Nitrogen Cycling in an Open Canopy Mountain Stream

Hellbaum, Paige
Streams receive, transform, sequester, and transport nitrogen (N). Stream N fluxes and transformations are of particular interest due to the large anthropogenic influence on N movement through the biosphere. Though regional estimates quantifying N fluxes have been proposed, these represent averages that may vary drastically given different local geomorphology and species composition. These fluxes are also highly variable at different time scales. In order to characterize short-term N cycling in a (relatively) pristine sub-alpine stream, we added K15NO3 to North Fork Little French Creek in Medicine Bow National Forest. We measured travel time and ecosystem metabolism in three reaches downstream of the isotope tracer addition. We collected plant, organic matter, invertebrate and water column samples over a period of 45 days before and after the 15N addition. The relative isotopic enrichment in these samples will allow us to calculate uptake length and the degree of biological N assimilation over time. Preliminary indicators for in-stream biological activity and turnover rates can be deduced from comparisons of travel time (salt tracer additions) and ecosystem metabolism (diel O2 measurement). Area-specific reconstructions of the fate of N in freshwater ecosystems will increase our understanding of the impact of human-induced changes on Wyoming's watersheds.
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