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 coastal region of the northern Baltic Sea. Heterotrophic nanoflagellates (HNAN) transferred carbon from picoplankton to ciliates, while ciliates Linked nano- and metazooplankton. Hydrodynamic conditions affected the planktonic community, and 3 different periods characterised by distinct community structure and carbon flow dynamics could be distinguished. All trophic groups were largely omnivorous, but the role of herbivory and feeding on heterotrophs varied during summer. In early June, an upwelling mixed the upper water layer. The water column was rich in detritus, remnants from the preceding spring phytoplankton bloom. Phytoplankton was dominated by >10 mu m phytoplankton, that was grazed by metazooplankton 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 during strong thermal stratification the biomass of <10 mu m phytoplankton increased, and protists became the most important herbivores. In addition to grazing, metazooplankton gained nanophytoplankton carbon through the MFW by feeding on ciliates. Strong horizontal currents due to upwelling destroyed the stable mid-summer community in late July. Primary production, the biomass of phytoplankton and metazooplankton declined abruptly, whereas the detritus pool increased again. Because all trophic groups fed largely on heterotrophs, the efficiency of the MFW in transferring bacterial carbon to metazooplankton was estimated to be at its highest. When integrated throughout the summer, primary production was 20.5 g m(-2), from which sedimentation, herbivory by protists and herbivory by metazooplankton accounted for 27, 40 and 26%, respectively. Bacterial production was 8 g m(-2), from which HNAN and ciliates grazed approximately 60%. Excepting the sedimentation in early June, carbon was mostly recycled within the planktonic community during the summer.