U.S. Water News Online
DAVENPORT, Iowa -- Two years after university researchers said the Gulf of Mexico's dead zone covered a record 8,006 square miles -- about the size of Massachusetts -- it has shrunk to about 3,300 square miles, a recent mapping shows.
The dead zone, a summertime phenomenon, is formed where Mississippi River freshwater enters the salty Gulf. Microscopic plants called phytoplankton feed on nitrogen and phosphorus in the river water, but when they die, they decompose and use up the oxygen in the Gulf.
The zone was smaller this year, about half the average size for the last decade, because of three tropical storms that re-oxygenated the water, said Don Scavia, a scientist with the National Oceanic and Atmospheric Administration.
While fishermen in the area weren't happy about the weather, they were pleased that the zone wasn't as large this year, Scavia said.
``The storms came through, stirred up water and broke it into smaller patches,'' he said. But, ``the overall trend and the overall problem are still there.''
Hypoxia occurs when algae, stimulated by nutrients such as nitrogen and phosphates from the Mississippi and Atchafalaya rivers, settles. Ultimately, it decays at the bottom of the Gulf, NOAA officials said.
Researchers believe hypoxia wasn't common in the area before the mid-1970s, Scavia said.
Between 1985 and 1992, the zone averaged 3,200 square miles, officials said. Between 1993 and 2001, the area doubled to an average size of 6,200 square miles. The zone extends about 375 miles west from the mouth of the Mississippi River.
Forecasters had thought the area would be bigger this year because of nutrient loads coming from the two rivers in May and June, Scavia said.
Iowa and Illinois contribute to the nutrients flowing down the Mississippi. Research projects are underway to determine how city dwellers and farmers can reduce runoff into streams and rivers.
The new forecast will allow those who study it to predict what will happen to the area in a specific year, as well as look back and study what did happen, said Nancy Rabalais, a researcher with Louisiana State University.
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