by Lenna Gonya

Created on: September 21, 2010   Last Updated: September 23, 2010

Dead zones are found in every part of the world. This condition, known as hypoxia occurs when the oxygen level in the water is below 2-3 ppm. The cause of these dead zone areas is excess nutrients that have collected, causing algae growth. The algae decomposes, using up the oxygen in the water, and ultimately endangers sea life.

In the case of the Gulf of Mexico, nitrogen and phosphorus, are washed down the Mississippi to the Gulf, where they are deposited. These nutrients that find their way down the river come from agricultural runoff, lawns, and even industrial waste.

Runoffs do not mix well with the sea water, staying near the surface and depleting oxygen levels further down in the water causing a condition called stratification. This stratification is caused by the runoff water which is warmer and less saline and remains near the surface.

The dead zone in the Gulf of Mexico is the largest in the United States. Known as the Mississippi-Atchafalaya River Basin, or MARB, and is a combination of the waters from the Mississippi and the Atchafalaya rivers. The Gulf, at this point, collects all of the nutrients and chemicals that wash down the 2,350 miles that comprises the Mississippi and its tributaries. In fact, water from 31 states drains into the Mississippi. This basin is the third largest in the world, after the Congo and the Amazon.

Through the years, the problem has been compounded by the fact that more and more industry and agriculture has appeared along the rivers and tributaries, and more and more wetlands have been destroyed. These wetlands and other natural formations once acted as a filter for contaminants. Land used for commercial purposes has eliminated important areas that once prevented erosion. And finally, the increased traffic on the river has produced water turbulence that has increased the amount of particles, that are normally included in the sediment of the river, to be displaced and incorporated into the water.

The effect of the formation of a dead zone may include the death of certain sea creatures that remain in the oxygen depleted water such as snails, and crabs. Fish can escape, however, some shellfish and other aquatic life cannot.

The Gulf is not the only body of water that is affected by this over abundant algae growth due to runoff. Even inland lakes are experiencing algae growth that cause contamination that renders the lake unsafe for swimming or fishing.

http://toxics.usgs.gov/hypoxia/hypoxic_zone.html

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