Influence of metazoan zooplankton on the microbial community before and after the onset of the spring clear-water phase in Lake Constance (Bodensee)

Shortly before and shortly after the onset of the spring clear-water phase, we took plankton samples from the epilimnion of Lake Constance and incubated them in the laboratory under in situ conditions. In three duplicated treatments, mesozooplankton gt 100 mu-m were removed, left at natural densities or enriched nine-fold. Before the clear-water phase, cyclopoid copepods dominated the mesozooplankton, bacteria probably were controlled by high heterotrophic flagellate densities, and the most important phytoplankton grazers apparently were in the fraction lt 100 mu (ciliates, rotifers). In the copepod enrichments, these small grazers were decimated and chlorophyll a and autotrophic picoplankton reached the highest concentrations. Heterotrophic flagellates did not respond clearly to the different treatments. After the onset of the clear-water phase, Daphnia dominated the mesozooplankton and controlled most components of the microbial food web as well as the phytoplankton. All biomass parameters of the phytoplankton, autotrophic picoplankton, heterotrophic flagellates, and ciliates increased when Daphnia were removed. Bacterial abundances did not respond clearly to the removal of mesozooplankton, because protozoans became important bacterial grazers when they were released from the control by Daphni