Carbon isotopic fractionation by plankton in the Southern Indian Ocean: relationship between δ13C of particulate organic carbon and dissolved carbon dioxide

Simultaneous measurements of chlorophyll a, carboxylase activities (Rubisco and βcarboxylases) in phytoplankton, organic carbon content and isotopic composition (δ13C(POC)) of suspended particulate organic matter, and partial pressure of carbon dioxide in surface seawater were performed during the end of summer 1992, in the South Indian Ocean. Results indicate that plankton populations display different carbon metabolisms on both sides of the subtropical convergence (STC). North of STC, planktonic communities appeared to be dominated by mixo/heterotrophs, while south of STC, physiological (βC/R ratio) or biomass (C(POC)/Chl) parameters are characteristic of autotrophic phytoplankton. The isotopic fractionation (εp) between dissolved CO2 and SPOM is different for these two populations. Only δ13C values of flora dominated by diatoms south of the STC part showed a strong correlation with dissolved CO2 concentrations which permitted a relationship between these parameters for the Southern Ocean.