SUMMER DYNAMICS OF THE COASTAL PLANKTONIC FOOD-WEB IN THE NORTHERN BALTIC SEA

Summer carbon now in the pelagic food web was studied within the coast al region of the northern Baltic Sea. Heterotrophic nanoflagellates (H NAN) transferred carbon from picoplankton to ciliates, while ciliates Linked nano- and metazooplankton. Hydrodynamic conditions affected the planktonic community, and 3 different periods characterised by distin ct community structure and carbon flow dynamics could be distinguished . All trophic groups were largely omnivorous, but the role of herbivor y and feeding on heterotrophs varied during summer. In early June, an upwelling mixed the upper water layer. The water column was rich in de tritus, remnants from the preceding spring phytoplankton bloom. Phytop lankton was dominated by >10 mu m phytoplankton, that was grazed by me tazooplankton and heterotrophic dinoflagellates. At the same time HNAN were largely bacteriovorous, thus the microbial food web (MFW) could transfer bacterial carbon to ciliates and metazooplankton. In July dur ing strong thermal stratification the biomass of <10 mu m phytoplankto n increased, and protists became the most important herbivores. In add ition to grazing, metazooplankton gained nanophytoplankton carbon thro ugh the MFW by feeding on ciliates. Strong horizontal currents due to upwelling destroyed the stable mid-summer community in late July. Prim ary production, the biomass of phytoplankton and metazooplankton decli ned abruptly, whereas the detritus pool increased again. Because all t rophic groups fed largely on heterotrophs, the efficiency of the MFW i n transferring bacterial carbon to metazooplankton was estimated to be at its highest. When integrated throughout the summer, primary produc tion was 20.5 g m(-2), from which sedimentation, herbivory by protists and herbivory by metazooplankton accounted for 27, 40 and 26%, respec tively. Bacterial production was 8 g m(-2), from which HNAN and ciliat es grazed approximately 60%. Excepting the sedimentation in early June , carbon was mostly recycled within the planktonic community during th e summer.