Potential Role of Major Phytoplankton Communities on pCO 2 Modulation in the Indian Sector of Southern Ocean

Spatial variations in the phytoplankton community compositions, carbon dioxide system and ancillary water column properties, were monitored across various frontal systems in the Indian Sector of Southern Ocean (ISSO) during austral summer 2013. Four major frontal systems, namely, the Agulhas Return Front (ARF), Southern Subtropical Front (SSTF), Subantarctic Front (SAF) and Polar Front (PF) were identified along the study area. Major groups of phytoplankton were distinguished adopting single marker pigment approach. Statistical computations showed the distribution of diatoms were influenced by the availability of inorganic nutrients (primarily silicate and nitrate) and mixed layer depths (MLD). Haptophytes were strongly dependent on phosphate availability, whereas, picoplankton flourished in water where regenerated nutrient ammonium was present. The lowest surface pCO 2 (267.26–291.5 µatm) along with in situ oxygen production (> 10 µM) was encountered at the two warmer fronts, ARF and SSTF, dominated by haptophytes and picoplankton. The colder Antarctic fronts, SAF and PF were dominated by diatoms where surface pCO 2 was relatively higher (> 350µatm). Poor statistical correlation among temperature and total chlorophyll with pCO 2 revealed complex interplay of multiple factors. Contribution of major phytoplankton groups towards pCO 2 drawdown was computed using a one-dimensional model describing the relative contributions of biological activities. In ISSO, calcifying and photosynthesizing haptophytes were observed to play a crucial role in the “biological pump” of CO 2 drawdown at ARF, SSTF and SAF, while the silicifying micro phytoplankton diatoms and picoplankton were more effective at PF.