for National Geographic News
Each year a swath of the Gulf of Mexico becomes so devoid of shrimp, fish, and other marine life that it is known as the dead zone.
Scientists have identified agricultural fertilizers as a primary culprit behind the phenomenon. Researchers are now focusing on shrinking the zone.
Dave Whitall is a coastal ecologist with the National Oceanic and Atmospheric Administration's Center for Coastal Monitoring and Assessment in Silver Spring, Maryland. He said the dead zone forms each April and lasts through the summer, adding that the zone "generally grows throughout the summer, reaching a peak in late July."
At its peak, the nearly lifeless water can span 5,000 to 8,000-plus square miles (13,000 to 21,000 square kilometers), an area almost the size of New Jersey.
The dead zone is the result of oxygen-depleted water. Fish, shrimp, and all other marine organisms that require oxygen to survive either flee the zone or die.
Whitall says the phenomenon is triggered by excess nutrients in the Mississippi and Atchafalaya River Basins. Streaming into the Gulf of Mexico along the Louisiana coast, the rivers drain about 40 percent of all U.S. land area and account for nearly 90 percent of the freshwater runoff into the Gulf.
The nutrients that flow into the Gulf of Mexico allow microscopic organisms called phytoplankton to bloom. When these algae die, they sink to the bottom of the ocean. There, they are decomposed by oxygen-consuming bacteria.
In the process, the bacteria consume most of the surrounding waterborne oxygen, leaving little oxygen for the other life-forms that depend on it.
While the excess nutrients that flow into the Gulf of Mexico come from a variety of sources, Whitall said, the main source is agricultural runoff, namely chemical fertilizers and animal manure.
"Nitrogen and phosphorus also come from human waste, via wastewater treatment plants and septic systems; domestic animal waste; and industrial sources," he said. Additional nutrient sources include power plants, cars, and agricultural and industrial ammonia emissions.
Dead zones are not unique to the Gulf of Mexico.
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