Big rivers typically get the credit for being powerful and mighty, but a sweeping national study shows that when it comes to pollution control, even little streams can pack a punch.
Stephen Hamilton, an aquatic ecologist at Michigan State University, studied nine streams that flowed through cities, forests and agricultural land in the Kalamazoo River watershed of southwestern Michigan as part of a nationwide team seeking to understand what happens to the nitrogen that is washed into the water.
The results, published in Nature, provide the most comprehensive understanding yet of how the complex network of rivers and streams -- mighty and small -- naturally process nitrogen from the waters before it ends up causing trouble downstream.
"This study presents a picture of unprecedented detail of the extent to which streams can remove nitrate," Hamilton said. "We also now have a better idea of what makes one stream more efficient at nitrate removal than another."
The stakes are high. Nitrogen gets into the water as runoff from fertilizers and wastes from human activities. Too much nitrogen 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.
Rivers and streams naturally can act as the "kidneys of our landscape," according to lead author 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.
Hamilton and his team from MSU and the University of Notre Dame spent three years conducting experiments in which they added small amounts of a harmless, nonradioactive isotope of nitrogen, N-15, into streams. They then were able to track the isotope as it traveled downstream and record what processes removed it from the water.
What they found, which was supported by experiments across 72 streams in eight regions across the United States and Puerto Rico, was that the nitrate was taken up from stream water by tiny organisms such as algae, fungi and bacteria. In addition, a considerable fraction was permanently removed from streams by a bacterial process known as denitrification, which converts nitrate to nitrogen gas that then 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."
In addition, the effectiveness of streams to remove nitrate was greatest if the streams were not overloaded by nitrogen sources such as fertilizers and wastes from human activities. If overloaded, a stream or river passes nitrogen downstream, where it can cause problems in oceans and coastal waterways.
This appears to put two imperatives at odds -- removing water quickly from urban areas or agricultural fields versus trying to reduce pollutants. But Hamilton said there are ways to satisfy both goals, such as directing waters into wetland ponds or buffer strips that allow nature time to gobble the nitrates.
The study, Hamilton said, now presents a comprehensive picture that can help guide stream and river management and land-use planning.
The study was funded by grants from the National Science Foundation and is a contribution to the Kellogg Biological Station's Long-Term Ecological Research program.
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