Coastal oceans are being starved of oxygen at an alarming rate, researchers are reporting, with vast stretches of water along the seafloor depleted of oxygen to the point that they can barely sustain marine life.
The main culprit, scientists say, is nitrogen-rich nutrients from crop fertilizers that spill into coastal waters by way of rivers and streams.
In a study to be published Friday in the journal Science, researchers say the number of marine "dead zones" around the world has doubled about every 10 years since the 1960s. At the same time, the zones along many coastlines have been growing in size and intensity. About 400 coastal areas now have periodically or permanently oxygen-starved bottom waters. Combined, they constitute an area larger than the state of Oregon.
"What's happened in the last 40, 50 years is that human activity has made the water quality conditions worse," Robert Diaz, the study's lead author and a professor at the Virginia Institute of Marine Science at the College of William and Mary, said in an interview. "Dead zones tend to occur in areas that are historically prime fishing grounds."
While the size of dead zones is small relative to the total surface of the earth covered by oceans, scientists say they represent a significant portion of the ocean waters that support commercial fish and shellfish species.
Low oxygen levels wipe out fish and crustaceans from bottom waters in places like the Gulf of Mexico, the Chesapeake Bay and the Baltic Sea on a seasonal basis, leaving little life other than microbes to survive. In recent years, dead zones have grown in places like coastal China and the Kattegat Sea, where the Norway lobster fishery collapsed. They have also cropped up unexpectedly in pockets off the coast of South Carolina and the Pacific Northwest.
This summer, the dead zone in the Gulf of Mexico covers a swath of ocean nearly the size of Massachusetts. It has more than doubled in size in the last 20 years.
"There are large areas of the Gulf where you can't catch any shrimp," said Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium. Rabalais has studied the dead zone there for more than two decades. "It's sort of a losing battle."
Nitrogen from agricultural runoff and sewage stimulates the growth of photosynthetic plankton on the surface of coastal waters. As the organisms decay and sink to the bottom, they are decomposed by microbes that consume large amounts of dissolved oxygen. Most animals that live at the bottom of the coastal ocean cannot survive as oxygen levels drop.
"The overwhelming response of the organisms in our coastal areas is to migrate or to die," Diaz said. "To adapt to low oxygen water, it has to be a part of your evolutionary history — it's not something you can develop in a 40- or 50-year time period."
Many dead zones are cyclical; they recur each year in the warm summer months. But over time, they can persist and permanently kill off entire species. They have also prevented the rebound of species under protection after overfishing, like in the Baltic Sea. Cod eggs rely on a delicate balance of oxygen content, salinity and water density and temperature to survive.
Low oxygen levels also kill off annelids and other sources of food for fish and crustaceans. In the Chesapeake Bay, Diaz estimates that dead zones prevent the production of 75,000 metric tons of fish food, enough to feed half of annual harvest of crabs from the bay for a year. The marine life lost each year to the dead zone in the Gulf of Mexico, he said, would supply about 75 percent of the brown shrimp catch with food for a year. In the Baltic Sea, dead zones lead to the loss of about 1.3 million metric tons of fish food each year.
"Once they recur, they are very hard to reverse," said Donald Boesch, president of the University of Maryland Center for Environmental Science "They have major consequences for the ability of fish populations to renew themselves."
Boesch, who was not affiliated with the study, said that "the global proliferation" of dead zones, once mainly a problem of the developed world, had been fueled by industrialization, changing eating habits and population growth, which has led to more fertilizer use and more waste in the world's watersheds.
Dead zones pose a serious threat to coastal ecosystems, said James Galloway, a professor of environmental science at the University of Virginia. "But the challenge is, how do you manage fertilizer use without compromising the ability of the world to feed people?"
Robert Howarth, a professor of ecology and environmental biology at Cornell, said that methods to reduce nitrogen-rich runoff exist, including planting winter rye or winter wheat in cornfields during the off-season so the spring rains do not cause the chemicals to leach into waterways.
Nevertheless, most experts agree that the changes needed to reverse the trend are dramatic. For example, scientists estimate that cutting the Gulf of Mexico's dead zone by a third would require a 45 percent decrease in nitrogen-rich runoff from the Mississippi River watershed, which extends into the croplands of the upper midwest.
"These are high expectations," said Alan Lewitus of the National Oceanic and Atmospheric Administration, which, in part, financed the study. "But it's one of the major problems we face in terms of the health of coastal ecosystems."
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