SMALL STREAM IMPORTANT IN CONTROLLING NITROGEN
BLACKSBURG, April 5, 2001 --- Streams are not gutters that simply deliver nutrients to lakes, oceans and bays. Streams are vibrant ecosystems, and the smallest streams remove as much as half of the inorganic nitrogen that enters them, according to researchers from more than a dozen institutions who studied streams from Puerto Rico to Alaska over the course of two years.
The results will be reported in the April 6 issue of Science, in the article "Control of Nitrogen Export from Watersheds by Headwater Streams" by Bruce J. Peterson and W.M. Wollheim of the Marine Biological Laboratory (MBL) in Woods Hole, Mass., Patrick J. Mulholland of Oak Ridge National Laboratory, Jack Webster and Maury Valett of Virginia Tech, Jennifer Tank of the University of Notre Dame, Walter Dodds at Kansas State University and others.
Human activities, such as fertilizer application and the burning of fossil fuels, result in excess nitrogen entering streams, changing water quality downstream, such as in the Chesapeake Bay or Gulf of Mexico. The approach to minimizing nitrogen in these waterways has been mainly terrestrial, since the processes responsible for nitrogen uptake and release in streams has been a mystery, says Webster, professor of biology at Virginia Tech. But an NSF-sponsored workshop in 1995 identified models and a tracer that might be used to develop a systematic approach.
A breakthrough came when MBL chemists made it easier to measure the stable isotope N15 (nitrogen 15), making it a useful and economically feasible tracer. Previously, N15 could not be detected in solution and analysis cost $30 per sample. Peterson and colleagues used new mass spectrometer techniques to improve sensitivity and reduce the cost to less than $10. "Now we have a way to track nitrogen through a stream's biological systems," says Valett. "N15 allowed us to add nitrogen in such small amounts that it does not change the nitrogen load already present."
Meanwhile, the scientists developed mathematical computer models of streams' biological processes that made it possible to compare the nitrogen cycle in different kinds of streams. The model was used to design the experiment then to track the data. "The model helped to generate hypotheses about the flow of nitrogen through these streams, and then was used to track the N15 tracer, as data from the field experiments were provided," says Tank of Notre Dame.
"The bottom line is streams have an impact. They can remove as much as 50 percent of the inorganic nitrogen. So anything we do to streams to modify them will impact the nitrogen that reaches rivers, lakes, bays and oceans," says Webster. The finding could have important consequences for land-use policies.
What happens to the nitrogen that is removed from streams? The Science article reports that some of the nitrogen is converted to nitrogen gas through denitrification processes and the rest becomes nutrition for algae, bacteria and fungi, which then become food for aquatic insects and fish. As the plant or organism dies, the nitrogen can then end up as slowly decomposing materials that settle in the stream or lake sediments, Virginia Tech?s Webster says.
He explains that plant life, particularly algae, is a very important nitrogen-user in some streams -- such as in Alaska, Arizona and Kansas, where there are few trees to block the sun. Alternatively, in forested streams, such as those in Oregon and North Carolina, nitrogen is removed by fungi and bacteria, which do not photosynthesize, but decompose dead organic material settled on the stream bottom, also a food resource for some aquatic insects.
"The smaller the stream, the more quickly nitrogen can be removed and the less distance it will be transported down the stream," Peterson says. Thus, taking greater care to insure that small streams can work effectively to clean the water will reduce the overall nitrogen load that makes its way into larger bodies of water. "It doesn't mean that you can ignore your sewage treatment plants, but if we can do better with our small streams and do some restoration activities it's going to have some benefits," he says. "What it means is that you have to take care of the streams on the landscape."
The research documents that pristine Midwest prairie streams are as retentive of nitrogen as those found in many other ecosystems. The project demonstrates that typical Flint Hills management of upland prairie sites leads to excellent water quality and nitrogen retention. It implies that prairie streams were historically very retentive of nitrogen and protected downstream water quality, says Dodds, the project coordinator at Konza Prairie Biological Station.
