Abstract. A CO₂ enrichment experiment (PeECE III) was carried out in 9 mesocosms in which the seawater carbonate system was manipulated to achieve three different levels of pCO₂. At the onset of the experimental period, nutrients were added to all mesocosms in order to initiate phytoplankton blooms. Primary production rates were measured by in-vitro incubations based on ¹⁴C-incorporation and oxygen production/consumption. Size fractionated particulate primary production was also determined by ¹⁴C incubation and is discussed in relation to phytoplankton composition. Primary production rates increased in response to nutrient addition and a net autotrophic phase with ¹⁴C-fixation rates up to 4 times higher than initial was observed midway through the 24 days experiment before net community production (NCP) returned to near-zero and ¹⁴C-fixation rates dropped below initial values. No clear heterotrophic phase was observed during the experiment. Based on the ¹⁴C-measurements we found higher cumulative primary production at higher pCO₂ towards the end of the experiment. CO₂ related differences were also found in size fractionated primary production. The most noticeable responses to CO₂ treatments with respect to primary production rates occurred in the second half of the experiment when phytoplankton growth had become nutrient limited, and the phytoplankton community changed from diatom to flagellate dominance. This opens for two alternative hypotheses that the effects are either associated with mineral nutrient limited growth, and/or with a change in phytoplankton species composition. The lack of a clear net heterotrophic phase in the last part of the experiment supports the idea that a substantial part of production in the upper layer was not degraded locally, but either accumulated or exported vertically.