The Gulf of Mexico Dead Zone’s hypoxic conditions have far reaching effects throughout the coastal and marine ecosystems.  Organisms living in the hypoxic zone experience direct mortality, an altered food web, and habitat changes and loss.  The loss of fisheries and oyster beds translates into an economic loss as commercial fishermen are forced to fish elsewhere or stop altogether, and recreational fishermen are no longer attracted to the area.  The species that do remain in this area are further threatened by over- harvesting and are less appealing to consumers fearing disease.  In addition, the same conditions, which produce the Dead Zone, also lead to other detrimental conditions such as Harmful Algal Blooms, which also cause many harmful effects.

The Mississippi River Basin covers forty-one percent of the continental United States, contains forty-seven percent of the nation’s rural population, and fifty-two percent of U.S. farms.  The waste from this entire area drains into the Gulf of Mexico through the Mississippi River.  Included in this agricultural waste are phosphorus and nitrogen, the primary nutrient responsible for algal blooms in the Dead Zone. 

Nitrogen and phosphorus were first used in fertilizers in the United States in the 1930s. Concentrations of nitrate and phosphate in the lower Mississippi have increased proportionately to levels of use of fertilizers by agriculture since the 1960s, when fertilizer use increased by over two million metric tons per year. Overall, nitrogen input to the Gulf from the Mississippi River Basin has increased between two and seven times over the past century.  In addition to agricultural waste, inadequately treated or untreated sewage and other urban pollution is also dumped into these waters.  Nitrogen is normally a limiting factor, meaning its restricted quantities limit plant growth and reproduction.  However, excessive amounts of nitrogen lead to eutrophication, the takeover of nutrient-rich surface water by phytoplankton or other plants.  If nutrient pollution is not greatly reduced, fish and shellfish may someday be permanently replaced by anaerobic bacteria.

http://www.tulane.edu/~bfleury/envirobio/enviroweb/DeadZone.htm