Population dynamics of bacterioplankton in an oligotrophic lake

The population ecology of bacterioplankton was studied over a 3 year period in Mirror Lake, an oligotrophic lake in the northeastern USA. Bacterial population density, biomass, and rates of biomass production in the epilimnion and hypolimnion were examined for their relationship with several environmental parameters. Bacterioplankton density fluctuated between 0.5 and 7 × l0 bacteria ml−1, with highest values in the anoxic hypolimnion. At all depths there was a trend towards a higher density of bacteria from spring to midsummer, followed by a decline in late summer to early autumn. Cocci tended to dominate bacterial cell shapes from winter to midsummer, after which rod-shaped cells became most abundant. Rod-shaped cells contributed the most to bacterioplankton biomass at all depths and times of year. The mean annual biovolume of all bacterioplankton was 0.12 μ cell−1. The mean annual areal bacterioplankton biomass was 11–12 mmol C m−2. The percentage of bacterial to phytoplankton biomass per volume in summertime was 27% in the epilimnion and 11% in the hypolimnion. Average annual and summertime bacterial production estimated using the [3H]thymidine method was similar to previous estimates of bacterial production measured in Mirror Lake using other methods. The average ratio of bacterial to net phytoplankton production per volume was 0.34 in the epilimnion, and between 0.65 and 1 1. depending on depth, in the hypolimnion during summer. Of several variables considered in regression analyses, only temperature explained >50% of the variance in bacterial production in both the hypolimnion and epilimnion. Above 14°C, however, bacterial production and growth rate in the epilimnion were not clearly related to temperature. During the period of midsummer hypolimnetic anoxia, despite colder temperatures in the hypolimnion, bacterial production was up to 10 times greater than in the epilimnion.