Martin Günter Joachim Löder

• This study investigates the role of microzooplankton grazers in the food web of the North Sea at Helgoland Roads. A 2.5 year monitoring showed that dinoflagellates and ciliates are key organisms in terms of planktonic biomass in the North Sea and constitute a highly diverse and abundant community with a clear seasonal trend. The evaluation of methodological sources of error in standard grazing experiments resulted in a new set-up technique, which was tested and shown to represent an improvement of the state-of-the-art method, conserving fragile microzooplankton species and grazer biodiversity. Grazing experiments during a spring bloom showed the overall important role of ciliates and dinoflagellates as phytoplankton grazers when compared to copepods. Their food selectivity shaped the bloom and led to a stabilized assemblage of the bloom forming diatoms. Copepods, in turn, selectively fed on microzooplankton and could therefore be regarded as the main top down controlling factor of this group. Interactions between different microzooplankton predators were investigated within a model system. In spite of theoretical assumptions that predicted the extinction of the smaller dinoflagellate predator, it was promoted by prey items that were pre-conditioned by its larger predator. The advantage for the smaller predator due to this commensalistic element in the predator relationsship was of such magnitude that it potentially enables a stable coexistence of both predators. This study contributed new insights on: (1) the species composition and seasonality of dinoflagellates and ciliates in the North Sea, (2) methodological improvements concerning microzooplankton grazing experiments, (3) the important role of microzooplankton as grazer and also prey and (4) interactive relationships between microzooplankton predators. It furthermore revealed that investigations on the role of microzooplankton in the food web will continue to be a challenge in the future.