Research confirms importance of buffers
KNOXVILLE, Tenn. — Take care of that little backyard stream. You could reap the benefits next time you draw some water from the tap or go for a swim at the beach.
An article published Tuesday in the journal Nature describes the critical role small streams play in removing nitrogen pollution from fertilizers, farm animal waste, septic tanks and fossil fuels before it reaches large reservoirs and the ocean.
“That stream in your backyard isn’t itself playing a dominate role, but when you add up all the small streams in any network they are very important,” said Patrick Mulholland of the Oak Ridge National Laboratory and University of Tennessee who was lead author on the study involving 31 researchers.
To date, most attention has been focused on downstream problems, he said. That’s where excess nitrogen can lead to an explosive growth of microscopic algae that can kill sea life. These red tides can be huge. One covered 25,000 square miles off the gulf coast of Florida in 2005.
“Nitrogen pollution is a very big problem and it is growing,” Mulholland said. “The areas are increasing both in space and in time.”
But apparently it could be worse. Research suggests only about a quarter of the nitrogen entering waterways from farm and city runoff or acid rain reaches oceans and lakes.
“So the question is, what happens to the rest of that nitrogen?” Mulholland said. “There has always been this speculation that maybe small streams or stream networks are the places we should be looking as being very important to retaining the nitrogen.”
To find out, Mulholland and his colleagues released and tracked an easily traceable nitrogen isotope, or nitrate, in 72 streams in North Carolina, Massachusetts, Kansas, Michigan, Oregon, Arizona, New Mexico and Puerto Rico. The streams ran past farms, cities and more unaffected areas. They ranged in size from 1 to 5 meters across.
The researchers found that algae, fungi and bacteria in the streams will consume a lot of the nitrate through natural processes — effectively storing it or removing it by converting it into harmless nitrogen gas.
Small streams, which can make up 80 percent of a water network, do a particularly good job of removing nitrogen when there isn’t too much of it, the researchers found. Larger streams do a better job when the nitrogen reaches moderate levels.
However, when nitrogen levels become high “removal becomes ineffective across all stream sizes,” the article says.
“You can overload the system. At some point, it cannot keep up with the added nitrogen,” Mulholland said.
Then the system “acts more like a pipe,” he said. “You basically transport the nitrogen right down to the (river’s) mouth.”
The study’s beginning-to-end conclusion was based on observations and modeling on the Little Tennessee River, which flows from North Carolina into Tennessee and feeds the Tennessee River system.
“So it is not like we have to preserve all our landscapes and make them pristine,” Mulholland said, so long as streams remain open and buffered from development. “We just need to try to preserve the integrity of the stream network, which is a very important filter.
A U.S. Forestry Service worker maintains a prescribed burn in Superior National Forest. Prescribed burns can stimulate growth of grasses, improve access to wooded areas and reduce high fuel levels that can lead to wildfires.