Streams are Important Nitrogen Filters in the Landscape
31 Mar, 2008 01:12 pm
If you plan to vacation at the beach this summer or open the tap for a clean glass of water, that small stream in your neighborhood may be more important than you think. Why is this? It's because streams are the kidneys of our landscape, helping filter pollutants that drain from their watersheds. We know this from our recently completed major study of 72 streams in 8 regions across the U.S. and Puerto Rico. The results of this study were reported in the March 13 issue of the journal Nature and represented the work of a team of 31 aquatic scientists from across the U.S.
In the first phase of our study, we added small amounts of an uncommon non-radioactive isotope of nitrogen, N-15, as nitrate, which is the most common form of nitrogen pollution. The nitrogen isotope was added during one day to each stream and we measured how far downstream the nitrate traveled and what processes removed it from the water. These experiments were conducted in many different types of streams, ranging from pristine streams in National Forests to streams flowing through agricultural and urban areas. We found that the nitrate was rapidly removed from stream water by tiny organisms such as algae, fungi and bacteria. In addition, a considerable amount was permanently removed from streams by a bacterial process known as denitrification, which converts nitrate to nitrogen gas (N2) that then escapes harmlessly to the atmosphere. However, we found that the proportion of the nitrate in water that stream organisms were able to remove declined when nitrate levels were high, as was the case for many streams in agricultural and urban areas.
In a second phase of the study, we developed a model based on our results that predicted nitrate removal as water flows through stream networks – from small streams and into larger streams and rivers and eventually to the river mouth. The model showed that the entire stream network was important in removing nitrogen pollution from stream water, from the smallest streams to the large rivers. In addition, the effectiveness of streams’ ability to remove nitrate from water was greatest if they were not overloaded by nitrogen pollution such as fertilizers and wastes from human activities. The largest removal occurred when nitrate entered small healthy streams and traveled through the network before reaching larger rivers. Because small streams account for about 80% of the total length of most stream networks and because removal of pollution by the stream network is a serial process, increasing with the length of streams through which water flows, small streams play a critically important part in nitrogen removal.
We concluded from our research that streams and rivers are effective filters that help reduce the amount of nitrate pollution that runs off the land into water before it can be transported to larger bodies of water and cause eutrophication in lakes, reservoirs and coastal oceans.
Our results have important implications for several policy issues confronting us today. First, the ecological integrity of streams and their riparian zones should be protected or, in many cases, restored so that they can provide the important ecological service of pollution control and protection of drinking water sources and biodiversity. Even the smallest streams are an important part of the pollution removal network and small streams are often the most at risk of being degraded or destroyed from urban development, road building, and agriculture and mining activities. Second, new trends such as the increasing importance of biofuels as a substitute for petroleum are worrisome if annual crops such as corn, which require large amounts of nitrogen fertilizers, are expanded and result in higher levels of nitrogen runoff into our streams. Biofuels aren’t necessarily a problem, however, if perennial crops such as switchgrass, which require much lower levels of fertilizer and protect against erosion, are used as the primary feedstock.
The study was funded by a grant from the National Science Foundation to the University of Tennessee and scientists from The University of Georgia, University of Wyoming, Michigan State University, Marine Biological Laboratory at Woods Hole, University of Notre Dame, Oregon State University, University of New Mexico, Kansas State University, Institute of Ecosystem Studies, Arizona State University, U.S. Forest Service Pacific Northwest Research Station, University of New Hampshire, Virginia Tech, Ball State University, U.S. Environmental Protection Agency, U. S. Geological Survey, and Washington State University.