Washington, August 15 : A global study has shown that the number of "dead zones"-areas of seafloor with too little oxygen for most marine life-has increased by a third between 1995 and 2007.
Professor Robert Diaz of the Virginia Institute of Marine Science, College of William and Mary led the study.
The study tallies 405 dead zones in coastal waters worldwide, affecting an area of 95,000 square miles, about the size of New Zealand.
The largest dead zone in the US, at the mouth of the Mississippi, covers more than 8,500 square miles, roughly the size of New Jersey.
According to Diaz and collaborator Rutger Rosenberg of the University of Gothenburg in Sweden, dead zones are now the key stressor on marine ecosystems and rank with over-fishing, habitat loss, and harmful algal blooms as global environmental problems.
Diaz began studying dead zones in the mid-1980s after seeing their effect on bottom life in a tributary of Chesapeake Bay near Baltimore.
His first review of dead zones in 1995 counted 305 worldwide. That was up from his count of 162 in the 1980s, 87 in the 1970s, and 49 in the 1960s.
He first found scientific reports of dead zones in the 1910s, when there were 4. Worldwide, the number of dead zones has approximately doubled each decade since the 1960s.
"There is no other variable of such ecological importance to coastal marine ecosystems that has changed so drastically over such a short time as dissolved oxygen," said Diaz and Rosenberg.
Dead zones occur when excess nutrients, primarily nitrogen and phosphorus, enter coastal waters and help fertilize blooms of algae.
When these microscopic plants die and sink to the bottom, they provide a rich food source for bacteria, which in the act of decomposition consume dissolved oxygen from surrounding waters.
Major nutrient sources include fertilizers and the burning of fossil fuels.
"Geologic evidence shows that dead zones were not a naturally recurring event in Chesapeake Bay or most other estuarine ecosystems," said Diaz. "Dead zones were once rare. Now they're commonplace. There are more of them in more places," he added.
Scientists refer to water with too little oxygen for fish and other active organisms as "hypoxic."
Diaz and VIMS colleague Linda Schaffner estimate that Chesapeake Bay now loses about 10,000 metric tons of carbon to hypoxia each year, 5% of the Bay's total production of food energy.
The Baltic Sea has lost 30% of its food energy-a condition that has contributed to a significant decline in its fisheries yields.
According to Diaz and Rosenberg, the key to reducing dead zones is to keep fertilizers on the land and out of the sea.