Small streams mitigate human influence on coastal ...( Healthy streams play a major role in...)

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 ultimatel
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