Storage of nitrogen-15 nitrate during snowmelt in 2 mountain streams separated by a lake

R.O. Hall, Jr.¹, M.A. Baker², W.A. Wurtsbaugh³, B. Koch¹, and C.D. Arp². ¹Dept. of Zoology and Physiology, University of Wyoming, Laramie WY 82071, ²Dept. of Biology, Utah State University, Logan, UT 84322, ³Dept of Aquatic, Watershed and Earth Resources, Utah State University, Logan, UT 84322

Streams can have high uptake rates for dissolved inorganic nitrogen (N), but their net storage of N may be high or low, depending on hydrologic conditions, landscape position and biotic processes. We measured N cycling in two streams separated by a 0.32 km² lake to estimate uptake, turnover, and storage of nitrogen in these 2 ecosystems. We added 66g of ¹⁵N as nitrate to a 1.8-km inlet reach above a lake during snowmelt. Discharge ranged from 0.5-1 m³/s. We sampled the inlet, lake, and 1.8-km outflow reach for 90 days. Twenty-two percent of tracer N was removed from the water column in the inlet stream, but <10% of this removed N was stored in benthic N pools. Twice the amount in benthic pools was quickly transported to the lake as seston, indicating that fine benthic organic N was dominated by transport rather than storage. The lake absorbed most of the ¹⁵N-nitrate, but exported seston and DON. A small fraction of exported N was retained in benthic pools in the outlet. Despite relatively high uptake of nitrate from the water column, transport processes were high during this flood period, so net retention of N in the stream channels was low.