Heterotrophic bacterial and viral dynamics in Arctic freshwaters:: results from a field study and nutrient-temperature manipulation experiments

Heterotrophic bacterial and viral concentrations (range, 0.7 x 10(4) to 206.2 x 10(4) ml(-1) and 0.05 x 10(6) to 128.9 x 10(6) ml(-1), respectively) were determined in several Arctic freshwater environments, including lakes and glacial ecosystems (78.55 degrees N, 11.56 degrees E). Our bacteria and virus results mirrored trends seen in temperate lakes, with an average virus-to-bacteria ratio (VBR) of 13 (range, 7.3-25.2) and viral concentrations and DOC positively correlated with bacterial concentrations (R = 0.964, P < 0.01 and R = 0.813, P < 0.05, respectively). Lysogenic bacteria, determined by induction with Mitomycin C, were not detected in any of the investigated Arctic freshwater environments. Nutrient-addition experiments at in situ and at elevated temperatures were performed to elucidate the factors which influenced the bacterial growth and the virus-bacteria interactions in Arctic freshwaters. Our results suggest that multiple limiting factors interacted and constrained bacterial growth. Bacterial concentrations and doubling times increased at elevated temperatures and appeared to be co-stimulated by phosphorus and carbon. However, viral concentrations showed a lack of response to nutrient addition thus indicating an uncoupling between bacteria and viruses in the experiment.