Microzooplankton (MZP) plays an important role as intermediary between the microbial loop and higher trophic levels. Due to high growth and grazing rates, MZP has a strong impact on phytoplankton biomass and community composition thus leading to dietary competition with mesozooplankton. Ocean acidification (OA) is anticipated to cause food quality alterations by changing algal stoichiometry in disfavor of grazers and the ability of MZP to buffer nutritional imbalances for higher trophic levels is expected to gain importance. Within the framework of an outdoor mesocosm experiment in the North Sea (BIOACID II), we investigated the effects of elevated CO2 on natural plankton communities. From a spring to a mid-summer situation, MZP abundance, biomass and species composition were analyzed. Grazing experiments provided additional information on the MZP grazing potential. Results show a predominance of small Strombidiids and a lower MZP biomass at high CO2 during the bloom. This finding is contradictory to the hypothesis that MZP growth will increase at high CO2 due to enhanced phytoplankton growth, thus pointing at more complex responses of MZP to ocean acidification.