Scientists alarmed by ocean dead-zone growth
The phenomenon is called hypoxia
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 Thursday.
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.
"Dead zones were once rare, but now they`re commonplace, and there are more of them in more places," he said.
Diaz and Rutger Rosenberg, a marine ecologist at Sweden`s Göteborg University, have just completed a global survey of the imperiled areas, and their report appears today 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.
The causes of hypoxia
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.
The scientists found only a few small dead zones along the California coast and none in San Francisco Bay now, an improvement over previous eras when conditions made it impossible for marine life to thrive there.
That was during the 1950s through the 1970s, said James E. Cloern , a marine biologist at the U.S. Geological Survey in Menlo Park who has been monitoring the bay`s health for more than 30 years.
The problem then, Cloern said, was the result of continuous discharges of poorly treated sewage from communities surrounding the bay and wastes from many cannery plants. But the issues were resolved when waste treatment facilities were updated all around the bay, he said.
San Francisco Bay also benefits from "strong tidal action" that mixes the water and also supports active communities of clams and mussels that help keep anything like a dead zone from developing, Cloern said.
"But things can change, and there`s no guarantee that we won`t be seeing blooms of algae in the future here, too, so we need to be really vigilant," he said.
According to Diaz`s survey, the few dead spots along the California coast develop only periodically where water circulation is limited. They include the inland portion of Elkhorn Slough near Moss Landing in Monterey County and Alamitos Bay at the mouth of the San Gabriel River near Long Beach.
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."
"The conclusion is inescapable that dead zones are now a key stressor in coastal waters," she said.
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.
`Nutrient inputs again rising`
"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.
From limited surveys in the past and their own continuing research, the two scientists counted 49 dead zones around the world in the 1960s, then 87 in the 1970s, and 162 in the 1980s. The dead zone global count is 405 today, Diaz said, and could well be climbing.San Fransisco Chronicle