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# Hypoxia

Hypoxia means "low oxygen." In estuaries, lakes, and coastal waters low oxygen usually means a concentr ation 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 cons equences of this enhanced growth are reduced sunlight penetrating th e water, a decreased amount of oxygen dissolved in the water, and a loss of habitat for aquatic animals and plants. The decrease in disso lved 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 fertilizer s applied to agricultural fields, golf courses, and suburban lawns; de position of nitrogen from the atmosphere; erosion of soil containi ng 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 gr owth. The associated organic matter, such as dead algal cells and other debris fr om the algae, sinks to the bottom where it decomposes, consuming ava ilable oxygen. Stra tification of fresh water from the Mississippi Ri ver 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

z Hypoxia Fact Sheet (pdf), Ecological Society of America

z Mississippi River Basin Challenges

- Hypoxia , U.S. Environmental Protection Agency

z Oxygen Depletion in Coastal Waters , National Oceanic and Atmospheric Administration

z Harmful Algal Bloom Research and Control Act of 1998

z Questions and Answers about Hypoxia , American Farm Bureau 

### Page 1 of 2 Hypoxia Definition Page

12/17/2009 htt p ://toxics.us g s. g ov/definitions/h yp oxia.htmlInformation on Hypoxia in the Gulf of Mexico

z Hypoxia in the Gulf of Mexico , U.S. Geological Survey

z Mississippi River/Gulf of Mexico Watershed Nutrient Task Force

z Science Assessment of the Causes and Consequences of Gulf of Mexico Hypoxia , National Ocean Service, National Oceanic and Atmospheric Administration

z Hypoxia Watch System for the Gulf of Mexico , National Oceanic and Atmospheric Administration

z Gulf of Mexico Ecosystems and Hypoxia Assessment , Center for Sponsored Coastal Ocean Research, National Oceanic and Atmospheric Administration

z Northern Gulf of Mexico Ecosystem Description, Hypoxia in the Gulf of Mexico (by Nancy N. Rabalais, Louisiana Universities Marine Consortium), National Oceanic and Atmospheric Administration

z Hypoxia In The Gulf Of Mexico an d Nutrients in the Midwest

, Illinois State Water Survey



## Related Headlines

z Nitrogen Isotope Study Answers Questions about the Transport of Nutrients in Streams

z New Report on Science for Hypoxia in Gulf of Mexico

z USGS Info Guides Gulf of Mexico Hypoxic Zone Action Plan

z USGS Leads a National Study on Hypoxia in the Gulf of Mexico

, Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin

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U.S. Department of the Interior | U.S. Geological Survey URL: http://toxics.usgs.gov/definitions/hypoxia.html Page Contact Information: Webmaster Page Last Modified:Thursday, 14-Dec-2006 16:22:47 EST 

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