Toxic Substances Hydrology Program
HypoxiaHypoxia means "low oxygen." In estuaries, lakes, and coastal waters low oxygen usually means a concentration of less than 2 parts per million. In many cases hypoxic waters do not have enough oxygen to support fish and other aquatic animals. Hypoxia can be caused by the presence of excess nutrients in water. Excess nutrients can cause intensive growth of algae. The consequences of this enhanced growth are reduced sunlight penetrating the water, a decreased amount of oxygen dissolved in the water, and a loss of habitat for aquatic animals and plants. The decrease in dissolved oxygen is caused by the degradation of dead plant material (algae), which consumes available oxygen. The overall effect is called eutrophication. Nutrients can come from many sources, such as fertilizers applied to agricultural fields, golf courses, and suburban lawns; deposition of nitrogen from the atmosphere; erosion of soil containing nutrients; and sewage treatment plant discharges. What is the Gulf of Mexico Hypoxic Zone? The Gulf of Mexico hypoxic zone is an area along the Louisiana-Texas coast where water near the sea floor has hypoxic conditions. Hypoxia can cause fish to leave the area and can cause stress or death to bottom dwelling organisms that can't move out of the hypoxic zone. The hypoxic zone is thought to be caused primarily by excess nitrogen delivered from the Mississippi River in combination with seasonal stratification (layering) of Gulf waters. Nitrogen promotes algal and attendant zooplankton growth. The associated organic matter, such as dead algal cells and other debris from the algae, sinks to the bottom where it decomposes, consuming available oxygen. Stratification of fresh water from the Mississippi River and saline water prevents the mixing of oxygen-rich water on the surface with oxygen-depleted water on the bottom. This lack of mixing limits the replenishment by oxygen and sustains the hypoxic zone. Information on Hypoxia
Information on Hypoxia in the Gulf of Mexico
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