Big rivers typically get the credit for being powerful and mighty, but a sweeping national study released today shows that even little streams can pack a punch when it comes to pollution control.
A nationwide team seeking to understand what happens to the nitrogen that is washed into the water as runoff from fertilizers and wastes from human activities has provided the most comprehensive understanding yet of how complex networks of rivers and streams—mighty and meager—naturally process nitrogen from the waters before it ends up causing trouble downstream. The scientists say the study presents a highly detailed picture that can help guide stream and river management and land-use planning.
Michigan State University water ecologist Stephen Hamilton and his team studied nine of the streams that flowed through cities, forests and agricultural land in the Kalamazoo River watershed of southwestern Michigan.
"We now have a better idea of what makes one stream more efficient at nitrate removal than another," Hamilton said.
"This study presents a picture of unprecedented detail of the extent to which streams can remove nitrate," Hamilton said. Too much nitrogen can cause noxious algal blooms and lead to oxygen depletion and death of fish and shellfish such as that reported last month in the Florida Keys. Rivers and streams act as natural filtering organs, or the "kidneys of our landscape," according to study leader Patrick Mulholland of the Oak Ridge National Laboratory and University of Tennessee. They can significantly improve the quality of water, thereby reducing the potential for problems in downstream environments.
The researchers spent 3 years conducting experiments in which they added small amounts of a harmless, traceable form of nitrogen into streams. They then tracked the nitrogen as it traveled downstream and recorded what processes removed it from the water.
They found that the nitrate was taken up from stream water by tiny organisms such as algae, fungi and bacteria, a result that was in line with those from a network of experiments across 72 streams in eight regions across the United States and Puerto Rico. Also, a considerable amount of the nitrate was permanently removed from streams by a bacterial process that converts nitrate to nitrogen gas that escapes harmlessly into the atmosphere.
Hamilton said they also learned that not all streams are created equal. Streams that are allowed to meander naturally through a complex channel were more efficient at filtering pollutants than streams that had been engineered to quickly convey water away from farmland or developments.
"What we often do to streams to make them more like drains diminishes their ability to reduce pollutants," Hamilton said. "Complexity—both biological and physical—helps streams be more effective at removing nitrogen."
Streams were most effective at removing nitrate when they were not overloaded with nitrogen from sources like fertilizers and wastes from human activities.
That appears to put two imperatives at odds—removing water quickly from urban areas or agricultural fields to avoid flooding versus trying to reduce pollutants. Directing waters into wetland ponds or buffer strips that allow nature time to gobble the nitrates can satisfy both goals, Hamilton said.
Results of the study are published in this week’s issue of the scientific journal Nature.
The study was funded by grants from the National Science Foundation and the Michigan Agricultural Experiment Station.
—By Sue Nichols/SU
This report is provided by the National Science Foundation, an independent federal agency that supports fundamental research and education across all fields of science and engineering, in partnership with U.S. News and World Report.