A new study has determined that tiny organisms residing in healthy streams and rivers play a critical role in removing nitrate, a form of nitrogen pollution caused by human activities.
The study, which was carried out by a team of 31 aquatic scientists across the United States, was the first to document just how much nitrogen can be filtered through tiny organisms or release into the atmosphere through a process called denitrification in rivers and streams.
The scientists conducted experiments in 72 streams across the United States and Puerto Rico that spanned a diversity of land uses, including urban, agricultural and forested areas.
They discovered that roughly 40 to 60 percent of nitrogen was taken up by the river system within 500 meters of the source where it entered the river - if that ecosystem was healthy.
According to Sherri Johnson, a research ecologist with the U.S. Forest Service, tiny organisms such as algae, fungi and bacteria that may live on rocks, pieces of wood, leaves or streambeds can "take up," or absorb about half of the nitrogen - on average - that humans currently put into the sampled river sites.
"The study clearly points out the importance of maintaining healthy river systems and native riparian areas," said Stan Gregory, a stream ecologist in the Department of Fisheries and Wildlife at Oregon State University, an a co-author of the study.
"It also demonstrates the importance of retaining complex stream channels that give organisms the time to filter out nitrogen instead of releasing it downstream," he added.
In their study, the scientists added small amounts of an uncommon, non-radioactive isotope of nitrogen N-5 to streams as a nitrate, which is the most prevalent form of nitrogen pollution. By adding the isotope, they were able to measure how far downstream the nitrate traveled, and analyze what processes removed it from the water.
In addition to the 40 to 60 percent taken up by tiny organisms, the researchers found denitrification accounted for about 19 percent of the nitrogen uptake across all the sites.
Denitrification takes place through an anaerobic metabolic process that converts the nitrogen to a harmless gas and releases it into the atmosphere.
In the second phase of the study, the scientists developed a model to study nitrate removal from water within river networks. These networks develop as small streams flow into larger streams and rivers.
'Our model showed that the entire stream network is important in removing pollution from stream water,' said Patrick Mulholland, lead author of the study. 'In addition, the effectiveness of streams to remove nitrate was greatest if the streams were not overloaded by pollutants such as fertilizers and wastes from human activities.'