ABSTRACT: A mesocosm approach was used to investigate the effects of ocean acidification (OA) on a natural plankton community in coastal waters off Norway by manipulating CO₂ partial pressure (pCO₂). Eight enclosures were deployed in the Raunefjord near Bergen. Treatment levels were ambient (~320 µatm) and elevated pCO₂ (~2000 µatm), each in 4 replicate enclosures. The experiment lasted for 53 d in May-June 2015. To assess impacts of OA on the plankton community, phytoplankton and protozooplankton biomass and total seston fatty acid content were analyzed. In both treatments, the plankton community was dominated by the dinoflagellate Ceratium longipes. In the elevated pCO₂ treatment, however, biomass of this species as well as that of other dinoflagellates was strongly negatively affected. At the end of the experiment, total dinoflagellate biomass was 4-fold higher in the control group than under elevated pCO₂ conditions. In a size comparison of C. longipes, cell size in the high pCO₂ treatment was significantly larger. The ratio of polyunsaturated fatty acids to saturated fatty acids of seston decreased at high pCO₂. In particular, the concentration of docosahexaenoic acid (C 22:6n3c), essential for development and reproduction of metazoans, was less than half at high pCO₂ compared to ambient pCO₂. Thus, elevated pCO₂ led to a deterioration in the quality and quantity of food in a natural plankton community, with potential consequences for the transfer of matter and energy to higher trophic levels