Stream ecosystems clean nitrogen from water

Small streams are able to protect large bodies of water from excessive eutrophication, according to a research project in which scientists from across the United States participated.

The research, which took place over a six week period in a dozen pristine Midwest prairie streams, used isotopes of nitrogen 15 to trace the movement of nitrogen through the streams’ biological systems. “When you add the tracer to the streams, within several hundred metres it’s all in the stream bottom; it’s all been taken up by the organisms that live in the stream,” said Walter Dodds of Kansas State University, one of the researchers on the project. According to the scientists, some of the nitrogen is converted to nitrogen gas, and the rest becomes nutrition for algae, bacteria and fungi, and enters into the food web.

“The bottom line is that streams have an impact. They can remove as much as 50% of the inorganic nitrogen,” said Dodds. “Conservation of small streams is an important step in mitigating the nitrogen that reaches rivers, lakes, bays and oceans.”

According to Bruce Peterson, of the Marine Biological Laboratory in Woods Hole, smaller streams remove nitrogen more quickly, which means that by taking greater care to insure that small streams are able to work more effectively, the nitrogen load reaching large bodies of water will be reduced. “It doesn’t mean that you can ignore your sewage treatment plants, but if we can do better with our small streams and do some restoration activities, it’s going to have some benefits,” he said.

“This demonstrates that the natural systems out there really allow for very clean water,” said Dodds. “It provides a point of comparison that shows we have a lot of water bodies in the state that are highly contaminated with nitrogen.”

“This was a most gratifying project because of the enthusiastic involvement of many of the top stream ecologists from around the United States who shared ideas and data unselfishly,” said Patrick Mulholland of Oak Ridge National Laboratory, and leader of the research. “I think it is an excellent model for how inter-site, multi-investigator research can and should be done to address some of the most fundamental environmental issues facing us today.”

A number of the researchers are now involved in a study of the denitrification process in streams, and another nitrogen tracer study is planned, this time in agricultural and urban landscapes.

The results of the study were published in the 6 April issue of Science

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