New insights into the primary production and the structure of the phytoplankton community in the South Indian Ocean using size fractionation experiments

Abstract. As part of the South Indian Ocean CARBon fluxes from the surface to the mesopelagic twilight zone (SOCARB) project, the phytoplankton biomass and net primary production (NPP), along with the biomass of phytoplankton chemotaxonomic groups, were assessed in contrasting biogeochemical areas of the South Indian and Southern Oceans in late austral summer 2023. A size fractionation approach was performed to characterize the size structure of primary production and phytoplankton chemotaxonomic groups biomass in three size classes: picophytoplankton (< 3 µm), nanophytoplankton (3–20 µm), and microphytoplankton (> 20 µm). Across the study area, NPP was dominated by microphytoplankton (56 % ± 21 %) while total chlorophyll a (TChla) was sustained by nano- (40 % ± 11 %) and microphytoplankton (37 % ± 18 %), notably by nanophytoplankton haptophytes and microphytoplankton diatoms. Our results highlighted the spatial variability of NPP and TChla size structures, mainly driven by temperature, salinity and macronutrients – mainly N, P. In the Subtropical and Subantarctic zones, NPP and TChla were sustained by pico- and nanophytoplankton with a diversified community (cyanobacteria, haptophytes, chlorophytes, pelagophytes). Conversely in the Polar Frontal and Antarctic zones, NPP and TChla were dominated by nano- and microphytoplankton with a less diversified community (diatoms, haptophytes). Our results also underline the intra-zonal variability of NPP and TChla through bottom-up processes, such as cyclonic eddy in the Subtropical zone or Si-depleted water mass intrusion in the Polar Frontal zone. Focusing on the links between NPP and TChla size structure across the study area, NPP was mainly driven by the biomass of nano- and microphytoplankton, more specifically by the biomass of nano- and microphytoplankton diatoms, haptophytes and dinoflagellates. This study paves the way for a better understanding of phytoplankton productivity and community size structure, which could contribute to a more detailed knowledge on their role in the biological carbon pump.