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Study Shows Streams Unable to Filter Increasing Amounts of Nitrogen

Streams and rivers are effective at removing nitrates at low concentrations but are unable to process increasing amounts of anthropogenic nitrogen, according to a recent paper in Nature by Bruce Peterson and Suzanne Thomas of the Ecosystems Center and other authors. The result is that estuaries, coastal oceans and inland basins are receiving nitrate pollution that causes algal blooms and fish kills.

Peterson, a senior scientist, and Thomas, a research assistant, are among 31 scientists in the study, part of the Lotic Intersite Nitrogen Experiment that was based on 72 streams in eight regions in the U.S. and Puerto Rico.

The approach utilized in this study, using the staple isotope 15N (nitrogen 15) to measure what happens to nitrogen that enters streams and rivers, was developed by Peterson in the mid-1990s in his Arctic streams research. With Wil Wollheim, former Ecosystems Center research assistant, and others, Peterson applied a method commonly used in terrestrial studies to streams. The N addition method has now been extended to streams throughout the world.

“Streams 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 Patrick Mulholland, lead author of the Nature report, which was published March 13.

Building on an earlier study by Peterson and colleagues (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 streams, including those 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 Peterson. In denitrification, bacteria help convert nitrate in the water to nitrogen gas that then escapes 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 river 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 15N tracer method 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.

Along with Mulholland, Peterson and Thomas, other collaborators on this study included scientists from the University of Georgia, Athens; Eco-Metrics; University of Wyoming, Laramie; Michigan State University; University of Notre Dame; Oregon State University; University of New Mexico; Kansas State University; Institute of Ecosystem Studies; Arizona State University; U.S. Forest Service; University of New Hampshire; Virginia Tech; and Ball State University.

Funding for the study was provided by a grant from the National Science Foundation to the University of Tennessee.


Citation: Mulholland, Patrick J. et al. Stream denitrification across biomes and its response to anthropogenic nitrate loading. Nature, 13 March 2008, Vol. 452, Issue 7184, 202-205.