Carbon isotopic fractionation by plankton in the Southern Indian Ocean: relationship between delta C-13 of particulate organic carbon and dissolved carbon dioxide

Simultaneous measurements of chlorophyll a, carboxylase activities (Rubisco and beta carboxylases) in phytoplankton, organic carbon content and isotop ic composition (delta(13)C((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 side s of the subtropical convergence (STC). North of STC, planktonic communitie s appeared to be dominated by mixo/heterotrophs, while south of STC, physio logical (beta C/R ratio) or biomass (C-(POC)/Chl) parameters are characteri stic of autotrophic phytoplankton. The isotopic fractionation (epsilon(p)) between dissolved CO2 and SPOM is different for these two populations. Only delta(13)C values of flora dominated by diatoms south of the STC part show ed a strong correlation with dissolved CO2 concentrations which permitted a relationship between these parameters for the Southern Ocean.