University of California Water Resources Center

There are more than 4000 lakes above 2500 m elevation in the Sierra Nevada of California. Recent developments in understanding of aquatic ecosystem function predict that metabolism and food web structure in these dilute, low-productivity lakes may be dominated by an active and diverse microbial community. The identity, activity, and importance of pelagic prokaryotic communities (bacterioplankton) in these ecosystems is little known, yet understanding the role of these organisms in lake metabolism and food web dynamics is critical to predicting the response of Sierra lakes to global change. This study combined range-wide surveys, multiannual seasonal monitoring, and experimental approaches to describe the biodiversity and biogeochemical role of bacterioplankton in high-elevation lakes of the Sierra Nevada. Particular attention was payed to determining the impacts of anthropogenic nutrient deposition and introduced salmonids on the structure and metabolism of microbial communities.

Our results indicate an abundant, remarkably diverse, and metabolically active bacterioplankton community. A three-year study of Emerald Lake, a representative cirque lake in Sequoia National Park, provided estimates of more than 250 bacterial phylotypes exhibiting predictable patterns of community phenology in response to seasonal transitions. Bacterial densities averaged 1 billion cells per liter throughout the year, and estimates of bacterioplankton biomass averaged 40% of total pelagic biomass. Rates of bacterial production were comparable to rates of net primary production, ranging from 1% during fall phytoplankton blooms to nearly 500% during snowmelt when terrestrial inputs of dissolved organic matter were highest. Bacterial metabolic properties were coupled to larger ecosystem dynamics, with growth efficiencies correlated with rates of primary productivity.

Phosphorus enrichment, but not nitrogen enrichment, produced rapid and sustained increases in bacterial production rates and produced significant alterations to bacterial community structure. The metabolic and community response to nutrient enrichment by the bacterioplankton was independent of phytoplankton responses, suggesting that microbial populations may be more sensitive indicators of eutrophication and ecosystem change. Among lakes throughout the Sierra Nevada, introduced salmonid fish had no significant relationship with multivariate bacterial community composition, but salmonid density did exhibit a significant positive correlation with bacterial phylotype richness and diversity, suggesting that trout introductions have altered bacterioplankton community structure. Within catchments bacterial communities exhibited decreasing similarity with increasing biogeographic distance, supporting the concept that macroecological principles of dispersal and isolation play a role in structuring communities of microorganisms.

This study highlights the importance of bacteria in lake ecosystems of the Sierra Nevada by providing spatially and temporally robust estimates of biomass, metabolism, diversity, and role in ecosystem biogeochemical processes. In addition to specifically defining impacts of introduced species and nutrient enrichment on microbial communities, it provides a sturdy ecological scaffolding for future studies of the role of microorganisms in high-elevation lake ecosystems.