Ocean "dead zones" spreading

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 where they can barely sustain marine life, researchers are reporting.

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

A study to be published today 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 have periodically or perpetually oxygen-starved bottom waters, many of them growing in size and intensity. Combined, the zones, one of which is in a bay off a Skagit County island, are larger than Oregon.

"What's happened in the last 40, 50 years is that human activity has made the water-quality conditions worse," said the study's leader author, Robert Diaz.

The trend portends nothing good for many fisheries, said 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."

Low oxygen, or hypoxia, is a significant measure of the downstream effect of chemical fertilizers used in agriculture. Air pollution is another factor.

Hypoxia has been seen for decades in such places as the Chesapeake Bay, Lake Erie, the Gulf of Mexico and Long Island Sound, but Diaz's survey has found new zones in Washington state's Samish Bay, Oregon's Yaquina Bay, prawn culture ponds in Taiwan, the San Martin River in northern Spain and some fjords in Norway, Diaz said.

A dead zone has been newly reported off the mouth of the Yangtze River in China, Diaz said, but the area probably has been hypoxic since the 1950s. "We just didn't know about it," he said.

"We're saying that hypoxia is now everywhere, it seems," Diaz said. "We can say that human activities really screwed up oxygen conditions in our coastal areas."

Douglas Rader, chief ocean scientist for the Environmental Defense Fund, said the chaos in the planet's nitrogen cycle is not only creating dead zones but also inciting the spread of toxic algae, such as the pfiesteria that has appeared in recent years in the Chesapeake.

"The next big challenge, after global warming, is going to be addressing the massive upset of the world's nitrogen cycle," Rader said.

While the size of dead zones is small relative to the total surface of the oceans, scientists said 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 such as the Gulf of Mexico, Chesapeake Bay and the Baltic Sea, leaving little life other than microbes to survive.

Among places where dead zones have grown in recent years are coastal China and the Kattegat Sea, where the Norway lobster fishery collapsed.

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 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 much of the creation of 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 at the bottom cannot survive.

"The overwhelming response of the organisms in our coastal areas is to migrate or to die," 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 summer. But over time, they can permanently kill off entire species within the zone. They also have prevented the rebounding of species under protection after overfishing, such as the Baltic Sea's cod. Low oxygen levels also kill food sources for fish and crustaceans.

Once dead zones recur, "they are very hard to reverse," said Donald Boesch, president of the University of Maryland Center for Environmental Science.

A few hypoxic ecosystems have improved in recent years due to better management of pollutants. Diaz identified the Indian River in Florida as showing signs of improvement. Globally, however, only 4 percent of the dead zones are recovering, the report said.

The Washington Post, The Associated Press and Seattle Times staff contributed to this report.