Within the BIOACID framework, the effects of elevated CO2 levels on microzooplankton were investigated. Apart from its importance as intermediary between the microbial loop and the traditional food web, its high specific growth and feeding rates makes microzooplankton an important competitor for larger mesozooplankton. The ability of microzooplankton to buffer nutritional imbalances of phytoplankton for higher trophic levels will gain even more importance with the anticipated ocean acidification, as this is expected to render the phytoplankton into low quality food by changing the carbon-to-nutrient ratio in disfavour of the grazers. The microzooplankton community was analysed both during indoor and outdoor mesocosm experiments. At the KOSMOS 2013 Gullmar Fjord field campaign, different CO2 levels were applied to the eutrophic North Sea water throughout the entire spring bloom period. During the indoor mesocosm studies at the GEOMAR, different temperature and CO2 scenarios were simulated. The BIOACID Autumn 2012 and Summer 2013 experiments allowed closer insights into the dynamics and seasonality pattern of Baltic Sea plankton communities from the Kiel Bight. Within all three studies, microzooplankton species composition and abundance, biomass and size classes were investigated. Apart from regular samplings which aimed at providing insights into the succession of the natural plankton communities as a whole, modified Landry-Hassett dilution series were performed to assess microzooplankton grazing activities. By choosing the most abundant copepod species as top predator, these small-scale experiments allowed the differentiation between micro- and mesozooplankton grazing impact and to evaluate food preferences of microzooplankton.