The phenology, biogeography, and metabolism of bacteria in high-elevation lakes of the Sierra Nevada, California

Bacterioplankton are a significant component of many aquatic food webs and regulate the cycling of organic matter in pelagic marine and freshwater environments. The composition of bacterial communities influences their role in ecosystems, but their high metabolic and phylogenetic diversity has hindered understanding of the relationship between community structure and function. I examined the community structure and metabolism of bacterioplankton in high-elevation lakes of the Sierra Nevada, California. I characterized spatial and temporal patterns in bacterial community composition in relation to physical, biogeochemical, and landscape parameters by comparing bacterial 16S ribosomal DNA sequence polymorphisms among environmental samples. I also examined how a suite of bacterial metabolic parameters, including cell densities, rates of production, and growth efficiencies, changed in concert with seasonal shifts in lake ecosystem properties. A three-year study of bacterioplankton abundance, productivity, respiration, and community composition throughout the water column of Emerald Lake (a cirque lake located at 2800 meters above sea level in Sequoia National Park representative of lakes throughout the Sierra Nevada) demonstrated interannually-predictable seasonal changes in bacterioplankton assemblages linked to climatic shifts in the timing of snowmelt and thermal stratification of the lake. Changes in bacterioplankton metabolism and community structure tracked temporal variations in organic matter source, engendering seasonal shifts in the net metabolic balance of the lake ecosystem. In 2005 I experimentally examined the response of bacterioplankton communities to nitrogen and phosphorus enrichment and found that bacterial community composition and metabolism shift in response to increased phosphorus, but not nitrogen. In 2006 I conducted a survey of the bacterioplankton community composition of 100 lakes in 17 chain lake catchments throughout the Sierra Nevada to examine community variability among lakes within catchments. This work demonstrated clear biogeographic patterns, indicating that bacterial communities found in terrestrial runoff were distinct from those found in lakes, and that this difference was maintained during peak snowmelt periods of rapid flushing, indicating strong environmental selection for freshwater community types. Together these results provide an initial characterization of the composition and metabolic role of bacterioplankton communities in high-elevation lake ecosystems of the Sierra Nevada.