New insights into the primary production and the structure of the phytoplankton community in the Southern Indian Ocean from the subtropical to the Antarctic zones

Assessing and understanding the factors that control the biological carbon pump (BCP), i.e. the transfer of organic carbon biologically fixed by primary production (PP) from the euphotic zone to the deep ocean, remains a major challenge in marine biogeochemistry. Among these factors, the intensity of PP and the structure of phytoplankton community play key roles in the biogeochemical fluxes of the BCP and depend on the physico-chemical conditions of the ocean. Although the BCP has received significant attention in the last decades, the magnitude of this process remains poorly quantified, notably for under-sampled areas such as the Indian sector of the Southern Ocean (ISSO). The latter hosts contrasting biogeochemical provinces, from low productive systems with High Nutrient Low Chlorophyll areas, to high productive regimes in the vicinity of the Subantarctic Islands as a consequence of natural iron fertilization.

This study aims to assess the links between the PP and phytoplankton community structure in the ISSO. We present results from the SOCARB (South Indian Ocean CARBon fluxes from the surface to the mesopelagic twilight zone) cruise conducted during the late austral summer of 2023. This includes: (i) PP from ¹³C method and on-deck incubations, (ii) relative phytoplankton chemotaxonomic groups from pigments data and total chlorophyll a (TChla) and (iii) phytoplankton size classes abundances from on board cytometry flow analyses. PP and pigments were size-fractioned (< 3 µm; 3-20 µm; > 20 µm) following the three considered phytoplankton classes (pico-; nano-; microphytoplankton) to quantify their impact on organic carbon production and to address the size structure of the phytoplankton community.

At the Antarctic zone (AZ) and the Polar Frontal zone (PFZ), integrated TChla – over the 0.01 % euphotic layer depth – was structured by TChla_NANO (46 ± 12 %) and TChla_MICRO (40 ± 14 %) and featured a community dominated by diatoms and haptophytes (68 ± 8 %). The Subantarctic zone (SAZ) differs from the rest of the Southern Ocean (i.e. south of the subtropical front) with a distinct community and a TChla structured in pico- (42 %) and nano- (36 %). In the South Indian Ocean, the Subtropical zone (STZ) exhibited a TChla structured by TChla_PICO (43 ± 8 %) and TChla_NANO (39 ± 9 %) with a diversified community. From linear correlations and relative contribution of phytoplankton groups to TChla, we show that PP in the AZ and PFZ is conditioned by diatoms and haptophytes algal biomass in both nano- and micro- size classes. In the STZ, PP is mainly conditioned by the algal biomass of cyanobacteria in the pico- and by haptophytes, chlorophytes and dinoflagellates in the nano- size classes. Our results also underline the intra-zonal variability of PP and TChla through bottom-up processes, such as cyclonic eddy in the STZ or water mass intrusion in the PFZ. This study paves the way for a better comprehension of phytoplankton productivity and community size structure, which could contribute to a more detailed knowledge on their role in the BCP.