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Streams and rivers have an important job in nature's ecosystem: To filter nitrate and prevent pollution of the world's oceans and lakes.
That job is becoming more and more difficult due to increasing nitrogen runoff from urban and agriculture land-use, according to a report by 31 stream ecologists from across the United States, including Bob Hall, an associate professor in the University of Wyoming's Department of Zoology and Physiology.
The report, titled "Stream denitrification across biomes and its response to anthropogenic nitrate loading," was published today (Thursday) in Nature, the world's foremost weekly scientific journal and the flagship journal for Nature Publishing Group (NPG).
"Our central finding was that as nitrate concentrations increase in streams, the streams become less efficient at using that nitrate," says Hall, who spent three summers studying nine streams in the Jackson area for the report. "So, basically, if you increase the supply, you decrease the demand."
The ecologists studied 72 reference, agricultural and urban streams in eight regions of the United States and Puerto Rico, using the stable, non-radioactive isotope Nitrogen-15 to track nitrogen movement and determine how it was removed from stream and river networks.
They found that nitrate is filtered by small organisms -- including algae, bacteria and fungi -- to build their biomasses and that certain bacteria convert nitrate through denitrification, a process that produces nitrogen gas that escapes harmlessly and permanently into the atmosphere.
The national median for denitrification, according to the report, is 16 percent.
"The Wyoming streams were good for the study, because we have the lowest levels of nitrate in streams in the country. There's very, very low levels of nitrogen, and that provided a benchmark for which to compare to the more polluted streams," says Hall, who worked with Jennifer L. Tank, a professor at the University of Notre Dame, to collect data from the Wyoming streams. "Even the streams that are used for agriculture in Wyoming have very low nitrate concentrations and therefore very high levels of nitrate reactivity."
He adds, "The water quality in the Jackson area is really good. And good water quality helps the stream do its job, which is to remove nitrogen. If you pollute them further, they will respond by not removing nitrogen as efficiently."
The study streams in Wyoming were located in Grand Teton National Park, on the Snake River Ranch in Wilson, and at the Jackson Hole Golf and Tennis Club and Teton Pines Resort.
The study also incorporated data gathered from streams in Arizona, Kansas, Massachusetts, Michigan, New Mexico, North Carolina, Oregon and Puerto Rico.
The study was funded through a grant from the National Science Foundation (NSF) to the University of Tennessee.