Streams Reduce Nitrogen Pollution, Scientists Find

Hillary Mayell
for National Geographic News
May 4, 2001

Dammed, drained, straightened, trampled on by cattle, small streams and creeks—the "gutters of the Earth"—don't get the respect they deserve. That may be about to change.

Nitrogen runoff into waterways has become an escalating environmental problem over the last 50 years, but a recent study found that small streams are extremely efficient at removing excess nitrogen—as much as half of the nitrogen that enters them.

"Wetlands, riparian areas, and small streams act as natural nitrogen traps," said Jack Webster, a biologist at Virginia Polytechnic Institute and co-author of a report on the study published April 6 in the journal Science. "Yet they're threatened everywhere by a whole host of disturbances—urbanization, suburbanization, building on floodplains, conversion to agricultural uses, and the effects of logging, to name a few."

Co-author Bruce Peterson, a senior scientist at the Marine Biological Laboratory in Woods Hole, Massachusetts, said the findings could have important implications for conservation strategies, which have generally sought to reduce levels of nitrogen by limiting the flow of it into streams.

"Land-use policies up until now have focused on terrestrial solutions for inhibiting nitrogen pollution," said Webster. "What this study shows is that we can reduce the amount of nitrogen that is being carried downstream by maintaining streams and wetland areas."

Nitrogen Pollution

Humans have more than doubled the amount of nitrogen naturally available to Earth's ecosystems over the past century, a panel of scientists reported in 1997.

The major sources of increased nitrogen include industrial processes that produce nitrogen fertilizers, the burning of fossil fuels in automobiles and power plants, and increased cultivation of soybeans, peas, rice, and other crops that naturally convert nitrogen gas to fixed nitrogen.

Human activities—burning forests and grasslands, and draining wetlands—also free nitrogen from long-term storage in soil and tree trunks.

"All of this excess nitrogen is being washed into waterways, carried downstream, and deposited in estuarine and marine systems, where it contributes to eutrophication," said Webster.

Just as nitrogen acts as an excellent fertilizer for agricultural crops, it also acts as a fertilizer in aquatic ecosystems. When too much nitrogen is washed into a waterway, it promotes an explosion of plant and algae growth, knocking the system out of balance. The plants and algae deplete the available oxygen supply.

Continued on Next Page >>


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