A global map of “dead zones”—where coastal waters contain too little oxygen to sustain life—shows (as black dots) a concentration in the Northern Hemisphere, where human activity has had the most effect.
As of August 2008, there were more than 400 known “dead zones,” scientists said, up from just over 300 in the 1990s. Image courtesy Science/AAAS
“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.
Related article: Are the oceans giving up?
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.
Government-supported scientists not involved with Diaz’s review are forecasting an expansion of the Gulf of Mexico dead zone to a record 8,800 square miles (23,000 square kilometers), an area larger than New Jersey.
(Related story: “Gulf of Mexico “Dead Zone” Is Size of New Jersey” [May 25, 2005])
Nancy Rabalais, executive director and professor at the Louisiana Universities Marine Consortium, said the paper “shows that there is a lot of lost production of [seafloor] animals-those living in the sediments-that could be food” for fishery stocks.
Diaz and Rosenberg note in the press release that dead zones tend to be overlooked until they start to affect organisms that people eat.
Another researcher at the Virginia Institute of Marine Science, fish pathologist Wolfgang Vogelbein, said high rates of disease [missing word?] Bay stripers are due to the stress they encounter trying to escape the Chesapeake Bay’s summertime dead zone.
Some local and regional governments have stepped in with conservation and cleanup efforts to combat dead zones.
Maryland, for instance, gives $18 million a year in grants to farmers who plant additional crops after their harvest to absorb leftover fertilizer before it ends up in the Chesapeake Bay.
Rabalais, who was not involved in the Diaz review, said she has seen little sustained effort to combat nutrient runoff in the Mississippi River.
“In the recent years of increased acreage of corn and biofuels, the amount of fertilizer used and the amount of nitrogen per volume of Mississippi River water has increased dramatically,” Rabalais said.
“What we have is this pulse of nutrients that are coming down our rivers every year,” Diaz added. “Somehow we have to find a way to stop that.
“The loss of fertilizer is an economic drain on the industry. It is not something the farming community wants to happen, and controlling it is the key to controlling the spread of dead zones.”
for National Geographic News
August 14, 2008
Source: National Geographic