'Dead zones' threaten fisheries
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.Skip to next paragraph
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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.
Excess nitrogen combined with placid summer weather results in an oxygen-poor bottom layer of water. The process works this way: In the top layer, the nitrogen and sun feed phytoplankton, which grow rapidly, then die and fall to the bottom. As they decay, they consume oxygen. Called eutrophication, the cycle depletes oxygen in isolated bottom waters. In 2002, one of the worst years since it was first documented in the 1970s, the northern Gulf's hypoxic zone reached more than 7,700 square miles. Despite its size, the problem is largely hidden from view, except to the trained eye.
"I see massive schools of stingrays, bottom dwellers, moving on the surface. Even shrimp come up 20 feet or so off the bottom trying to get to oxygen," says Nancy Rabalais, a marine biologist at the Louisiana Universities Marine Consortium in Chauvin, La. Only because they are desperate to breathe do such bottom-dwelling creatures flee upward, risking becoming easy prey.
More mouths to feed
Such scenes will become more common worldwide, scientists predict. As populations grow, nitrogen and phosphorous-caused eutrophication will more than double in coastal areas by 2050, predicts a 2001 study published in Science magazine.
"There's been a big increase in these hypoxic zones that correlates strongly with increased use of nitrogen fertilizers, particularly in the '60s and 1970s," says Robert Howarth, a coauthor of the Science study and professor of environmental biology at Cornell University in Ithaca, N.Y. "About half of the nitrogen fertilizer used on Earth in all of history has been used in the last 15 years."
One positive trend: Total global fertilizer use seems to be growing more slowly than in the past few decades. It plateaued in 1990 then declined after the collapse of the Soviet Union. In the mid-'90s, global growth resumed, but much more slowly. For the decade, nitrogen fertilizer rose only slightly from 79 million to 82 million tons.