Year-round patterns in bacterial production and biomass in Lake Simcoe, Ontario, Canada: Are heterotrophic bacteria a significant contributor to low hypolimnetic oxygen?

Abstract

Bacteria serve an important function in aquatic environments and are associated with nutrient regeneration, carbon (C) metabolism, and secondary production. This is the first study to measure heterotrophic bacterial production, abundance, biomass, and biovolume in Lake Simcoe, Ontario. Excessive phosphorus loading resulting in low hypolimnetic oxygen concentrations has impacted the cold-water fishery in Lake Simcoe. We tested the hypothesis that bacteria contributed to dissolved oxygen declines in Lake Simcoe and examined the environmental factors impacting bacterial activity. Spatial and temporal variations in the microbial community were measured from June 2010 to July 2011. A dual-isotope method (3H-TdR and 14C-leu) was used, resulting in mean (± standard deviation) annual epilimnetic bacterial production estimates of 0.130 ± 0.173 and 0.268 ± 0.304 μg C L-1 h-1, respectively. The mean annual bacterial abundance was 1.49 ± 1.53 cells × 109 L-1, with a mean biomass of 1.21 ± 1.34 μg C L-1 and a mean biovolume of 0.0043 ± 0.0030 μm3 cell-1. These estimates had distinct seasonal patterns, with consistently lower bacterial activity in the winter relative to the spring, summer, and fall. Differences between epilimnetic and hypolimnetic bacterial activity were inconsistent for the 4 bacterial parameters measured. Lake temperature, chlorophyll a, and dissolved organic C concentrations were the most significant factors influencing the annual epilimnetic patterns in the examined bacterial parameters. Annual bacterial production was low in Lake Simcoe and does not seem to be a major contributor to the low hypolimnetic oxygen concentrations in the lake.