The Gulf of Mexico's dead zone--the huge swath of oxygen-starved, nearly lifeless ocean that appears off the Louisiana coast every summer--was smaller this year than last, the first time the zone's maximum extent has shrunk since 1993. The news could herald a welcome retreat of the deadly patch of water, which some observers say is harming the state's $3 billion fishery.
Over the last 20 years, scientists have shown that the dead zone is a product of the Mississippi River. It starts each spring when floods wash nutrients, such as nitrogen from farm fertilizers, off riverside lands and into the gulf. The river's warmer, lighter water spills dozens of kilometers offshore, sliding over the heavier, saltier ocean water, forming a lidlike layer. Fueled by sunlight and the dissolved nitrogen, massive algae blooms then thrive in the surface water, attracting tiny grazing crustaceans. Dead algae and the grazer's fecal pellets sink to the bottom, where they are devoured by oxygen-consuming bacteria.
Hypoxia sets in when oxygen levels in the isolated bottom water drop below 2 milligrams per liter, too little to support most marine life. Organisms that can swim, such as fish and shrimp, flee. But less mobile creatures, such as clams, often die. The hypoxic zone disappears by October, after storms stir up the gulf's waters.
In 1993, scientists became alarmed after severe floods caused the dead zone to double in size to more than 17,000 square kilometers, and their worries grew after it failed to shrink substantially in subsequent years. This summer, however, Nancy Rabalais of the Louisiana Universities Marine Consortium in Chauvin and Eugene Turner and colleagues report that the zone has retreated to about 12,288 square kilometers. But a week-long monitoring cruise in July also revealed a disturbing development: "The low oxygen extended much farther from shore, into deeper waters than we normally see it," says Rabalais, who adds that further studies are needed to explain the consequences of that finding.
The shrinkage doesn't mean the dead zone's threat to fisheries is over, says ecologist John Downing of Iowa State University in Ames. "These things have their ups and downs from year to year," he says. It will be important, he cautions, "to make sure we continue to measure this."