Description: In the frame of the German BIOACID project, we investigate the impacts of acidification and temperature changes on the quantity and quality of the marine planktonic food web. We aim to understand the interaction between warming and acidification effects on primary producers (phytoplankton) and consumers (copepods) when embedded in natural plankton communities. Laboratory experiments with single species and two-species food chains have shown temperature effects on production, growth rates, biomass accumulation and biochemical composition. In the latter case, the content of polyunsaturated fatty acids (PUFAs) is of particular interest because of their decisive role in food quality for herbivores. Since the effects of CO2-enrichment and temperature have usually been studied in isolation, we were also interested in interactive effects of both aspects of climate change. Therefore, we used a factorial design with two temperature and two acidification levels for our mesocosm experiment with Baltic Sea phytoplankton. Phytoplankton responded mainly to temperature. Average as well as maximum bloom biomass phytoplankton carbon) decreased significantly with increasing temperature. Impacts of CO2 on phytoplankton biomass could not be detected, while zooplankton responded both to temperature and CO2. Copepod abundance decreased with increasing temperatures but increased with higher pCO2. Biochemical composition, in particular fatty acids, also responded to the experimental treatments, as shown by the ratio of PUFAs, saturated FA (SFA) and individual FA to total FA (TFA). Again, phytoplankton was affected by temperature. The average TFA content was highest under warm temperature. The PUFA:TFA ratio increased with blooming under both temperature regimes. Copepods showed also a more complex picture. The TFA content of individuals did not change in response to temperature and CO2 while the composition of lipids changed. The ratio SFA:TFA increased with warmer temperature while the PUFA:TFA ratio was higher at cold temperatures. An interaction effect of temperature and CO2 could only be seen in the SFA:TFA, which increased with warming but only in colder temperatures SFA:TFA ratio increased with higher CO2.