Availability of phosphate for phytoplankton and bacteria and of labile organic carbon for bacteria at different pCO2 levels in a mesocosm study
1Marine Microbiology Res. Group (MMRG), Dept. of Biology, Univ. Bergen, Bergen, Norway
2Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB-Neuglobsow), Department of Limnology of Stratified Lakes, Alte Fischerhuette 2, 16775 Stechlin, Germany
3Leibniz Inst. for Marine Sciences (IFM-GEOMAR), Düsternbrooker Weg 20, 24105 Kiel, Germany
*now at: Laboratoire d'Océanographie et de Biogéochimie, UMR6535-CNRS, Centre d'Océanologie de Marseille, Campus de Luminy-Case 901, 13288 Marseille CEDEX09, France
**now at: Faculty of Science and Technology, Department of Mathematics and Natural Sciences, University of Stavanger, 4036 Stavanger, Norway
Abstract. 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 μatm: 1×CO2; 750 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) 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.