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Small streams mitigate human influence on coastal ecosystems

March 13, 2008

Healthy streams play a major role in minimizing the amount of human-generated pollutants, such as nitrogen, that are delivered downstream. This ecosystem service is valuable; excess nitrogen degrades lakes and coastal oceans by stimulating algal blooms and depleting the water of oxygen. This phenomenon, also known as eutrophication, threatens fisheries around the world.

This week, a team of scientists that included Cary Institute ecologists Dr. Stuart E.G. Findlay and Dr. Amy Burgin published a Nature paper cautioning against policies and practices that result in intensive nitrogen loading to streams. Thirty-one researchers participated in the effort, which was headed by Dr. Patrick Mulholland of the Oak Ridge National Laboratory. Their recommendation is based on findings that stream networks are effective nitrogen filters, but they cannot keep pace when they are directly overloaded by nitrogen from fertilizer or human waste.

Using a non-radioactive nitrogen isotope (N-15), the research team traced the fate of nitrate additions in 72 streams across multiple land use conditions (e.g., urban, agricultural, natural conditions) in the continental U.S. and Puerto Rico. The tracer let scientists measure how far the nitrate traveled and what processes removed it from the water. At low-to-moderate addition levels, streams were effective at minimizing nitrate export downstream; under high-loading rates this efficiency collapsed.

Within the streams, nitrate was removed by two pathways. It was taken from the stream water by tiny aquatic organisms, such as algae, fungi, and bacteria. Or it was permanently removed by a bacterial process called denitrification. This occurs when microbes in the stream bed convert nitrate to nitrogen gas (N2), allowing it to return in an inert form to the atmosphere.

The researchers also developed a model that predicts nitrate removal as water flows from small streams to larger rivers, and ultimately the ocean. The model showed that removal was most effective when nitrate entered small healthy streams and traveled through a stream network before reaching larger water bodies. Participants concluded that, by minimizing the amount of nitrogen pollution exported downstream, streams play a key role in reducing eutrophication in lakes and coastal waters.

By protecting and restoring stream ecosystems, we can maintain or enhance their nitrogen removal capabilities. But, as with any service, overuse can lead to failure. Findlay notes, "We should be cautious about using a stream's cleansing ability to solve human-generated water quality problems. It is akin to treating the symptoms of a problem rather than the underlying cause, which includes intense agricultural practices and poorly planned development. Our results show streams can help us use natural resources, but this capacity has its limits."

This study was funded by a grant from the National Science Foundation.

Cary Institute