• Healthy headwaters generate healthy rivers says research
ALBUQUERQUE, NM, March 14, 2008 -- Streams are filters for our landscapes, helping to remove and transform pollutants that drain from their watersheds. As reported in the journal Nature, a team of 31 aquatic scientists across the United States, including University of New Mexico researchers Cliff Dahm and Chelsea Crenshaw, studied how streams remove excess nitrogen that results from human activities.
Researchers found that the efficiency of nitrogen removal by biological organisms in small streams deteriorates as nitrate concentrations increase in the water - a pattern that held across nitrate concentrations varying by up to six orders of magnitude and across eight ecological communities or biomes.
"Streams are very effective in reducing the amount of nitrates downstream," said Dahm, a professor in the biology department. "Nitrogen removal in streams is important because it reduces the potential for eutrophication -- the excessive growth of algae and aquatic plants -- in downstream lakes, reservoirs, rivers and coastal marine waters. Successfully maintaining quality headwater streams maintains healthier lakes and rivers."
Eutrophication is linked to problems such as harmful algal blooms and oxygen depletion in places such as the Gulf of Mexico, where nitrogen additions from the Mississippi River create a vast zone of oxygen depletion with adverse effects on coastal ocean ecosystems. Excess nitrogen helps increase the potential for eutrophication.
Dahm and Crenshaw, a UNM graduate student, collaborated with researchers at Arizona State and measured nine streams and rivers in the southwestern United States. They found that nitrate levels were much lower than areas with more intense agriculture.
"There's not the same level of nitrate pollution," said Dahm. "Wastewater sources were more discrete and distinguishable point sources than in croplands. In southwestern waters, nitrate levels are more clearly linked to human population centers and waste water treatment plants."
In the first phase of their study, the scientists added small amounts of an uncommon, non-radioactive isotope of nitrogen, N-15, to streams as nitrate, which is the most prevalent form of nitrogen pollution. The added isotope allowed them to measure how far downstream the nitrate traveled and what processes removed nitrate from the water.
These experiments in 72 streams across the U.S. and Puerto Rico spanned a diversity of land uses, including nine urban, agricultural and forest streams in the southwestern United States. The scientists found that the nitrate was taken up from stream water by microorganisms such as algae, fungi and bacteria. In addition, a fraction was permanently removed from streams by a bacterial process known as denitrification, which converts nitrate to nitrogen gas that then escapes harmlessly to the atmosphere.
In a second phase of the study, the scientists developed a model based on the field experimental results that predicts nitrate removal as water flows through small streams into larger streams and rivers. The model showed that the entire stream network was important in removing nitrogen pollution from stream water.
"The effectiveness of streams to remove nitrate was greatest if the streams were not overloaded by nitrogen sources such as fertilizers, wastes from human activities, and waste water treatment plants," said Dahm.
The largest removal occurred when nitrate entered small healthy streams and traveled throughout the network before reaching larger rivers. The scientists concluded from their research that streams and rivers could be effective filters to help reduce the amount of nitrate pollution exported from landscapes, and thereby reduce eutrophication problems.
"Healthy headwaters generate healthy rivers," said Dahm. "About half of the state lives along the Rio Grande in central New Mexico. We have to protect our headwaters to have a healthy river."
The study was funded by a grant from the National Science Foundation.
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