In midsummer, the northern Gulf of Mexico, where the Mississippi River empties into it, may shimmer like any other swath of sea. But a few score feet below, bottom-dwelling fish and other creatures struggle just to breathe.
This area - one of the world's biggest coastal "dead zones" - is rapidly being joined by a growing number of "hypoxic," or oxygen-depleted areas around the world. At least 146 such zones have been documented through 2000 - from the northern Adriatic Sea to the Gulf of Thailand to the Yellow Sea, according to a United Nations Environment Program (UNEP) report released in March. And their number has been doubling every decade since 1960, it adds. At risk: coastal fisheries near the most populous regions.
A handful of efforts are under way that could mitigate the effects. But because of lag times involved, the problem is likely to get worse before it gets better.
"I'm convinced this is going to be the biggest environmental issue in the aquatic marine realm in the 21st century," says Robert Diaz, a marine biologist and professor at the Virginia Institute of Marine Science, who coauthored the study undergirding the UNEP report. "It won't take too much for these annual lower-oxygen events to expand throughout the year and actually eliminate fisheries."
Dead zones often grow where populations grow. But the real driver is the spread of nitrogen, many observers say, caused by runoff of nitrogen-based fertilizers, sewage outflows, and nitrogen deposits from burning fossil fuels. Some waters remain oxygen-depleted year-around. In other waters, the problem appears periodically.
In the northern Gulf of Mexico, one of the best-known and best-studied dead zones, hypoxia occurs seasonally from April to September. The zone's size depends on the weather and how much flow the Mississippi brings each year. Its waters are laden with fertilizer runoff from farms and lawns across the Midwest. Sewage and fossil-fuel emissions exhaust (from power plants and autos) are also factors, says a 1999 University of Alabama study sponsored by the fertilizer industry.
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