ANAHEIM, Calif.(AP) -- A 7,000-square-mile dead zone in the Gulf of Mexico caused by the runoff of fertilizer is getting worse and the only solution may be to change farming practices throughout the Corn Belt, experts say. Ê Every spring and summer, nitrogen from agricultural fertilizer washes down the Mississippi River and into the northern Gulf of Mexico. The nutrient-rich waters trigger a bloom of algae which strips the water of oxygen.
Researchers attending the recent national meeting of the American Association for the Advancement of Science said that by midsummer, an area of the Gulf extending from Louisiana to Texas becomes so oxygen-deprived that most fish and shrimp escape to healthier waters, leaving behind a vast ``dead zone.''
Nancy N. Rabalais of Louisiana State University said the average size of the dead zone has doubled since 1992 and is now persisting from May until October in some areas.
Shrimp, menhaden, and other valuable species have not been affected because these animals simply flee the bad water, said Rabalais, but bottom-dwellers, such as worms, starfish, and some single-celled animals are killed. At the same time, some microbes that prefer low-oxygen waters explode, forming a white, cotton-like mass that floats on the surface.
``We don't know what the long-term effects of this will be on the ecology of the Gulf,'' said Rabalais. ``The hypoxia (lack of oxygen) has decimated a number of organisms living in the sediments'' and such a change is bound to affect the ecology of the Gulf, she said.
The Mississippi River drains about 40 percent of the United States and carries more than a billion tons of nitrogen, much of it washed from the agricultural fields in the Midwestern farm belt, said Otto Doering, a professor at Purdue University.
Some of the nitrogen dumped into the Gulf comes from natural sources and from cities and industry, but ``agriculture uses 6.5 million metric tons of nitrogen a year and is clearly a major player.''
Doering said farmers could reduce nitrogen runoff by 20 percent by changing farm management practices. He said the goal could be reached if farmers stopped fertilizing in the fall and if major wetlands were restored along the Mississippi River watershed.
Wetlands retain the nitrogen and the chemical then tends to disappear into the atmosphere, he said. The wetlands also would increase wildlife habitat and improve water quality.
Using less fertilizer would reduce crop yield and a 20 percent reduction is about the limit before there would be serious economic consequences for farmers and the nation, according to a scientific assessment by Doering and others.
``Beyond 20 percent, there is a serious disruption in terms of high food prices, an increasing drop in exports and a loss of farmland,'' he said.
Doering said it may take regulation or a tax on nitrogen fertilizer to force changes in farming practices. Just how this will be done, he said, has not been determined.
``Farm groups may need to be compensated,'' he said. ``We don't know how this would affect the individual farmers.''
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