Guntram Weithoff, Andreas Lorke, Norbert Walz

• Water column mixing is known to have a decisive impact on plankton communities. The underlying mechanisms depend on the size and depth of the water body, the nutrient status, and the plankton community structure and they are well understood for shallow polymictic and deep stratified lakes. Two consecutive mixing events of similar intensity under different levels of herbivory were performed in enclosures in a shallow, but periodically stratified, eutrophic lake, in order to investigate the effects of water column mixing on bacteria abundance, phytoplankton abundance and diversity, and rotifer abundance and fecundity. When herbivory by filter-feeding zooplankton was low, water column mixing provoking a substantial nutrient input into the euphotic zone, led to an intense net increase of bacteria and phytoplankton biomass. Phytoplankton diversity was lower in the mixed enclosures than in the undisturbed ones owing to the larger contribution of a few fast-growing species. After the second mixing event at high biomass of filter-feedingWater column mixing is known to have a decisive impact on plankton communities. The underlying mechanisms depend on the size and depth of the water body, the nutrient status, and the plankton community structure and they are well understood for shallow polymictic and deep stratified lakes. Two consecutive mixing events of similar intensity under different levels of herbivory were performed in enclosures in a shallow, but periodically stratified, eutrophic lake, in order to investigate the effects of water column mixing on bacteria abundance, phytoplankton abundance and diversity, and rotifer abundance and fecundity. When herbivory by filter-feeding zooplankton was low, water column mixing provoking a substantial nutrient input into the euphotic zone, led to an intense net increase of bacteria and phytoplankton biomass. Phytoplankton diversity was lower in the mixed enclosures than in the undisturbed ones owing to the larger contribution of a few fast-growing species. After the second mixing event at high biomass of filter-feeding crustaceans, the increase of phytoplankton biomass was lower than after the first mixing, and diversity remained unchanged as the enhanced growth of small fast-growing was prevented by zooplankton grazing. Bacteria abundance did not increase after the second mixing, when cladoceran biomass was high. Rotifer fecundity indicated a transmission of the phytoplankton response to the next trophic level. Our results suggest that water column mixing in shallow eutrophic lakes with periodic stratification has a strong effect on the plankton community by enhanced nutrient availability rather than resuspension or reduced light availability. This fuels the basis of the classic and microbial food chain via enhanced phytoplankton and bacteria growth, but the effects on biomass may be dampened by high levels of herbivory.…