Streams are natural filters

Streams are natural filters that help remove and transform pollutants that drain from surrounding watersheds, including excess nitrogen from human activities. Scientists know this as a result of many hours of getting their hands - if not dirty, at least very wet - monitoring streams nationwide.

'Nitrogen removal in streams is important because it reduces the potential for eutrophication - the excessive growth of algae and aquatic plants in downstream lakes and coastal marine waters,' said Jack Webster, professor of biology at Virginia Tech. 'Eutrophication in the Chesapeake Bay has damaged the oyster industry in Virginia and in the Gulf of Mexico, the Mississippi River has created a vast zone of oxygen depletion with adverse effects on fisheries.'

Webster, two of his Virginia Tech colleagues, and four former Virginia Tech students are among 31 authors of an article in the 13 March issue of Nature that reports the researchers' findings on how stream systems are able to remove nitrogen.

The study, lead by Oak Ridge National Laboratory (ORNL), looked at 72 streams in the U.S. and Puerto Rico over the course of three years. Virginia Tech's Stream Team conducted measurements on nine streams in North Carolina - including forest streams in the southern Appalachian Mountains, agricultural streams where they had to protect equipment from curious cows, and urban streams, including one that ran through a golf course and another that ran through a construction site. Eight other teams worked at the other 63 streams.

The research process meant 24-hour monitoring. 'The Stream Team involvement was very important,' said Webster.

In the first phase of the study, the scientists added small amounts of a non-radioactive isotope of nitrogen to streams as nitrate, the most prevalent form of nitrogen pollution. They then measured how far downstream the nitrate traveled and how what processes removed it from the water.

The scientists found that the nitrate was taken up from stream water by algae and microorganisms. In addition, a fraction was permanently removed from streams by denitrification, a bacterial process that converts nitrate to nitrogen gas, which harmlessly joins an atmosphere already predominantly composed of nitrogen gas.

In the second phase of the study, the scientists developed a model that predicts nitrate removal as water flows through small streams and into larger streams and rivers. 'Our model showed that the entire stream network is important in removing pollution from stream water,' said Patrick Mulholland, lead author of the study, a member of ORNL's Environmental Sciences Division, and a faculty member at the University of Tennessee. 'In addition, the effectiveness of streams to remove nitrate was greatest if the streams were not overloaded by pollutants such as fertilizers and wastes from human activities.'

The largest removal occurred when nitrate entered small healthy streams and traveled throughout the network before reaching large rivers. The scientists concluded from their research that streams and rivers are effective filters that help reduce the amount of nitrate pollution exported from landscapes and thereby reduce eutrophication problems, Webster said.

Authors of the article, 'Stream denitrification across biomes and its response to anthropogenic nitrate loading,' are Mulholland; Ashley M. Helton and Geoffrey C. Poole of the University of Georgia (UGA); Robert O. Hall Jr. of the University of Wyoming; Stephen K. Hamilton of Michigan State University; Bruce J. Peterson of Marine Biological Laboratory at Woods Hole; Jennifer L. Tank, a Virginia Tech Ph.D. graduate now at the University of Notre Dame; Linda R. Ashkenas of Oregon State University; Lee W. Cooper of the University of Tennessee; Clifford N. Dahm of the Univesity of New Mexico; Walter K. Dodds of Kansas State University, Stuart E. G. Findlay of the Institute of Ecosystem Studies, Millbrook, NY; Stanley V. Gregory of Oregon State; Nancy B. Grimm of Arizona State University; Sherri L. Johnson of the U.S. Forest Service, Corvallis, Ore.; William H. McDowell of the University of New Hampshire; Judy L. Meyer of UGA; H.Maurice Valett, associate professor of biological sciences at Virginia Tech; Webster; Clay P. Arango and Jake J. Beaulieu of Notre Dame; Melody J. Bernot of Ball State University; Amy J. Burgin of Michigan State; Chelsea L. Crenshaw, a Virginia Tech master's of science graduate now at the University of New Mexico; Laura Taylor Johnson, who was a Virginia Tech undergraduate and is now at Notre Dame, B. R. (Bobbie) Niederlehner, laboratory specialist at Virginia Tech; Jonathan M. O'Brien of Michigan State; Jody D. Potter of the University of New Hampshire; Richard W. Sheibley of Arizona State; Daniel J. Sobota, who was a Virginia Tech undergraduate now at Oregon State; and Suzanne M. Thomas of Woods Hole.

There were many more people involved than even the list of co-authors reflects, Webster said. 'This project was part of collaboration among a group of people who have worked together since 1995,' he said. 'The undergraduate research with the Stream Team at Virginia Tech has been important in guiding students toward graduate school and careers.' Learn more about the Virginia Tech Stream Team at www.biol.vt.edu/research/streamteam/.

The National Science Foundation funded the research. The authors also thanked the U.S. Forest Service, National Park Service, and many private landowners for permission to conduct experiments on their lands.