Around 1910, when scientists began studying the marine areas of low oxygen known as dead zones, there were only four of them worldwide. Now, there are 405 dead zones in the world's coastal waters, covering a total area of 95,000 square miles, according to the latest research published today in the journal "Science." A global study led by Virginia Institute of Marine Science Professor Robert Diaz shows that the number of dead zones has increased by a third between 1995 and 2007.
"Dead zones were once rare. Now they're commonplace. There are more of them in more places," Diaz says. Worldwide, the number of dead zones has roughly doubled each decade since the 1960s, his research shows.
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."
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 agricultural fertilizers and the burning of fossil fuels. The largest dead zone in the United States today, at the mouth of the Mississippi River, covers more than 8,500 square miles, an area roughly the size of New Jersey.
A dead zone also underlies much of the main channel of Chesapeake Bay, each summer occupying about 40 percent of its area and up to five percent of its volume. Geologic evidence shows that dead zones were not "a naturally recurring event" in Chesapeake Bay or most other estuarine ecosystems, says Diaz. The first dead zone in Chesapeake Bay was reported in the 1930s. Scientists refer to water with too little oxygen for fish and other active organisms as "hypoxic." Diaz says that many ecosystems experience a progression in which periodic hypoxic events become seasonal and then, if nutrient inputs continue to increase, persistent.
Earth's largest dead zone, in the Baltic Sea, is hypoxic year-round. Chesapeake Bay experiences seasonal, summertime hypoxia through much of its main channel. 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 link between oxygen-poor bottom waters and a chronic outbreak of a bacterial disease among striped bass.
Several Chesapeake Bay researchers, including VIMS fish pathologist Wolfgang Vogelbein, believe that the high prevalence of mycobacteriosis, found in more than 75 percent of the Bay stripers, occurs because they are weakened by the stress of encountering the Bay's summertime dead zone. When the dead zone forms, it forces the stripers from the cooler bottom waters they prefer into warmer waters near the surface. Diaz and Rosenberg say an even more fundamental effect of hypoxia is the loss of energy from the Bay's food chain.
Without bottom-dwellers such as clams and worms, their predators lose an important source of nutrition. Diaz and Rosenberg say the key to reducing dead zones is "to keep fertilizers on the land and out of the sea." Farmers concerned with the high cost of buying and applying nitrogen to their crops share that goal. "Scientists and farmers need to continue working together to develop farming methods that minimize the transfer of nutrients from land to sea."says Diaz.