Particulate biogenic silica and carbon production rates and particulate matter distribution in the Indian sector of the Subantarctic Ocean

International audienceThe silicon and carbon cycles in the Subantarctic region have been investigated in the summer 1999 during the Antares 4 cruise in the Indian sector of the Southern Ocean. Both biogenic silica (BSi) biomass and production values were low and indicative of the end of the productive period. Biogeochemical parameters revealed that the siliceous components of the phytoplankton were scarce. The study area was characterized by nanoplanktonic communities. Depth-integrated values of BSi showed a north-south positive gradient: average values were 45.65 mmol m−2 in the Polar Frontal Zone (PFZ) and 19.82 mmol m−2 in the Subtropical Zone (STZ). Diatom frustules appeared slightly deteriorated, and BSi distribution suggested particulate matter sedimentation especially at the southern boundary of the study area, where the highest concentrations were found (750 nmol l−1). BSi tended to accumulate along the Subtropical Front, with values reaching up to 420 nmol l−1. Depth-integrated BSi production rates showed an inverse pattern compared to BSi: the maximum and the minimum values were found, respectively, in the STZ (0.72 mmol Si m−2d−1) and in the PFZ (0.25 mmol Si m−2 d−1). The Subantarctic region, located between the PFZ and the STZ, showed intermediate values for both BSi standing stock (31.58 mmol m−2) and production rates (0.50 mmol Si m−2 d−1). A production regime based on orthosilicic acid regeneration is hypothesized in the STZ where surface temperatures appear high enough to sustain higher BSi dissolution rates. During summer, BSi production in the frontal zone of the Indian sector is comparable to the lowest production rates of oligotrophic waters