The role of sea ice biota for the ecosystem in the northwestern Weddell Sea

The western Weddell Sea along the northward branch of the Weddell Gyre is a region of major outflow of various water masses, thick sea ice, and biogeochemical matter, linking the Antarctic continent to the world oceans. This region is undergoing dramatic changes due to the breakup of ice shelves along the Antarctic Peninsula, however, its perennial sea ice cover is among the thickest on Earth and thus make it hard accessible and extensive information on biogeochemical variables and biodiversity are still lacking. During the interdisciplinary Weddell Sea Ice (WedIce) project, carried out in the northwestern Weddell Sea on board the German icebreaker RV Polarstern in spring 2019, in-situ snow and ice sampling including biodiversity and biogeochemical studies were carried out. We investigated mostly second- and third-year ice and station work was accompanied by airborne ice-thickness surveys. Preliminary results show mean ice thicknesses between 2.6 and 5.4 m, increasing from the Antarctic Sound towards the Larsen B region. Due to the summer’s thaw, an average of 0.14 m of superimposed ice was found in all studied ice cores. Although there was rotten ice below a solid, ~30 cm thick surface-ice layer, pronounced gap layers, typical for late summer ice in the marginal ice zone, were rare and high algal biomass, dominated by Phaeocystis, was only found north of the Antarctic Sound. However, diatom dominated standing stocks of integrated sea ice algae biomass are among the highest in Antarctica and the spatial variability in standing stocks and vertical distribution are likely governed by the origin of the sampled ice floe. Despite high macro-nutrient concentrations of the water, the biomass of the flagellate dominated phytoplankton was negligible for primary production in the entire region. It thus can be concluded that despite changing light condition for the phytoplankton, sea ice derived carbon represents an important source for higher trophic levels in this Region.