Eutrophication

EutrophicationWhen too many nutrients, such as phosphorus and nitrogen, are in the water, a condition known as eutrophication occurs. The process begins with an accelerated growth of algae with the end result being that oxygen in the water is depleted as plant matter decays, killing fish and shellfish.

Sources of excess nutrients include urban and agricultural runoff, often called “non-point source” pollution. Nutrient levels have remained constant over the last 15 year period, while other indicators of eutrophication have increased. Chlorophyll levels, which indicate how much algae is growing, are high in the northern part of the Barataria basin and have increased substantially in past decades. Similar increases were observed in Terrebonne and Barataria Bays.

Beyond the algal blooms, eutrophic waters are characterized by a dominance of fish, like gar and shad, and have a potential for noxious and toxic phytoplankton blooms. At present, toxic and noxious phytoplankton have been observed in Bayou Petit Caillou, in the Terrebonne Bay estuary and in Four League Bay. To date, these tiny plants have not caused harm to human health, but they have discolored the water and caused some fish kills. Since 1980, there have been 188 reported fish kills in the estuary due to the presence of nutrients, toxics, and other contaminants. In September of 1994, an algal bloom resulted in a fish kill of almost 200,000 fish.

Probable Causes

  • Sewage treatment plants
  • Septic tanks
  • Urban runoff
  • Agricultural runoff
  • Mississippi River diversions
  • Channelization of runoff directly into the estuary
  • Channel dredging and loss of wetlands resulting in reduced capacity of the estuary to filter out nutrients

Probable Impacts

  • Algal blooms, floating masses of algae. and noxious odors
  • Reduced recreational value of beaches and water bodies
  • Anoxic conditions resulting in fish kills
  • Changes in species composition and population
  • Decreases in wildlife populations with sport and commercial value
  • Reduced recreational and commercial value of wetlands and the estuaries
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