Diatom species fluxes in the seasonally ice-covered Antarctic Zone: New data from offshore Prydz Bay and comparison with other regions from the eastern Antarctic and western Pacific sectors of the Southern Ocean

The Antarctic Zone, the southernmost belt of the Antarctic Circumpolar Current, plays an important role in the control of atmospheric carbon dioxide concentrations. In the last decade, a number of studies have highlighted the importance of diatom assemblage composition in influencing the magnitude of the organic carbon and biogenic silica fluxes exported out of the mixed layer in Southern Ocean ecosystems. Here we investigate the relationship between the makeup of the diatom assemblage, organic carbon and biogenic silica export and several significant environmental parameters using sediment trap records deployed in different sectors of the Antarctic Zone. The study is divided in two parts. We first present unpublished diatom species flux data collected by a sediment trap in the offshore waters of Prydz Bay (Station PZB-1) over a year. The results of this study revealed a major export peak of diatom valves in Austral summer and two small unexpected secondary flux pulses during full winter conditions. The summer diatom sinking assemblages were largely composed of small and rapidly dividing species such as Fragilariopsis cylindrus, Fragilariopsis curta and Pseudo-nitzschia lineola, while winter assemblages were dominated by Fragilariopsis kerguelensis most reflecting its persistent strategy and selective preservation. In the second part of the study, we compare the annual diatom assemblage composition and biogeochemical fluxes of Station PZB-1 with flux data documented in previous sediment trap studies conducted in other sectors of the Antarctic Zone in order to investigate how diatom floristics influence the composition and magnitude of particle fluxes in the Antarctic Zone. The lack of correlation between the annual diatom valve, organic carbon and biogenic silica fluxes across stations indicates that other factors aside from diatom abundance play a major role in the carbon and silica export in AZ. Among these factors, the composition of the diatom assemblage appears to be critical, as suggested by the strong and significant correlation between Bio-SiO₂ and the valve fluxes of F. kerguelensis alone, that this species is the main Bio-SiO₂ vector from the surface layer to the deep ocean in the AZ waters, regardless of its relative abundance. Lastly, the good correlation between the annual fluxes of the group of small Fragilariopsis species with satellite-derived chlorophyll-a concentration estimates over the study stations, suggest that high abundances of these species in the Southern Ocean paleorecords could be used as a proxy of high algal biomass accumulation.