In the latest sign of trouble in the planet's chemistry, the number of oxygen-starved “dead zones” in coastal waters around the world has roughly doubled every decade since the 1960s, killing fish, crustaceans and massive amounts of marine life at the base of the food chain, according to a study released Thursday.
“These zones are popping up all over,” said Robert Diaz, a professor at the Virginia Institute of Marine Science who led of the study published online by the journal Science.
Diaz and co-author Rutger Rosenberg of the University of Gothenburg in Sweden counted more than 400 dead zones globally, ranging from massive ones in the Baltic Sea and the Gulf of Mexico to small ones that episodically appear in river estuaries.
The amount of “biomass” that is missing because of low oxygen levels in the Chesapeake Bay would be enough to feed half the commercial crab harvest for a year, Diaz estimated.
Low oxygen, known as hypoxia, is a significant measure of the downstream effect of chemical fertilizers used in agriculture. Air pollution is another factor. The nitrogen from the fertilizer or pollution feeds the growth of algae in coastal waters, particularly during summer. The algae eventually dies and sinks to the bottom, where the organic matter decays in a process that robs the bottom waters of oxygen.
Hypoxia has been seen for decades in such places as the Chesapeake Bay, Lake Erie, the Gulf of Mexico and Long Island Sound, but Diaz's survey has found new zones in the Florida Keys, Puget Sound and tidal creeks in the Carolinas.
“We're saying that hypoxia is now everywhere, it seems,” said Diaz. “We can say that human activities really screwed up oxygen conditions in our coastal areas.”
Douglas Rader, chief ocean scientist for the Environmental Defense Fund, said the chaos in the planet's nitrogen cycle is not only creating dead zones but also inciting the spread of toxic algae, such as the pfiesteria that has appeared in recent years in the Chesapeake.
“The next big challenge, after global warming, is going to be addressing the massive upset of the world's nitrogen cycle,” Rader said.
A few hypoxic ecosystems have improved in recent years due to better management of pollutants. Diaz identified the Indian River in Florida as showing signs of improvement. Dead zones in New York's Hudson River and East River have actually disappeared, the study found.
Globally, however, only 4 percent of the dead zones are recovering, the report said.