Study: Ocean Dead Zones Increased by a Third Over 12 Years
A global study led by Professor Robert Diaz of the Virginia Institute of Marine Science, College of William and Mary, shows 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.
Diaz and collaborator Rutger Rosenberg of the University of Gothenburg in Sweden say that 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."
The study, which appears in the Aug. 15 issue of the journal Science, 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 United States, at the mouth of the Mississippi River, covers more than 8,500 square miles, roughly the size of New Jersey.
Diaz and Rosenberg write "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."
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, says Diaz. "Dead zones were once rare. Now they're commonplace. There are more of them in more places." The first dead zone in Chesapeake Bay was reported in the 1930s.
Earth's largest dead zone, in the Baltic Sea, experiences hypoxia year-round. Chesapeake Bay experiences seasonal, summertime hypoxia through much of its main channel, occupying about 40 percent of its area and up to 5 percent of its volume.
Diaz and Rosenberg note that hypoxia tends to be overlooked until it starts to affect organisms that people eat. A possible indicator of hypoxia's adverse effects on an economically important finfish species in Chesapeake Bay is the purported link between oxygen-poor bottom waters and a chronic outbreak of a bacterial disease among striped bass.
The researchers say the key to reducing dead zones is "to keep fertilizers on the land and out of the sea." Diaz says that goal is shared by farmers concerned with the high cost of buying and applying nitrogen to their crops. "They certainly don't want to see their dollars flowing off their fields into the Bay," says Diaz. "Scientists and farmers need to continue working together to develop farming methods that minimize the transfer of nutrients from land to sea."