PARENT SESSION
Oral Session 1: Aquatic Ecology I: Rivers, Nutrients, and Water Quality.
Presiding: D Kashian
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room A 105.

Missing nitrate in streams: Hydrological and biological sinks in a mountain stream.

Hall, Robert^*,1, Baker, Michelle², Arp, Christopher², Koch, Benjamin¹, ¹ University of Wyoming, Laramie, WY, USA² Utah State University, Logan, WY, USA

ABSTRACT- Although we know that streams can be sinks for nitrate nitrogen (N), we do not know the long term fate of this missing nitrate and if indeed streams can be net sinks for N. We measured nitrate uptake and storage during a snow melt flood and later at summer baseflow in a meandering glacial valley stream by using ¹⁵N tracer additions. Based on these additions we calculated mass balances of the nitrate tracer. During high flow, 33% of the nitrate was taken up in the stream, and during low flow 75% was removed. Uptake velocities (demand relative to concentration) was similar for both additions. Denitrification of this added nitrate was not detectable. Only 4-20% of the N was taken up by benthic pools in the stream. Invertebrates represented one of the largest benthic standing stocks of stored ¹⁵N with 50% of the N in sediment surface. The bulk of the ¹⁵N nitrate tracer left the stream into the fringing alluvial riparian zone, which we estimated based upon mass-balance of co added bromide, a conservative tracer of water. Missing Br at the end of our solute addition corresponded to that of the missing ¹⁵N; this Br (but not the ¹⁵N) leaked backed into the stream for weeks following the addition showing a long-term storage pool for water. Streams can be long terms sinks for N, not only from denitrification, but also by hydrologic exchange with the riparian zone.

Key words: nitrate, tracer, stream, snowmelt