Fertilizer Runoff Overwhelms Streams and Rivers--Creating Vast "Dead Zones"

The water in brooks, streams and creeks from Michigan to Puerto Rico carries a heavy load of pollutants, particularly nitrates from fertilizers. These nitrogen and oxygen molecules that crops need to grow eventually make their way into rivers, lakes and oceans, fertilizing blooms of algae that deplete oxygen and leave vast "dead zones" in their wake. There, no fish or typical sea life can survive. And scientists warn that a federal mandate to produce more biofuel may make the situation even worse.

Researchers led by aquatic ecologist Patrick Mulholland of the Oak Ridge National Laboratory in Tennessee report in Nature that streams and other waterways are losing their ability to filter excess nitrates from fertilizers and sewage. They discovered this by releasing a concentrated nitrate solution carrying an unusual isotope of nitrogen into 72 different streams—ranging from heavily altered urban waterways to pristine rivulets—and then tracked the isotope to find out how much made it downstream. The amount at the end indicated each stream's ability to naturally remove the pollutant—a measure of its health.

"We found that they continue to take up nitrate, but they remove a smaller fraction of the overall nitrate as you overload them," Mulholland says. "This is probably the reason we're seeing hypoxia [low oxygen levels] and other problems in coastal waters."

Typically, bacteria remove excess fertilizer from water through a chemical process known as denitrification, which enables them to convert nitrate to nitrogen that is then released into the atmosphere as a gas. The team found, however, that bacteria in the streams they studied only eliminated an average of 16 percent of the nitrogen pollution; bacteria in the most undisturbed streams performed the best, removing as much as 43 percent.

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