We studied the effect of ocean acidification (OA) on a coastal North Sea plankton community in a long-term mesocosm CO₂-enrichment experiment (BIOACID II long-term mesocosm study). From March to July 2013, 10 mesocosms of 19 m length with a volume of 47.5 to 55.9 m³ were deployed in the Gullmar Fjord, Sweden. CO₂ concentrations were enriched in five mesocosms to reach average CO₂ partial pressures (pCO₂) of 760 μatm. The remaining five mesocosms were used as control at ambient pCO₂ of 380 μatm. Our paper is part of a PLOS collection on this long-term mesocosm experiment. Here, we here tested the effect of OA on total primary production (PP_T) by performing ¹⁴C-based bottle incubations for 24 h. Furthermore, photoacclimation was assessed by conducting ¹⁴C-based photosynthesis-irradiance response (P/I) curves. Changes in chlorophyll a concentrations over time were reflected in the development of PP_T, and showed higher phytoplankton biomass build-up under OA. We observed two subsequent phytoplankton blooms in all mesocosms, with peaks in PP_T around day 33 and day 56. OA had no significant effect on PP_T, except for a marginal increase during the second phytoplankton bloom when inorganic nutrients were already depleted. Maximum light use efficiencies and light saturation indices calculated from the P/I curves changed simultaneously in all mesocosms, and suggest that OA did not alter phytoplankton photoacclimation. Despite large variability in time-integrated productivity estimates among replicates, our overall results indicate that coastal phytoplankton communities can be affected by OA at certain times of the seasonal succession with potential consequences for ecosystem functioning.