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Hypoxia 101

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What is hypoxia and what causes it?

Hypoxia means low oxygen and is primarily a problem for estuaries and coastal waters. Hypoxic waters have dissolved oxygen concentrations of less than 2-3 ppm. Hypoxia can be caused by a variety of factors, including excess nutrients, primarily nitrogen and phosphorus, and waterbody stratification due to saline or temperature gradients. These excess nutrients, eutrophication, promote algal growth. As dead algae decompose, oxygen is consumed in the process, resulting in low levels of oxygen in the water.

Nutrients can come from many sources, including any of the following:

  • Fertilizers from agriculture, golf courses, and suburban lawns
  • Erosion of soil full of nutrients
  • Discharges from sewage treatment plants
  • Deposition of atmospheric nitrogen

The hypoxic zone in the Gulf of Mexico forms every summer and is a result of excess nutrients from the Mississippi River and seasonal stratification (layering) of waters in the Gulf. Nutrient-laden freshwater from the Mississippi River flows into the Gulf of Mexico. This freshwater is less dense and remains above the more dense saline Gulf water. In addition to the saline gradient caused where the freshwater and saline water meet, the freshwater is warmer than the deeper ocean water, further contributing to the stratification. This stratification prevents the mixing of oxygen-rich surface water with oxygen-poor water on the bottom of the Gulf. Without mixing, oxygen in the bottom water is limited and the hypoxic condition remains.


A diagram of how hypoxia and eutrophication occur in a coastal area.

Click to see larger image.

View a short movie on hypoxia and eutrophication Source: www.gulfhypoxia.net

HowStuffWorks video on hypoxia in the Gulf of Mexico Source: videos.howstuffworks.com

Visit the Gulf of Mexico Dead Zone site, developed by the Science Museum of Minnesota Source: http://www.smm.org/deadzone/ and http://www.smm.org/deadzone/

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Why is it important to reduce hypoxic zones?

Direct effects of hypoxia include fish kills, which deplete valuable fisheries and disrupt ecosystems. Mobile animals (e.g., adult fish) can typically survive a hypoxic event by moving to waters with more oxygen. Less mobile or immobile animals, such as mussels or crabs, cannot move to waters with more oxygen and are often killed during hypoxic events. Ultimately, hypoxia causes a severe decrease in the amount of life in hypoxia zones. Hypoxia also affects the ability of young fish or shellfish to find the food and habitat necessary to become adults. As a result, fish and shellfish stocks may be reduced or become less stable because less young reach adulthood. Hypoxia can also affect species that rely on fish for food. Such species might have to leave an area to find the necessary food to survive.

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Where does hypoxia occur?

Hypoxia occurs naturally in many aquatic environments throughout the world, such as deep basins in the ocean. Hypoxic waters have occurred throughout history, but they are occurring in shallow coastal and estuarine waters more frequently as anthropogenic sources of nutrients increase.

The Gulf of Mexico hypoxic zone is located in the northern Gulf of Mexico off the coast of Louisiana and Texas. The zone was first documented in 1972. Hypoxia occurs in many places throughout the world. For more information, refer to www.gulfhypoxia.net/overview.

A bar graph depicting the size of the hypoxic zone in square kilometers from 1985 to 2008. The graph also depicts the 5-year hypoxic zone average size from 2004 to 2008 of 17,076 square kilometers, and the Action Plan Goal hypoxic zone size of 5,000 square kilometers.

Figure adapted from data provided by N. Rabalais
Click to see larger image

A graphical version of an areal map that shows the extent of hypoxia in the Gulf of Mexico in 2008. The hypoxic zone is depicted with a spectrum of colors depending on the severity of hypoxia in a particular area.

Figure from N. Rabalais and B. Babin
Click to see larger image

A recent study in Science found that there are over 400 hypoxic zones in the world (Diaz and Rosenberg, 2008). The hypoxic zone in the Gulf of Mexico is the largest in the United States.

Map from Diaz and Rosenberg showing hypoxic zones throughout the world.

Figure from Diaz and Rosenberg, 2008 Click to see larger image

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Sources