Population dynamics of bacterioplankton in an oligotrophic lake

The population ecology of bacterioplankton was studied overa 3 year period in Mirror Lake, an oligotrophic lake in thenortheastern USA. Bacterial population density, biomass, andrates of biomass production in the epilimnion and hypolimnionwere examined for their relationship with several environmentalparameters. Bacterioplankton density fluctuated between 0.5and 7 x l0 bacteria ml^–1, with highest values in the anoxichypolimnion. At all depths there was a trend towards a higherdensity of bacteria from spring to midsummer, followed by adecline in late summer to early autumn. Cocci tended to dominatebacterial cell shapes from winter to midsummer, after whichrod-shaped cells became most abundant. Rod-shaped cells contributedthe most to bacterioplankton biomass at all depths and timesof year. The mean annual biovolume of all bacterioplankton was0.12 µ cell^–1. The mean annual areal bacterioplanktonbiomass was 11–12 mmol C m^–2. The percentage ofbacterial to phytoplankton biomass per volume in summertimewas 27% in the epilimnion and 11% in the hypolimnion. Averageannual and summertime bacterial production estimated using the3H]thymidine method was similar to previous estimates of bacterialproduction measured in Mirror Lake using other methods. Theaverage ratio of bacterial to net phytoplankton production pervolume was 0.34 in the epilimnion, and between 0.65 and 1 1.depending on depth, in the hypolimnion during summer. Of severalvariables considered in regression analyses, only temperatureexplained >50% of the variance in bacterial production inboth the hypolimnion and epilimnion. Above 14°C, however,bacterial production and growth rate in the epilimnion werenot clearly related to temperature. During the period of midsummerhypolimnetic anoxia, despite colder temperatures in the hypolimnion,bacterial production was up to 10 times greater than in theepilimnion.