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 two years.
The results were reported in the April 6, 2001, 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, Tech graduate Jennifer Tank, now of the University of Notre Dame, 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 black box," 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.
"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?
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, Webster says.
He explains that plant life, particularly algae, is a very important nitrogen user in streams where there are few trees to block the sun. Alternatively, in forested streams, 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 ensure 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. "It means that you have to take care of the streams on the landscape."
The research teams sampled water, plant life, bacteria, fungi, and insects for six weeks at 12 sites. Tank, a Virginia Tech graduate working on the project as a post doc, 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."
It has taken since 1998 to synthesize the data from about 1,500 samples per stream. "It is the first and largest coordinated data set for stream ecosystems," says Valett.
"This was a most gratifying project because of the enthusiastic involvement of many of the top stream ecologists from around the United States," says Mulholland, the project leader. "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 fun."
The research was funded by the National Science Foundation. Researchers want to conduct another study involving 72 streams, this time to include streams in agricultural and urban landscapes.
Links to photos of some streams are at www.research.vt.edu/resmag/streams/ and Science is at www.sciencemag.org.
For more information, see the Stream Team.
