Coastal waters worldwide contain more than 400 dead zones that, combined, make up an area the size of New Zealand, according to a study published online Thursday in the journal Science. Robert Diaz, a marine biologist at the College of William and Mary Virginia Institute of Marine Science and lead author of the study, said fertilizer runoff from farms is a major source of the problem.
Fertilizer, animal waste and car exhaust leach into storm-water runoff and spill excess nutrients, primarily nitrogen and phosphorous, into coastal seawaters. Algae and phytoplankton on the seafloor feed off the nutrients, which cause them to bloom wildly. As bacteria consume the blooms, they suck oxygen from the water, depleting whole stretches of ocean. Scientists call this oxygen depletion hypoxia.
“All of the systems are reacting in concert to a general over-fertilization of the rivers and bays in the coastal area,” Diaz said. “Most of this in the marine environment is due to excess nitrogen.”
Diaz’s research shows that the number of dead zones have roughly doubled each year since the 1960s. Dead zones alter the habitat for crab, shrimp, fish and lobster, often forcing them to shallow areas.
“They’re bumping shells with each other, eating each other, and having bad interactions, because it’s so crowded,” Diaz said. “There’s not enough food to eat, because they’re limited by hypoxia.”
The largest dead zone in the U.S. exists in the Gulf of Mexico and spans 8,500 square miles — that’s roughly the size of New Jersey. The Chesapeake Bay, which has the third largest dead zone in the country, would contain enough food to serve half the commercial crab harvest for the year, were it not hypoxic, according to Diaz.
The sources of nitrogen vary by region. Much of the nitrogen pollution in the northeastern United States comes from car exhaust or pollution from power plants. In the South, the source is usually agriculture — fertilizer from farms or animal waste from feed lots.
But solutions exist to curb fertilizer runoff, said Robert Howarth, a professor at Cornell University who studies nitrogen sources and dead zones.
“You can change the crops you’re growing,” Howarth said. “Corn is maybe the biggest culprit. It uses a lot of fertilizer and it’s not that efficient at using that fertilizer. If you switch to perennial grasses, that helps tremendously.”
Farmers can cut the loss of nitrogen threefold by planting a cover crop during the winter season such as winter wheat or rye. Roots from the plants hold in the nitrogen and prevent it from running off the farm in streams during the spring thaw.
Cutting back on car exhaust by using less gas and switching to hybrid cars could also alleviate some of the stress on the ocean.
“I’d mostly like to project that it’s not a hopeless problem,” Horwath said. “If society wants to pay attention, it can be fixed.”