The increasing CO2 concentration in the atmosphere caused by burning fossil fuels leads to increasing pCO2 and decreasing pH in the oceans worldwide. These changes may severely affect the performance and trophic interactions of marine organisms and, thus, ecosystem functioning. Calanoid copepods are key components of pelagic food webs, linking primary production and higher trophic levels. Therefore, our studies aim at a comprehensive understanding of the response of dominant species to ocean acidification. In incubation experiments, direct physiological and ecological effects of elevated pCO2 were investigated in copepodites (stage V) of Calanus finmarchicus, Calanus glacialis and C. hyperboreus; synergistic effects of pCO2 and temperature were studied in female C. hyperboreus. In addition, effects of ocean acidification on zooplankton communities were tackled in large-scale mesocosm experiments, comparing a boreal and a high latitude ecosystem. With few exceptions, we did not find significant direct effects of elevated pCO2 on copepods and our data suggest that this group is generally robust to CO2 concentrations predicted for the end of the century. There is, however, evidence that algal communities may change at elevated pCO2 suggesting that ocean acidification has the potential via indirect effects to influence life cycle events and population dynamics of copepods.