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'The influence of upstream lakes on rates and stability of whole stream metabolism in subalpine watersheds of the Sawtooth Mountains, central Idaho.'
Scarlett Vallaire (scarlett.vallaire@usu.edu), Utah State University Michelle Baker (michelle.baker@usu.edu), Utah State University; Wayne Wurtsbaugh (wayne.wurtsbaugh@usu.edu), Utah State University

Lakes within mountain watersheds provide a source of nutrients and organic matter to downstream reaches and can significantly alter downstream hydrology, geomorphology, and temperature. Lake outflows may therefore represent biologically important sites of ecosystem production and stability, especially in seasonally dynamic, snowmelt-driven systems. Whole stream metabolism was monitored within paired inflow and outflow reaches of 5 subalpine watersheds in the Sawtooth Mountain Lake District (central Idaho) from snowmelt through baseflow of 2008. Results show significantly higher rates of gross primary production (GPP) in lake outflows during both snowmelt and baseflow seasons, and lakes also appeared to reduce variation in outflow GPP during periods of high flows associated with peak snowmelt flood pulses. Distinct snowmelt controls on GPP were unclear, but rates were likely limited by a combination of temperature and bed dynamics, and following snowmelt disturbance, baseflow rates of GPP across all sites were largely explained by stream-water total phosphorous concentrations. Rates of ecosystem respiration (ER) varied by watershed but not by landscape position, and were most strongly related to watershed hydrogeomorphic parameters throughout both seasons. Net ecosystem productivity was largely driven by ER, and while most sites remained net heterotrophic, differences in process response resulted in outflows that tended more towards autotrophy.