The importance 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. It features a deep shelf and the second largest ice shelf (Larsen C) in the WS, and its perennial sea ice cover is among the thickest on earth. This region is undergoing dramatic changes due to the breakup of ice shelves along the Antarctic Peninsula, which results in oceanographic conditions unprecedented in the past 10,000 years. Since this region is difficult to access, comprehensive physical and biogeochemical information is still lacking. During the interdisciplinary Weddell Sea Ice (WedIce) expedition to the northwestern Weddell Sea on board the German icebreaker RV Polarstern in spring 2019, oceanographic and biogeochemical studies were conducted together with in-situ snow and ice sampling. Most stations visited contained second- and third-year ice. Additional airborne ice-thickness surveys revealed a mean ice thicknesses between 2.6 and 5.4 m, increasing from the Antarctic Sound towards the Larsen B region. Usually rotten ice was present below a solid, ~30 cm thick surface-ice layer, however, pronounced gap layers, typical for late summer ice in the marginal ice zone, were rare. The associated high algal biomass was only found north of the Antarctic Sound. Nevertheless, diatom-dominated standing stocks of integrated sea ice algae biomass were among the highest, previously described in Antarctic waters. In contrast, despite overall high macro-nutrient concentrations in the water, the biomass of the flagellate dominated phytoplankton was negligible for primary production in the entire region. Overall, it seems that despite changing light conditions for the phytoplankton due to the loss of ice shelves, the sea ice-derived carbon represents an important control variable for higher trophic levels in the western Weddell Sea.