Abstract
Bacterioplankton abundance, [H]thymidine incorporation, CO(2) uptake in the dark, and fractionated primary production were measured on several occasions between June and August 1982 in eutrophic Lake Norrviken, Sweden. Bacterioplankton abundance and carbon biomass ranged from 0.5 x 10 to 2.4 x 10 cells liter and 7 to 47 mug of C liter, respectively. The average bacterial cell volume was 0.185 mum. [H]thymidine incorporation into cold-trichloroacetic acid-insoluble material ranged from 12 x 10 to 200 x 10 mol liter h. Bacterial carbon production rates were estimated to be 0.2 to 7.1 mug of C liter h. Bacterial production estimates from [H]thymidine incorporation and CO(2) uptake in the dark agreed when activity was high but diverged when activity was low and when blue-green algae (cyanobacteria) dominated the phytoplankton. Size fractionation indicated negligible uptake of [H]thymidine in the >3-mum fraction during a chrysophycean bloom in early June. We found that >50% of the H activity was in the >3-mum fraction in late August; this phenomenon was most likely due to Microcystis spp., their associated bacteria, or both. Over 60% of the CO(2) uptake in the dark was attributed to algae on each sampling occasion. Algal exudate was an important carbon source for planktonic bacteria. Bacterial production was roughly 50% of primary production.