for National Geographic News
"Dead zones" are on the rise, says a new study that identified stark growth in the number of coastal areas where the water has too little oxygen to sustain marine life.
There are now more than 400 known dead zones in coastal waters worldwide, compared to 305 in the 1990s, according to study author Robert Diaz of the Virginia Institute of Marine Science.
Those numbers are up from 162 in the 1980s, 87 in the 1970s, and 49 in the 1960s, Diaz said. In the 1910s, four dead zones had been identified.
Diaz and co-author Rutger Rosenberg, of the University of Gothenburg in Sweden, said in a press release that dead zones are now "the key stressor on marine ecosystems" and "rank with overfishing, habitat loss, and harmful algal blooms as global environmental problems."
Their study appears in the August 15 issue of the journal Science.
Dead zones occur when excess nutrients—usually nitrogen and phosphorus—from agriculture or the burning of fossil fuels seep into the water system and fertilize blooms of algae along the coast.
As the microscopic plants die and sink to the ocean floor, they feed bacteria, which consume dissolved oxygen from surrounding waters. This limits oxygen availability for bottom-dwelling organisms and the fish that eat them.
(Related story: "Ocean Dead Zones Growing; May Be Linked to Warming" [May 1, 2008])
Many marine ecosystems experience low oxygen levels between spring and fall, Diaz said. But the lack of oxygen becomes persistent if nutrient levels stay high.
Earth's largest dead zone, in the Baltic Sea, experiences oxygen deprivation year-round, the press release said.
The second largest dead zone surrounds the mouth of the Mississippi River in the Gulf of Mexico. Despite decades of efforts to clean up U.S. rivers and lakes, high nitrogen levels are currently combining with strong water flow to make that dead zone larger than it has ever been.
SOURCES AND RELATED WEB SITES