Abstract

ABSTRACT: We measured phytoplankton and bacterial biomass and production at weekly intervals during summer in water samples from 2 sites in the shallow, eutrophic Roskilde Fjord, Denmark. In addition we measured Oz-uptake on unfiltered water and size-fractions < 100 pm and < 1 pm. Phytoplankton gross production in the water column was 6.2 g O2 m 2 d ' at Stn 1 and was balanced by pelagic community respiration (Rc, 3.8 g 0; m 2 d ' ) and sediment respiration (2.5 g 0; m"' d-l). Phytoplankton gross respiration (3.0 g O2 m 2 d l ) was temporarily exceeded by pelagic community respiration (3.1 g O2 m 2 d ' ) plus sediment respiration (2.0 g 0; m 2 d l ) at Stn 2 where there is additional production by Littoral plant communities. Phytoplankters and bacteria together accounted for 72 to 85 % of Re and zooplankters for the remainder Phytoplankters respired a large proportion (ca 30%) of their gross production and were apparently mainly grazed by benthic suspension feeders. Phytoplankters dominated pelagic respiration (50% of Re) at Stn 1, which has most phytoplankton, while bacteria dominated (44 % of Re) at Stn 2. We ascribe the relatively larger respiratory activity of bacteria at Stn 2 to additional supply of organic matter from littoral plant communities and frequent sediment resuspension. The importance of bacteria in the pelagic food web was supported by other findings. Bacterial biomass approached phytoplankton carbon biomass at Stn 2 and bacterial net production in the water column was 10 % (Stn 1) and 35 " % (Stn 2) of phytoplankton gross production. Bacterial net production and respiration were linearly related in the bacterial size-fraction (< 1 pm) with a bacterial growth yield of 47 %. We argue that the conversion factors applied to calculate bacterial net production and the mean growth yield attained are reasonable values considering the other measurements of pelagic carbon pools and processes.