The influence of upstream lakes on rates and stability of whole stream metabolism in subalpine watersheds of the Sawtooth Mountains, central Idaho

Lakes within mountain watersheds provide a source of nutrients and organic matter to downstream reaches and can significantly alter downstream hydrology, geomorphology, and temperatures. Lake outflows may therefore represent biologically important sites of ecosystem production and stability, especially in seasonally dynamic, snowmelt-driven systems. Whole stream metabolism was measured in paired lake inflow and outflow stream reaches of 5 subalpine watersheds in the Sawtooth Mountain Lake District (central Idaho) from snowmelt through baseflow of 2008. Results showed significantly higher rates of gross primary production (GPP) within lake outflows, when compared to inflows, during both snowmelt and baseflow seasons, and lakes also appeared to reduce variation in rates of GPP during periods of high flows associated with peak snowmelt flood pulses. Factors controlling GPP during snowmelt were unclear, but production was likely influenced by streambed dynamics, and following snowmelt disturbance, baseflow rates across all sites were positively correlated with stream-water total phosphorous concentrations. Rates of ecosystem respiration (ER) varied by watershed but not by landscape position relative to lakes, and were most strongly negatively correlated with discharge and total watershed area throughout both seasons. ER dominated over GPP, and therefore most sites remained heterotrophic throughout the entire sampling period; however, because rates of GPP were higher in outflow streams without a subsequent increase in ER, these streams did tend more towards autotrophy throughout both hydrologic seasons.