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Scientists alarmed by ocean dead-zone growth
San Francisco Chronicle


August 17, 2008

Dead zones where fish and most marine life can no longer survive are spreading across the continental shelves of the world's oceans at an alarming rate as oxygen vanishes from coastal waters, scientists reported this week.

The scientists place the problem on runoff of chemical fertilizers in rivers and fallout from burning fossil fuels, and they estimate there are now more than 400 dead zones along 95,000 square miles of the seas -- an area more than half the size of California.

The number of those areas has nearly doubled every decade since the 1960s, said Robert J. Diaz, a biological oceanographer at the Virginia Institute of Marine Science.

Diaz and Rutger Rosenberg, a marine ecologist at Sweden's Goteborg University, have just completed a global survey of the imperiled areas, and their report appears in the journal Science.

The phenomenon that drives life away from so many coastal habitats is called hypoxia -- the lack of enough oxygen in bottom waters for fish and other valuable marine life to thrive, the report notes.

Hypoxia is caused by tons of nitrogen and phosphorus in fertilizers that run from farms and spill into the seas from rivers and streams as well as by fallout from power plants that burn fossil fuels.

The chemicals become prime nutrients that fertilize rich blooms of microscopic algae near the surface layers of coastal waters. The algae eventually die, sink to the bottom layers of the ocean and become food for masses of bacteria that decompose and consume the oxygen around them. The result is the dead zone, devoid of most marine life forms.

The largest dead zone on Earth is in the Baltic Sea, according to the survey, and the largest in the United States lies at the mouth of the Mississippi River, where the water is "hypoxic" over an area of 8,500 square miles -- roughly the size of New Jersey.

Diaz's institute is part of the College of William and Mary in Williamsburg, Va., and he has been surveying the world's dead zones, starting with nearby Chesapeake Bay, for more than 20 years.

Jane Lubchenco, former president of the American Association for the Advancement of Science and a leading marine biologist on ocean ecology at Oregon State University, said by e-mail that the report is "a sobering documentation of the growing threat of nutrient pollution in coastal waters around the world."

But she added that the problem is solvable.

"The evidence suggests that if the spigot of nutrients can be turned off, coastal systems can recover," she said. "Doing it can be accomplished by using fertilizers more efficiently, preventing human and animal sewage from entering rivers, and replanting vegetation (along riverbanks) to absorb excess nutrients."

Diaz and Rosenberg cited the Black Sea as an example of the improvements that can be made when solutions are applied. Until the 1990s, the shallow northwest continental shelf there was a major dead zone, but then nutrients declined as fertilizer use diminished for several years.

"However, nutrient inputs are again rising (there) as agriculture expands and a return to hypoxic conditions may be imminent," the scientists wrote in their report.

About half the known dead zones develop once a year during the summer after the algae bloom widely, the water is warmest and water layers along coasts are most distinctly separated, Diaz and Rosenberg reported.


E-mail David Perlman at dperlman(at)
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