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America's 10 worst man-made environmental disasters

By: Laura Moss on June 14, 2010, 12:50 p.m.
Gulf dead zone

Photo: NOAA

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Gulf dead zone

An area in the Gulf of Mexico — beginning at the Mississippi River Delta and extending to Texas — is a wildlife dead zone for many months of the year. During the summer, this area, which is more than 7,000 miles wide, is devoid of wildlife except for the bodies of shrimp, crabs and other marine life that die because of oxygen depletion in polluted water.

This underwater wasteland is the world’s second largest dead zone and is caused by drainage from the Mississippi River, which deposits massive amounts of pesticides, fertilizers and animal waste from the central United States. Nitrogen in the chemicals and animal waste spur the growth of algae, which is eaten by zooplankton. Those microscopic creatures then excrete pellets that decay on the ocean floor, a process that depletes oxygen.

Although the dead zone fades in winter, not all the organic matter decays, which means the zone will get larger even if the same amount of nitrogen is released next year. The zone has grown steadily over the past few decades, and scientists are concerned the Gulf oil spill could have widened the massive dead zone.