Healthy streams with vibrant ecosystems play a critical role in removing excess nitrogen caused by human activities, according to a major new national study published in Nature.
The research, by a team of 31 aquatic scientists across the United States, was the first to document just how much nitrogen that rivers and streams can filter through tiny organisms or release into the atmosphere through a process called denitrification.
"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."
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.
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, according to Sherri Johnson, a research ecologist with the U.S. Forest Service, and a courtesy professor of fisheries and wildlife at OSU.
"Streams are amazingly active places, though we don't always see the activity," Johnson said. "When you have a healthy riparian zone, with lots of native plants and a natural channel, the stream has more of an opportunity to absorb the nitrogen we put into the system instead of sending it downriver."
The study is important, scientists say, because it provides some of the best evidence of the extent to which healthy rivers and streams can help prevent "eutrophication" -- the excessive growth of algae and aquatic plants fueled by too much nitrogen. Eutrophication has been linked to harmful algal blooms and oxygen depletion in such places as the Gulf of Mexico, where the Mississippi River empties its nitrogen-rich waters, adversely affecting fishing and shrimp industries.
In their study, the scientists added small amounts of an uncommon, non-radioactive isotope of nitrogen -- N-15 -- to streams as a nitrate, which is the most prevalent form of nitrogen pollution, Gregory said. 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.
Slower moving streams with little oxygen have higher rates of denitrification, though they have other pitfalls, including increased risk to fish and humans because of the "microbial stew" they foster, Gregory pointed out.
"The overall amount of denitrification by streams and rivers was lower than what many scientists had anticipated," he said. "We had hoped it would be higher. That makes it even more essential to maintain healthy riparian zones so the organisms have the opportunity to process the nitrogen."
Oregon had even lower levels of denitrification than the national average. Johnson said the combination of high-gradient streams, oxygenated water and porous stream beds is not conducive to the denitrification process.
"A lot of streams in Oregon have subsurface water flowing beneath the streambed through the gravel," she pointed out. "This 'hyporheic' flow intermixes with the river water and limits the anaerobic processes. It also underscores the importance of maintaining healthy in-stream communities so the nitrogen is taken up by the ecosystem in other ways."
Gregory says too many river systems have lost their natural channels to human activities and have essentially become "pipelines" for drainage. The original, braided channels many rivers had were complex, played a major role in slowing and filtering the river water, and provided natural habitat for native and migrating fish.
Past studies by Gregory and others have pointed out how these pipeline river channels harm fish and their eggs during floods. The new study suggests that these pipelines also limit the potential of the river to absorb nitrogen that humans add to the system through a variety of activities.
The Oregon studies focused on Oak Creek basin in Corvallis, the Calapooia River near Albany, and the McKenzie River near Eugene. Each study basin looked at the streams in forested, agricultural and urban areas.
Among the other authors were Linda Ashkenas, a senior research assistant at OSU, and Dan Sobota, who did his doctoral work at Oregon State.
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