CHICAGO (Reuters) - A meandering stream appears to play a powerful role in filtering out pollutants like nitrogen, and understanding this role could help prevent oxygen-depleting blooms of algae that threaten fish and shellfish downstream, researchers said on Wednesday.

A man looks at a stream near an emerald mine in Muzu, northern Colombia, in this photo taken April 10, 2003. A meandering stream appears to play a powerful role in filtering out pollutants like nitrogen, and understanding this role could help prevent oxygen-depleting blooms of algae that threaten fish and shellfish downstream, researchers said on Wednesday. REUTERS/Eliana Aponte

The research was part of a project to determine whether rivers actively process pollutants and remove them from the ecosystem, or simply act as drain pipes that flush polluted waters into lakes or out to sea.

“They are most definitely processors,” said Stephen Hamilton, an aquatic ecologist at Michigan State University in Lansing, who led one of several teams studying the problem.

The study, which appears in the journal Nature, looked at how 72 streams across eight regions in the United States and Puerto Rico neutralize nitrogen.

“There is a remarkable amount of processing that takes place,” Hamilton said in a telephone interview. “We were able to see how streams vary in that nitrogen processing.”

If overloaded, however, they found the streams were less efficient at removing the nitrogen that enters the stream through agricultural runoff, acid rain and human waste.

Too much nitrogen in the water can cause excessive growth of algae and aquatic plants in lakes and coastal marine waters, which deplete oxygen stores, killing fish and other marine life. Such so-called “dead zones” already are seen in the Chesapeake Bay, the Gulf of Mexico and the Baltic Sea.

NOT SO LAZY RIVER

To measure this clean-up effort, the researchers added a small amount of a harmless, radioactive isotope of nitrogen into the streams. This acted as a tracer, allowing the researchers to track its path.

Hamilton’s team stationed itself in the headwaters of the Kalamazoo River, dribbled the tracer into the water, then field workers took samples as it made its way downstream for a distance of about 3,000 feet.

“Most of the nitrogen we found in a stream was taken up by the stream organisms in a fairly short distance downstream,” he said.

The nitrogen was gobbled up by tiny organisms such as algae, fungi and bacteria. But, a large portion of it was permanently pulled from the streams by a process known as denitrification, which converts nitrate to nitrogen gas that escapes into the atmosphere.

“We were able to quantify the fate of how much nitrogen goes into each of these potential pathways,” he said.

“That allows us to understand how some streams do better than others and that opens up the possibility of understanding how we can manage streams to promote denitrification or importantly, how we are managing them to discourage it,” he said.

Hamilton said the trick is to allowing lazy, meandering rivers to do their job instead of diverting them into straight drainage ditches that act more like water pipes and less like filters.

“We’ve been very industrious and successful in draining vast amounts of wetland. We’ve re-engineered our streams to conduct the water and any nutrients in the water out as fast as possible,” he said.

“We think it’s fair to say as a group there are a lot of ways we could do that better.”