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5 December 2013 11:26 am ,
Vol. 342 ,
An animal rights group known as the Nonhuman Rights Project filed lawsuits in three New York courts this week in an...
Researchers have been hot on the trail of the elusive Denisovans, a type of ancient human known only by their DNA and...
Thousands of scientists in the Russian Academy of Sciences (RAS) are about to lose their jobs as a result of the...
Dyslexia, a learning disability that hinders reading, hasn't been associated with deficits in vision, hearing, or...
Exotic, elusive, and dangerous, snakes have fascinated humankind for millennia. They can be hard to find, yet their...
Researchers have sequenced and analyzed the first two snake genomes, which represent two evolutionary extremes. The...
Snake venoms are remarkably complex mixtures that can stun or kill prey within minutes. But more and more researchers...
At age 30, Dutch biologist Freek Vonk has built up a respectable career as a snake scientist. But in his home country,...
- 5 December 2013 11:26 am , Vol. 342 , #6163
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Dead Zone Dies Back
18 August 1998 7:30 pm
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."