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Go Hypoxia 

Hypoxia 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

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 

Hypoxia Definition Page Page 1 of 2

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Information 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 and 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 

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Page Last Modified:Thursday, 14-Dec-2006 16:22:47 EST 

Hypoxia Definition Page Page 2 of 2

http://toxics.usgs.gov/definitions/hypoxia.html 12/17/2009