Availability of phosphate for phytoplankton and bacteria and of glucose for bacteria at different pCO₂ levels in a mesocosm study

Abstract. Availability of phosphate for phytoplankton and bacteria and of glucose 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 μatm: 1×CO₂; 700 μatm: 2×CO₂; 1050 μatm: 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 μm 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) rather than a P-limitation for the phytoplankton and bacterial community at the three different pCO₂ levels. Specific phosphate affinity and specific APA tended to be higher in the 3×CO₂ than in the 2×CO₂ and 1×CO₂ mesocosms during the phosphate depletion period, although no statistical differences were found. Glucose turnover time was correlated significantly and negatively with bacterial abundance and production but not with the bulk DOC concentration. This suggests that even though constituting a small fraction of the bulk DOC, glucose was an important component of labile DOC for bacteria. Specific glucose affinity of bacteria behaved similarly at the three different pCO₂ levels with measured specific glucose affinities being consistently much lower than the theoretical maximum predicted from the diffusion-limited model. This suggests that bacterial growth was not severely limited by the glucose availability. Hence, it seems that the lower availability of inorganic nutrients after the phytoplankton bloom reduced the bacterial capacity to consume labile DOC in the upper mixed layer of the stratified mesocosms.