Defining Eutrophication and Hypoxia

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Nutrient loading from sources in the Mississippi River basin has resulted in a large zone of hypoxia in the Gulf of Mexico creating what is widely referred to as the "dead zone. Eutrophication is defined as the addition of nutrients to an aquatic system. At first glance this may seem like a good thing; increased nutrient levels result in increased primary productivity and thus a more productive system. Unfortunately eutrophication can have many negative effects on ecosystems that are normally nutrient low systems. Eutrophication is a process that occurs naturally in bodies of water; however human activities have greatly increased the rate of the process of eutrophication (Horne and Goldman p. 496). Eutrophication results in an increase in primary productivity, often in the form of huge algal blooms. These algal blooms may shade out plants that would normally live lower in the water column resulting in a loss of biodiversity. Eutrophication can also result in the development of hypoxic conditions.

Waters that have a concentration of less than 2 mg/L (20% oxygen saturation in the Gulf of Mexico) are defined as hypoxic (Montgomery, 1969). Low oxygen concentrations disrupt benthic and demersal communities and can eventually lead to mass mortality (Boesch and Rabalais, 1991; Diaz and Rosenburg,1995) and changes in natural migration patterns (Gazey et. al., 1982). In estuarine ecosystems, it is generally recognized that high levels of nutrients, particularly nitrogen increases primary production. When the Mississippi River system floods in early spring, heavy discharge of nutrients coupled with favorable atmospheric light conditions causes phytoplankton to bloom (Wiseman et. al., 1997). Masses of organic carbon sink to the bottom layers in the form of individual cells and zooplankton fecal matter forming an organic substrate which oxygen-using bactera metabolize during summer months. Because of the halocline produced from the vast freshwater discharge, re-oxygenated water from the surface is unable to mix with the oxygen depleted bottom water causing stratification and intensification of hypoxia. Hypoxia and stratification remain until the beginning of the fall storm season when high winds, currents and wave action mix the various strata (Wiseman et. al., 1997). Currently, the largest coastal zone of hypoxic water in the United States is in the northern Gulf of Mexico on the Louisiana/Texas continental shelf. Here, at the mouth of the Mississippi and Atchafalya Rivers, the story of eutrophication, hypoxia, and the resulting ecological stresses impacting the coastal ecosystem begin.

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