The Louisiana Environment

The Gulf of Mexico Dead Zone
Red Tides

by Elizabeth Carlisle

The Dead Zone

    The Gulf of Mexico hypoxic zone is a seasonal phenomena occurring in the northern Gulf of Mexico, from the mouth of the Mississippi River to beyond the Texas border.  It is more commonly referred to as the Gulf of Mexico Dead Zone, because oxygen levels within the zone are too low to support marine life.  The Dead Zone was first recorded in the early 1970's. It originally occurred every two to three years, but now occurs annually.  In the summer of 1999 the Dead Zone reached its peak, encompassing 7,728 square miles.

    Hypoxic conditions arise when dissolved oxygen levels in the water fall below two milligrams per liter of water, too low to sustain animal life in the bottom strata of the ocean. The Dead Zone forms each spring as the Mississippi and Atchafalaya Rivers empty into the Gulf, bringing nutrient rich waters that form a layer of fresh water above the existing salt water. It lasts until late August or September when it is broken up by hurricanes or tropical storms.  The nutrients provide favorable conditions for excessive growth of algae that utilize the water’s oxygen supply for respiration and when decomposing.

    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.

   The Dead Zone reappears every spring as conditions for algal blooms become more favorable.  Rivers carry greater quantities of water in the spring, along with greater quantities of dissolved nutrients, as the snow melts in northern areas and rainfall increases.  Sunlight also increases in intensity and duration during this period, accompanied by warmer weather and fewer storms, all of which encourage algal growth.  Decreasing storms in late spring and early summer result in calmer water, which prevents the bottom strata of low-oxygen water from mixing with oxygenated surface water.  Organisms living at greater depths, including most marine animals, cannot acquire necessary oxygen.  This timing is especially bad, as the summer months are a time of active reproduction by fish and benthic (bottom-dwelling) invertebrates.  In turn, the Dead Zone is broken up in late August or September by hurricanes or tropical storms.

    As the fresh, nutrient-enriched water from the Mississippi and Atchafalaya Rivers spread across the Gulf waters, favorable conditions are created for the production of massive phytoplankton blooms.  A bloom is defined as an  “increased abundance of a species above background numbers in a specific geographic region”.  Incoming nutrients stimulate growth of phytoplankton at the surface, providing food for unicellular animals.  Planktonic remains and fecal matter from these organisms fall to the ocean floor, where they are eaten by bacteria, which consume excessive amounts of oxygen, creating eutrophic conditions.  Hypoxic waters appear normal on the surface, but on the bottom, they are covered with dead and distressed animal, and in extreme cases, layers of stinking, sulfur-oxidizing bacteria, which cause the sediment in these areas to turn black.  These hypoxic conditions cause food chain alterations, loss of biodiversity, and high aquatic species mortality.

Red Tide

     In addition to these direct effects, hypoxia may explain another phenomena observed in the northern Gulf of Mexico: red tides. These high concentrations of toxic phytoplankton share a complex relationship with hypoxia. The presence of nitrogen and phosphorus, as well as the disrupted food chain of the Dead Zone, create favorable conditions for cyanobacteria, microflagellates and dinoflagellates, organisms responsible for the formation of red tides. These algal blooms in turn kill additional marine species by paralyzing their respiratory systems.  Of the thousands of species of microscopic algae comprising the base of the marine food chain, approximately eighty-five species have been documented as being toxic.

     The term “red tide” is somewhat of an inappropriate name for the phenomena of these toxic algal blooms, although they are characterized by the discoloration of the water as they dominate the planktonic community.  The term is misleading because other non-toxic blooms can also cause the discoloration of the water, and conversely, negative effects can occur when toxic algal concentrations are low and the water is clear.  Therefore, the scientific community now employs the term “harmful algal bloom”, or HAB, in place of “red tide”.  Algae species associated with harmful blooms produce potent toxins, which are liberated when eaten, while other species kill without toxins.  For example, the serrated spines of certain nontoxic algae can lodge in fish gills, causing irritation which leads to over-production of mucus, and eventually death.  Many algae species are also capable of forming cysts that remain in sediment until environmental conditions are conducive to the occurrence of a bloom.  Cysts on the ocean floor are directly toxic to filter feeders like oysters.

