Linking Lakes to the Landscape: Ecosystem Metabolism of High-Elevation Lakes in the Sierra Nevada, California

Ecosystem metabolism quantifies trophic relationships and is a fundamental measure of ecosystem function. Despite its importance, little is known regarding the ecosystem metabolism of high-elevation lakes. I examined the ecosystem metabolism of Emerald Lake, a small high-elevation lake representative of over 4000 similar Sierra Nevada lakes. Using a combination of free-water and in situ incubation chamber measurements of dissolved oxygen, I characterized spatial and temporal variability in metabolic rates, evaluated the relative contributions of pelagic and littoral benthic habitats to whole lake metabolism, and identified key environmental drives of metabolism. I also characterized the mechanisms underlying diel variability in rates of respiration. I use a synoptic survey spanning the Sierra Nevada to explore landscape influences on lake biogeochemistry. Finally, I characterized the impact of landscape inputs that resulted from a rainstorm on the biogeochemistry and ecosystem metabolism of Emerald Lake.;Emerald Lake was slightly autotrophic over the course of consecutive ice-free seasons. Seasonal mean rates of gross primary production and respiration in littoral habitats were twice as high as pelagic habitats, but on average the pelagia contributed two-thirds of whole lake metabolism. The net autotrophy of the water column was high enough to compensate for benthic heterotrophy. During periods of stratification, gross primary production and respiration increased with depth and heterotrophy occurred in association with high chlorophyll a and particulate matter concentrations below the thermocline; zones of water column heterotrophy disappeared with the onset of lake mixing each autumn.;Bacterioplankton metabolism dominated community respiration and was tightly coupled to gross primary production. Diel variability in respiration resulted from bacterioplankton metabolizing different pools of dissolved organic matter, with higher daytime concentrations fueling higher respiration rates.;Landscape characteristics were important in structuring a wide range of biogeochemical properties of lakes, including nutrients and dissolved organic matter concentrations. The terrestrial inputs associated with a late season rainstorm dramatically altered the ecosystem metabolism of Emerald Lake. The simultaneous reduction in primary production and increase in respiration throughout the water column caused the lake to shift from autotrophy to heterotrophy, demonstrating the vulnerability high-elevation lakes to ecosystem perturbations associated with climate change.