Annual particulate matter and diatom export in a high nutrient, low chlorophyll area of the Southern Ocean

Abstract

Upper ocean plankton assemblages are known to influence the export of carbon and biominerals from the mixed layer. However, relationships between plankton community structure and the magnitude and stoichiometry of export remain poorly characterized. We present data on biogeochemical and diatom export fluxes from the annual deployment of a sediment trap in a High Nutrient, Low Chlorophyll (HNLC) area upstream of the Kerguelen Plateau (KERFIX station). The weak and tidal-driven circulation provided favorable conditions for a quantitative analysis of export processes. Particulate organic carbon (POC) fluxes were highest in spring and summer. Biogenic silica (BSi) fluxes displayed similar seasonal patterns, although BSi:POC ratios were elevated in winter. Fragilariopsis kerguelensis dominated the annual diatom export assemblage (59.8% of the total valve flux). We identified clusters of diatom species that were positively or negatively correlated to the BSi:POC ratio. Our results indicate that the differential role of certain diatom species for carbon and silicon export, previously identified from iron-fertilized productive areas, is also valid in HNLC regimes. Although annual POC export below the mixed layer of the HNLC site is twofold lower that the one previously reported in a naturally iron-fertilized area of the Kerguelen Plateau, the fraction of seasonal net community production exported is similar at both sites (~1.5%). These findings suggest that natural iron fertilization increases the strength but not the efficiency of carbon export from the mixed layer.