The effect of marginal ice-edge dynamics on production and export in the Southern Ocean along 170°W

We present a synthesis of the rates of gross, new, net and primary production along with particulate organic carbon (POC) flux at 100 m from four cruises along 170°W in the Southern Ocean. Concurrent satellite pigment data, a primary productivity model, and a nitrate mass balance are used to extrapolate daily production estimates in space and time to seasonal and annual rates. From this analysis, we gain a better understanding of the timing, magnitude and impact of the phytoplankton blooms in this region. One of the dominant features with respect to plankton biomass is the association of high chlorophyll levels with the retreat of the sea ice which begins in October just south of the Polar Front. Our primary production model and satellite pigment data suggest higher production and flux levels characterize the marginal ice zone than previous estimates. Elevated rates of new production in ice impacted regions are restricted to relatively short periods following retreat of the ice edge. Export is found to lag the onset of production by up to 1 month. The ratio of POC flux at 100 m to primary production when averaged over the entire season is quite high, increasing from 15% to 25% in the Subantarctic zones to 35–40% near the Polar Front and as high as 50–65% in the southernmost stations, just north of the Ross Sea gyre. Comparisons of phytoplankton community structure and Fe stress indicators suggest that blooms at the Polar Front are initially dominated by large centric diatoms, but are replaced by smaller pennate diatom and non-diatom species as Fe levels decrease. Further south, where Fe levels are never as high and large diatoms are not found, we still observe relatively high biomass and elevated production rates and POC fluxes during the short growing season.