Availability of phosphate for phytoplankton and bacteria and of labile organic carbon for bacteria at different pCO₂ levels were studied in a mesocosm experiment (PeECE III). Using nutrient-depleted SW Norwegian fjord waters, three different levels of pCO₂ (350 ?atm: 1×CO₂; 750 ?atm: 2×CO₂; 1050 ?atm: 3×CO₂) were set up, and nitrate and phosphate were added at the start of the experiment in order to induce a phytoplankton bloom. Despite similar responses of total particulate P concentration and phosphate turnover time at the three different pCO₂ levels, the size distribution of particulate P and ³³PO₄ uptake suggested that phosphate transferred to the >10 ?m fraction was greater in the 3×CO₂ mesocosm during the first 6?10 days when phosphate concentration was high. During the period of phosphate depletion (after Day 12), specific phosphate affinity and specific alkaline phosphatase activity (APA) suggested a P-deficiency (i.e. suboptimal phosphate supply) but not a P-limitation for the phytoplankton and bacterial community at the three different pCO₂ levels. Although specific phosphate affinity and specific APA tended to be higher in 3×CO₂ than in 2×CO₂ and 1×CO₂ mesocosms during the phosphate depletion period, no statistical differences were found. Responses of specific glucose affinity for bacteria were similar at the three different pCO₂ levels. Measured specific glucose affinities were consistently much lower than the theoretical maximum predicted from the diffusion-limited model, suggesting that bacterial growth was not limited by the availability of labile dissolved organic carbon. These results suggest that availability of phosphate and glucose was similar at the three different pCO₂ levels.