Influence of the diatom assemblage structure on the export of C and Si around the Kerguelen plateau

Following up on two previous programs (KEOPS and KEOPS 2), the late summer diatom bloom was investigated in February-March 2018 in the naturally Fe-fertilized region of the Kerguelen plateau during the MOBYDICK cruise. Diatoms are not equal in the export of C and Si, which is due to different life cycles, mortality processes, degree of silicification, but also by their extraordinary morphological and size diversity. Based on the data we collected during MOBYDICK, we aim to highlight the importance of the diatom diversity in controlling the stock and fluxes of C and Si, from the surface to the deep layers. We first describe the diatom assemblage structure in the surface layer, as well as the C contents associated with the main taxa, both above and outside the naturally fertilized area. The sinking particulate flux was investigated through the use of a bottlenet, a new device allowing to filter above 20 µm all particles within discrete layers of the mesopelagic zone. Our results show very different diatom assemblages both spatially and vertically. This is particularly obvious above the plateau where Corethron inerme was dominating the C biomass in the top 70 m, before being rapidly replaced by Eucampia antarctica, which formed a deep silica maximum below. Onboard, we were able to observe distinct mortality modes, ranging from parasitic and viral infection of diatoms, to predation by larger zooplankton organisms. Interestingly, an intermediate level of small microzooplanktonic (50-80 µm) grazers (various Protocystisspecies) belonging to the Phaeodarian group (Rhizaria) was actively grazing on Fragilariopsis kerguelensis, otherwise supposed to be resistant to grazing. Resting spores and winter forms of various diatom species were also observed, potentially leading to distinct export modes. Together, those data will help to better understand the role played by the diatom diversity in modulating the C and Si biogeochemical cycles.