Controls on Bacterial Productivity in Arctic Lakes and Streams.

Abstract

This dissertation investigates heterotrophic bacterial production in arctic lakes and streams in northern Alaska. Temperature, dissolved organic matter, and inorganic nutrients all control bacterial production, but interactions of these controls with each other and with bacterial community composition is poorly understood. These interactions were examined using lab and field observations and experiments to provide a better understanding of factors influencing bacterial activity in nature.

DNA analysis indicated that shifts in the composition of bacterial communities were driven more by temperature than by differences in dissolved organic matter source. Aquatic bacterial communities incubated at different temperatures had different rates of production, and two distinct optima (12 and 20 °C) were evident after three days. Therefore, predicting the impact of warmer temperature on bacterial productivity is more complex than simple Q10 responses, and requires consideration of the interaction with community composition.

Bacterial nutrient limitation and response to storm events (changes in water temperature and nutrient concentrations) were investigated in mesocosm experiments. Nutrient additions increased bacterial production up to seven times greater than the control, while warmer temperatures shortened the bacterial response time to added nutrients. Community composition shifted rapidly (2 days) in response to nutrient addition in all habitats, but exhibited habitat-specific responses to temperature. Although nutrients were more important, temperature and nutrient levels interact to control the onset and magnitude of increased bacterial growth and the corresponding shifts in community composition.

Metacommunity processes of species sorting (e.g., competition) and mass effects (dispersal) were investigated at an 18 ha area lake. Inlet and outlet community composition was most similar (61.5%) after large storm events, indicating the importance of dispersal. However, transplant experiments and DNA analyses indicated that resident lake populations out-compete many bacterial populations in stream water entering the lake. While mass effects may be important during storm events, species sorting appears to be the predominant mechanism controlling community composition and function.

Despite being considered a single functional group, the heterotrophic bacteria examined here exhibit community-specific responses to drivers and shifts in dominant community members that occur on ecologically relevant time scales. This highlights the importance of community composition to productivity.