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As spring arrives across the country, tourists returning to beaches will face the reality of "red tide" – harmful blooms of algae that make water unfit for swimming and pose risks to humans and sea life.

What they may not realize is that the small streams running through their neighborhoods play a critical role in filtering out the nitrogen that feeds the algae blooms.

A new study published in this week's edition of the journal Nature by 31 scientists from across the country sheds new light on streams' role as a nitrogen filter and uncovers data that show increases in nitrogen caused by human activities can make it harder for the streams to do their jobs.

"The filtering is a serial process, and it's bigger than any one stream," said Patrick Mulholland, the study's lead author and a researcher with the University of Tennessee and Oak Ridge National Laboratory. "What you see in your backyard, though, matters to the health of coastal oceans."

Excess nitrogen in streams is caused in large part by human activities, particularly overuse of nitrogen-based fertilizers, said Mulholland, and as nitrogen accumulates in increasingly larger bodies of water, it feeds the harmful algae growth that leads to red tide.

In addition, the excessive growths of algae consume large amounts of oxygen when they die and decompose, sometimes enough to make the water unable to support many forms of aquatic life. This problem has been especially pronounced in recent years in the Gulf of Mexico, impacting regional fisheries.

Mulholland and his colleagues, including UT Knoxville professor Lee Cooper, studied how a special, easily traceable form of nitrate made its way through 72 different streams across the U.S. and in Puerto Rico. They found that algae, fungi and bacteria in the streams consumed the nitrate, in essence causing the stream to store the nitrogen.