Increasing nitrogen pollution overwhelms filtering capability of streams

MBL, WOODS HOLE, MA—Increasing nitrogen runoff from urban and agriculture land-use is interfering with our streams’ and rivers’ natural processes for reducing this pollutant before it endangers delicate downstream ecosystems, reports a nationwide team of 31 ecologists, including two from the MBL (Marine Biological Laboratory) Ecosystems Center.

The findings, published in the March 13 issue of Nature, are based on a major study of 72 streams in 8 regions across the U.S. and Puerto Rico. “It was a collaborative effort by many leading aquatic ecologists working to solve a complex problem regarding the role of streams in reducing pollution,” says lead author Patrick Mulholland of the Oak Ridge National Laboratory and University of Tennessee.

Just how important are streams” “They are effective filters that can help prevent nitrate pollution from reaching lakes and coastal oceans, where it can cause noxious algal blooms and lead to oxygen depletion and death of fish and shellfish, as has been recently reported in the Gulf of Mexico,” says Mulholland.

Building on an earlier study (Science, April 6, 2001) that demonstrated that even the smallest streams can filter up to half of the inorganic nitrogen that enters them, the scientists launched the new study to learn how increased nitrogen pollution is affecting this process. They analyzed data collected from a variety of waterways, including streams in urban and agricultural settings, where land-use dominates the landscape and degrades water quality.

“Our findings demonstrate that streams containing excess nitrogen are less able to provide the natural nitrogen removal service known as denitrification,” says Bruce Peterson, a senior scientist at the MBL Ecosystems Center and one of the study’s authors. In denitrification, bacteria help convert nitrate in the water to nitrogen gases that then escape to the atmosphere.

“The new research demonstrates that although denitrification rates increase as nitrate concentrations increase, the efficiency of denitrification and nitrate assimilation decline as nitrogen loading increases,” adds Peterson. “This means humans can easily overload stream and rivers networks to the point that nitrate removal is not sufficient to prevent eutrophication downstream, the scenario where algae grow out of the control and oxygen may fall to unhealthy levels.”

To gauge the effects of high levels of nitrogen runoff on waterways, the scientists used the stable isotope 15N (nitrogen 15) to track nitrogen movement through each study stream. They also developed ecological models to study nitrate removal from water within river networks, which develop as small streams flow into larger streams and rivers. The models showed that the entire stream network is important in removing nitrogen from stream water.

The ecologists say these and other findings in the Nature study underscore the importance of controlling human-generated nitrogen runoff, and provide critical information to land-use managers contemplating large-scale land conversions for projects including corn farming for biofuels production.