Marine ecosystem community carbon and nutrient uptake stoichiometry under varying ocean acidification during the PeECE III experiment

Changes to seawater inorganic carbon and nutrient concentrations in response to the deliberate CO<sub>2</sub> perturbation of natural plankton assemblages were studied during the 2005 Pelagic Ecosystem CO<sub>2</sub> Enrichment (PeECE III) experiment. Inverse analysis of the temporal inorganic carbon dioxide system and nutrient variations was used to determine the net community stoichiometric uptake characteristics of a natural pelagic ecosystem perturbed over a range of <i>p</i>CO<sub>2</sub> scenarios (350, 700 and 1050 μatm). Nutrient uptake showed no sensitivity to CO<sub>2</sub> treatment. There was enhanced carbon production relative to nutrient consumption in the higher CO<sub>2</sub> treatments which was positively correlated with the initial CO<sub>2</sub> concentration. There was no significant calcification response to changing CO<sub>2</sub> in <i>Emiliania huxleyi</i> by the peak of the bloom and all treatments exhibited low particulate inorganic carbon production (~15 μmol kg<sup>−1</sup>). With insignificant air-sea CO<sub>2</sub> exchange across the treatments, the enhanced carbon uptake was due to increase organic carbon production. The inferred cumulative C:N:P stoichiometry of organic production increased with CO<sub>2</sub> treatment from 1:6.3:121 to 1:7.1:144 to 1:8.25:168 at the height of the bloom. This study discusses how ocean acidification may incur modification to the stoichiometry of pelagic production and have consequences for ocean biogeochemical cycling