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Communication at the NABS Annual meeting, Keystone, 2000
(301) NITRATE RETENTION IN FOUR HEADWATER STREAMS.
C.S. Fellows1, C.N. Dahm1, and H.M. Valett2. 1Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131, USA, 2Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA
Headwater streams are potentially important contributors to retention of nitrate across the landscape. Surface-subsurface water exchange is often significant in these streams, providing the opportunity for subsurface as well as benthic retention. We investigated the relative roles of surface and subsurface processes in nitrate retention in 4 streams representing a gradient of surface-subsurface exchange (transient storage, As/A, ranged from 0.1 to greater than 1.0). To assess the relative importance of photoautotrophy in nitrate uptake, we co-injected a conservative tracer (chloride) and nitrate during the day and at night. As/A and other hydrologic parameters were quantified by modeling the chloride data. Nitrate uptake lengths ranged from 120m to 1700m, and values of the mass transfer coefficient (vf, a measure of uptake standardized for discharge) ranged from 2.5 mm/hr to 46.0 mm/hr. The site with the greatest value of As/A also had the greatest value of vf, but the relationship between these two variables was not consistent among sites. Three of the 4 sites had greater nitrate retention during the day than at night, and the magnitude of the difference varied. Even for streams with large subsurface contributions to nitrate retention, scaling day-time measurements to longer time intervals likely overestimates total retention.
Presented at 2:30 PM on Thursday, June 1, 2000 in Groundwater and Surface Water Interactions I
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