In order to assess the effects of ocean acidification (OA) on natural plankton communities, a large-scale mesocosm experiment was conducted in Gullmar Fjord (North Sea) from a spring bloom to a mid-summer situation. Here, the emphasis was on the pivotal role of microzooplankton (MZP) as trophic intermediary between the microbial loop and higher trophic levels. Due to its high specific growth and grazing rates, MZP has a strong impact on phytoplankton standing stocks which can lead to dietary competition between MZP and mesozooplankton. Furthermore, the ability of MZP to act as trophic upgraders for mesozooplankton by buffering nutritional imbalances of algae might gain importance given the expected decrease in algal food quality with OA. We present data on MZP communities (abundance, biomass, taxonomic composition) with emphasis on phytoplankton-MZP-mesozooplankton interactions. Overall, two phytoplankton peaks occurred (1st peak: around day 30, 2nd peak: around day 50). While there was no direct numerical response of MZP biomass to increases in phytoplankton biomass during the 1st peak, a clear numerical response of MZP to the 2nd peak was observed. However, no significant differences in MZP biomass with regard to the different CO2 scenarios occurred. In general, the ciliate community was dominated by small Strombidiids and no clear CO2 impacts on the ciliate community composition could be detected. Additional grazing experiments conducted during the 1st phytoplankton peak showed negative phytoplankton growth rates throughout. MZP grazing could only be detected in some of the low pCO2 treatments, pointing towards complex responses of MZP communities to OA.