POPULATION-DYNAMICS OF BACTERIOPLANKTON IN AN OLIGOTROPHIC LAKE

The population ecology of bacterioplankton was studied over a 3 year p eriod in Mirror Lake, an oligotrophic lake in the northeastern USA. Ba cterial population density, biomass, and rates of biomass production i n the epilimnion and hypolimnion were examined for their relationship with several environmental parameters. Bacterioplankton density fluctu ated between 0.5 and 7 x 10(6) bacteria ml(-1), with highest values in the anoxic hypolimnion. At all depths there was a trend towards a hig her density of bacteria from spring to midsummer, followed by a declin e in late summer to early autumn. Cocci tended to dominate bacterial c ell shapes from winter to midsummer, after which rod-shaped cells beca me most abundant. Rod-shaped cells contributed the most to bacteriopla nkton biomass at all depths and times of year. The mean annual biovolu me of all bacterioplankton was 0.12 mu m cell(-1). The mean annual are al 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 summert ime bacterial production estimated using the [H-3]thymidine method was similar to previous estimates of bacterial production measured in Mir ror Lake using other methods. The average ratio of bacterial to net ph ytoplankton production per volume was 0.34 in the epilimnion, and betw een 0.65 and 1.1, depending on depth, in the hypolimnion during summer . Of several variables considered in regression analyses, only tempera ture explained >50% of the variance in bacterial production in both th e hypolimnion and epilimnion. Above 14 degrees C, however, bacterial p roduction and growth rate in the epilimnion were not clearly related t o temperature. During the period of midsummer hypolimnetic anoxia, des pite colder temperatures in the hypolimnion, bacterial production was up to 10 times greater than in the epilimnion.