The research teams sampled water, algae and other plant life, bacteria, fungi and insects for six weeks at each site. "This study provided clear, controlled methods across all of the sites, and all of the groups were willing to do this," says Valett, who was the leader at the Gallina Creek study in New Mexico, but has since joined Virginia Tech's faculty.
Tank, a Virginia Tech graduate working on the project as a post doc, trained the teams at each site. She also provided lively progress reports along with her instructions for the next set of streams. She noted that almost every stream seemed to be hit with a large storm on day 21. Exceptions were in New Mexico and Arizona. But the Arizona site had its own adventures. Located only 35 miles from Phoenix, Sycamore Creek required that a permanent camp be set up "to ensure that equipment was not stolen, shot at, or stepped on by cows," Tank noted. At least two people were required to be in the field full-time for 43 days. "Thanks to the large lab at Arizona State University, and their relatives and friends -- 36 people in all, we were able handle this problem; however, everybody was extremely happy when the addition (of N15) was over."
Upon completion of the field work in 1998, a number of papers were published about each site, but it has taken since 1998 to synthesize the data from about 1,500 samples per stream for the 12 streams. "It is the first and largest coordinated data set for stream ecosystems," says Valett.
The article in Science reports on the rates of nitrogen uptake, storage, regeneration, and export in headwater streams.The focus was the free mineral form of nitrogen, specifically nitrate (NO3) and ammonium (NH4).
"This was a most gratifying project because of the enthusiastic involvement of many of the top stream ecologists from around the United States who shared ideas and data unselfishly," says Mulholland, the project leader. "I think it is an excellent model for how inter-site, multi-investigator research can and should be done to address some of the most fundamental environmental issues facing us today. It was rewarding and it was fun."
"The collaboration was key," agrees Webster. "That was one of this project's strengths. There were about 30 investigators from 10 programs, plus many graduate and undergraduate students, who helped with the field work."
Several researchers involved in this research are now working on a project to better understand the denitrification process and are studying nitrogen dynamics in more detail at three of the 12 streams. Mullholland, of Oak Ridge National Lab, is leading the effort to obtain funding for another large N15 tracer project, involving about 20 scientists and a total of 72 streams (nine at each of eight sites). "The original streams were located in relatively pristine areas -- primarily national forests, some nature preserves and reservations. Next we would like to include streams in agricultural and urban landscapes" Webster says.
The research was funded by the National Science Foundation with a $1.4 million grant plus the resources of each unit. Mulholland, who is with the Environmental Sciences Division at Oak Ridge National Laboratory, was the project leader. The lab is a U.S. Department of Energy facility managed by UT-Battelle. Authors of the Science article are Peterson, Wollheim, Mulholland, Webster, J.L. Meyer of the Institute of Ecology, University of Georgia; Tank; E. Marti, Centre d'Estudis Avancats de Blanes, Spain; W. B. Bowden, Landcare Research, New Zealand; Valett; A.E. Hershey, Department of Biology, University of North Carolina; W.H. McDowell, Department of Natural Resources, University of New Hampshire; W.K. Dodds, Division of Biology, Kansas State University; S.K. Hamilton, Kellogg Biological Station, Michigan State University; Stanley Gregory, Department of Fisheries and Wildlife, Oregon State University, and D.J. Morrall, another Virginia Tech graduate, now at Proctor & Gamble Company Exper
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Photos of the streams can be seen online at
In addition to the researchers listed above, Dr. Jennifer Tank can be reached at [email protected]Dr. Maurice Valett can be reached at 540-231-2065 or [email protected]Dr. Watler Dodds can be reached at 785-532-6998 or [email protected]
Homepages:Dr. Peterson-- Dr. Webster -- Dr. Valett --
Ron Walli, Oak Ridge National Laboratory, (865) 576-0226 or [email protected]
Susan Trulove, Virginia Tech, (540) 231-5646 or [email protected]
Keener A. Tippin, Kansas State, (785) 532-6415 or [email protected]
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