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.

The worst-case scenario is the creation of so-called dead zones, where nothing can live. The Gulf of Mexico, Chesapeake Bay, and Black Sea all have dead zones, with devastating consequences to marine life and the livelihoods of those who depend on the fisheries.

Nitrogen pollution has also been implicated in manatee deaths in Florida, noxious algal blooms, and the loss of marine habitat from overgrowth of algae, especially in coral reefs and seagrass beds. These near-coast habitats serve as nurseries for commercial and non-commercial fisheries, leading to long-term declines in coastal marine fisheries.

New Insight

Until now, scientists had not fully understood the processes of nitrogen uptake and release in streams. Newly available technology enabled the researchers in this study to track nitrogen through a stream's biological systems and find out what happens to it. The two-year study involved more than 100 researchers collecting data on 12 streams from Puerto Rico to Alaska. Small streams handle nitrogen in two ways, Webster explained. First, the algae, bacteria, and fungi in a stream use nitrogen as a nutrient. They grow quickly and become food for aquatic insects and fish.

"Then you have the dead gunk that falls to the bottom," said Webster. "That is converted by denitrifying bacteria back to a gas, which is vented back into the atmosphere."

The researchers found that the faster flowing water of rivers and channelized streams carries those sediments rather than letting them fall to the bottom.

According to the results, the smaller a stream is, the more efficient it is at "trapping" nitrogen; the nitrogen is removed more quickly and travels shorter distances downstream.

"What it means," said Webster, "is that you have to take care of the streams on the landscape."

© 1996-2008 National Geographic Society. All rights reserved.