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 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."