetlands related to flood control measures and construction of shipping channels; chemical pollution; and introduced species.
  • While there is concern that hypoxic conditions in the northern Gulf of Mexico are resulting in a decline in Gulf of Mexico fisheries productivity, studies have been unable to detect an impact. While this can be seen as positive, scientists have also noted that this does not mean that an impact hasn't occurred, or that serious detrimental changes to fisheries will not occur if conditions continue or worsen. Rather, the complexity inherent in ecological systems, and the confounding effect of numerous stressing agents - such as fisheries and wetland loss - make it extremely difficult to delineate clear cause-and-effect relationships.
  • Recent analyses consider that a 50% reduction of nitrogen loading to the Gulf is possible if a variety of efforts are implemented. These include: modification of farm practices towards the more efficient use of fertilizer and manure; creation and restoration of wetlands and riparian ecosystems; reflooding of former wetlands; controls on nitrogen discharges from sewage treatment plants and industries; and changes in flood control measures throughout the Mississippi Basin. The reversal of hypoxic conditions is, however, likely to be slow and many decades of scientific monitoring may be required in order to verify system recovery.
  • The increase in the availability and mobility of nitrogen has only more recently become an issue of global significance and scientific concern. Effects worldwide include threats to human health in the form of high nitrate levels in drinking water, losses of important trace nutrients in soil, the formation of acid rain and photochemical smog, and a variety of impacts in coastal waters including increases in harmful algal blooms and declines in commercial fisheries. In addition, nitrogen in the form of nitrous oxide is a contributor to the 'greenhouse effect' as associated with climate change.

Further Reading

CENR. 2000. Integrated Assessment of Hypoxia in the Northern Gulf of Mexico. National Science and Technology Council Committee on Environment and Natural Resources, Washington, DC.

Chesney, E.J. et al.  2000.  Louisiana estuarine and coastal fisheries and habitats: Perspectives from a fish's eye view.  Ecological Applications 10(2): 350-366.

Dodds, W.K.  2006.  Nutrients and the ''dead zone'': the link between nutrient ratios and dissolved oxygen in the northern Gulf of Mexico.  Frontiers in Ecology and Environment 4(4): 211-217.

Donner, S.D.  2007.  Surf or turf: A shift from feed to food cultivation could reduce nutrient flux to the Gulf of Mexico.  Global Environmental Change - Human and Policy Dimensions 17(1): 105-113.

Mitsch, W.J. et al.  2001.  Reducing nitrogen loading to the Gulf of Mexico from the Mississippi River Basin: Strategies to counter a persistent ecological problem.  BioScience 51(5): 373-388.

Rabalais, N.N. et al.  2002.  Gulf of Mexico hypoxia, aka ''The dead zone''.  Annual Review of Ecology and Systematics 33: 235-263.

Rabalais, N.N. et al.  2002.  Beyond science into policy: Gulf of Mexico hypoxia and the Mississippi River.  BioScience 52(2): 129-142.

Smil, V.  1997.  Global population and the nitrogen cycle.  Scientific American July: 76-81.

Tilman, D. et al.  2001.  Forecasting agriculturally driven global environmental change. Science 292(5515): 281-284.