    Many toxic algae produce potent neurotoxins which can be transferred through the food web, affecting or killing higher life forms, including zooplankton, shellfish, fish, birds, marine mammals, such as whales and porpoises, and humans.  These toxins accumulate in shellfish, such as clams, oysters, mussels, and scallops in levels that are potentially lethal to consumers, including humans.  Toxins can also accumulate in the viscera of commercially important fish, including herring, mackerel, and sardines.  These toxins endanger human health if consumed, causing allergic reaction (skin and respiratory), nervous disorders, and liver disorders. In addition, these fat-soluble toxins accumulate in human body tissue, suggesting the possibility for long-term damage even in consumers who do not become obviously sick after eating contaminated seafood.  Toxins, whether contained in algae or released into open water, can move through ecosystems in a manner similar to the flow of energy or carbon, as a wide variety of animals are known to accumulate biotoxins and act as intermediate vectors to consumers at higher trophic levels; therefore these toxins can have significant and widespread impacts.  As algal toxins move through marine food webs, a broad spectrum of effects on aquatic organisms in both inshore and offshore habitats results from both chronic and acute exposure, and has been more evident in recent years.

     The blooms present in the Dead Zone are primarily nontoxic, and therefore pose no direct threat to other marine organisms and humans.  Indirectly, however, they cause conditions that lead to oxygen depletion, making the Gulf uninhabitable for other organisms, and leading to social and economic loss for humans.  The Gulf of Mexico yields approximately forty percent of annual U.S. commercial fishing, as well as being home to many recreational fishing activities.  There is growing concern over the safety of seafood as a result of the contamination and chemical pollution of fishing waters.  One half of the shellfish producing areas along the gulf coast have either been permanently closed or declared indefinitely off-limits by health officials as a result of pollution.  The same concerns have caused the closure of many oyster beds.  Raw shellfish, such as oysters, clams, and mussels, are at the greatest risk for contamination by bacteria and viruses from pollution.  Direct costs include adverse health effects and lost sales of fish and shellfish products, but there are also indirect costs, such as restricted development or investment decisions in coastal aquaculture due to the potential for algal blooms.  Commercial and recreational fisheries in the gulf generate 2.8 billion dollars annually.   This industry could be seriously affected by reduced food sources for fish and shrimp in hypoxic waters, which would lead to a reduction in the abundance of fish and shrimp and declines in shrimp catch and catch efficiency due to the expansion of hypoxia.

     Nutrient abatement in large systems, such as the Gulf of Mexico Dead Zone, has been slow, due to the accumulated materials in sediments. Abatement can be accomplished with current technology, but would require improvements in nutrient retention by farms throughout the Mississippi River Basin.  Remediation programs elsewhere have shown that: marine degradation has occurred slowly, therefore recovery is slow; multi-level, multi-institutional support is needed for effective nutrient management; large-scale ecosystem restoration is technically achievable; climate variability can cover the restoration process; and the benefits of restoration will profit many areas of society.  Plans undertaken to reduce gulf hypoxia would also result in cleaner air, enhance ground and surface water quality, promote beneficial growth management, reduce topsoil loss, provide additional wetland habitat, and the more efficient management of agricultural resources.  However, Rabalias estimates that a forty to fifty percent reduction in the nitrogen input of agriculture would be necessary to return to pre-1950-levels in Mississippi and Atchafalaya drainage.  The benefits of nutrient reduction are not in question; the problem remains to be how such plans would be implemented and who would cover the financial burden initially caused by such drastic changes.  These changes would primarily be carried out by the Mississippi Basin agricultural industry, which would reap few of the benefits, and therefore is not eager to implement necessary changes.  In addition, changes in the detrimental actions of farmers are unlikely to occur because fewer than eleven percent of the polled residents of the Mississippi Basin were even aware that the problem exists.

     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.

Further Reading

Anderson, Donald M.  “Toxic Algal Blooms and Red Tides: A Global  Perspective.”  Red Tides: Biology, Environmental Science, and  Toxicology.  Ed. Tomotoshi Okaichi et al. New York: Elsevier, 1989.  11-16.

Coleman, Elizabeth.  “Predicting Red Tide.”  Coast and Sea.  16 (1997):  17-19.

Malakoff, David.  “Death by Suffocation in the Gulf of Mexico.” Science.  281 (1998): 190-92.

Shimizu, Y.  “Toxicology and Pharmacology of Red Tides: an Overview.” Red Tides: Biology, Environmental Science, and Toxicology.   Ed. Tomotoshi Okaichi et al. New York: Elsevier, 1989. 17-21.

Turner, Eugene and Nancy Rabalais.  “Changes in Mississippi River  Water Quality this Century.”  Bioscience. 41 (1991): 140-147.


Nancy Rabalais, LUMCON at


Agriculture Industry's View

Gulf of Mexico Hypoxia Conference

Harmful Algal Blooms (HABS)

LUMCON Press Release- 1998

Nutrient Enrichment

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