貼Rapid Growth Found in Oxygen-Starved Ocean'Dead Zones'- NY

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http://www.nytimes.com/2008/08/15/us/15 ... ef=science



Rapid Growth Found in Oxygen-Starved Ocean ‘Dead Zones’



By BINA VENKATARAMAN

Published: August 14, 2008

Many coastal areas of the world’s oceans are being starved of oxygen at an alarming rate, with vast stretches along the seafloor depleted of it to the point that they can barely sustain marine life, researchers are reporting.

The main culprit, scientists say, is nitrogen-rich nutrients from crop fertilizers that spill into coastal waters by way of rivers and streams.

A study to be published Friday in the journal Science says the number of these marine “dead zones” around the world has doubled about every 10 years since the 1960s. About 400 coastal areas now have periodically or perpetually oxygen-starved bottom waters, many of them growing in size and intensity. Combined, the zones are larger than Oregon.

“What’s happened in the last 40, 50 years is that human activity has made the water quality conditions worse,” the study’s leader author, Robert J. Diaz, said in an interview.

The trend portends nothing good for many fisheries, said Dr. Diaz, a professor at the Virginia Institute of Marine Science at the College of William and Mary. “Dead zones,” he said, “tend to occur in areas that are historically prime fishing grounds.”

Indeed, while the size of dead zones is small relative to the total surface of the oceans, scientists say they account for a significant part of ocean waters that support commercial fish and shellfish species.

Seasonally, low oxygen levels wipe out fish and crustaceans from dead-zone bottom waters in places like the Gulf of Mexico, Chesapeake Bay and the Baltic Sea, leaving little life other than microbes. Among places where dead zones have grown in recent years are coastal China and the Kattegat Sea, where the Norway lobster fishery collapsed. The zones have also cropped up unexpectedly in pockets off the coast of South Carolina and the Pacific Northwest.

The dead zone in the Gulf of Mexico this summer covers a swath nearly the size of Massachusetts. That zone has more than doubled in size in the last 20 years.

“There are large areas of the gulf where you can’t catch any shrimp,” said Nancy N. Rabalais, executive director of the Louisiana Universities Marine Consortium, who has studied the dead zone there for more than two decades. “It’s sort of a losing battle.”

Scientists attribute dead zones to a process that begins when nitrogen from agricultural runoff and sewage stimulates the growth of photosynthetic plankton on the surface of coastal waters. As the organisms decay and sink to the bottom, they are decomposed by microbes that consume large amounts of oxygen. As oxygen levels drop, most animals that live at the bottom cannot survive.

“The overwhelming response of the organisms in our coastal areas is to migrate or to die,” Dr. Diaz said. “To adapt to low oxygen water, it has to be a part of your evolutionary history. It’s not something you can develop in a 40- or 50-year time period.”

Many dead zones are cyclical, recurring each year in the summer months. But over time, they can permanently kill off entire species within the zone. They have also prevented the rebounding of species that are under protection after overfishing, like the Baltic Sea’s cod.

Low oxygen levels also kill off annelid worms and other sources of food for fish and crustaceans.

Once dead zones recur, “they are very hard to reverse,” said Donald F. Boesch, president of the University of Maryland Center for Environmental Science, adding that “they have major consequences for the ability of fish populations to renew themselves.”

Dr. Boesch, who like Dr. Rabalais was not affiliated with the study, said that “the global proliferation” of dead zones, once mainly a problem of the developed world, had been fueled by industrialization, changing eating habits and population growth, which have led to more fertilizer use and more waste in the world’s watersheds.

Dead zones pose a serious threat to coastal ecosystems, said James N. Galloway, a professor of environmental science at the University of Virginia. “But the challenge is, How do you manage fertilizer use without compromising the ability of the world to feed people?” Dr. Galloway said.

Robert W. Howarth, a professor of ecology and environmental biology at Cornell, said methods to reduce nitrogen-rich runoff existed, including the planting of winter rye or winter wheat rather than leaving fields fallow after fall harvest. Such planting would cause much fertilizer to be absorbed by the winter crops rather than being leached into waterways by spring rains.

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