Unravelling the responses of shallow soft sediment assemblages to rapid glacier retreat in an Antarctic fjord: Carbon and nitrogen cycling

BRAECKMAN U; PASOTTI F; HOFFMANN R; VAZQUEZ SC; TORSTENSSON A; WENZHÖFER F; VANREUSEL A

Simposio; XII SCAR Biology Symposium; 2017

At the northern tip of the Western Antarctic Peninsula lies Potter Cove (PC), a fjord-like small embayment (about 3 km), influenced by the dynamics of the Fourcade Glacier. Since the 1950s, this glacier has been actively retreating, releasing the underlying soft sediments from the ice, exposing them to glacier calving disturbances (e.g. brash ice and ice scouring), increasing discharge of sediment-laden melt waters and to wave action. Several contrasting shallow (<20m) benthic habitat types are present within the bay. Although bathymetric and granulometric characteristics are relatively similar within the cove, the sediment-inhabiting fauna community composition is very patchy and variable, ranging from colonist to medium-developed benthic assemblages as a result of the locally altered conditions.Efficient carbon cycling is especially crucial in this very productive bay. The large annual primary producer biomass (mainly benthic microalgae and large macroalgae) needs to be recycled to the basic nutrients. Since benthic communities are - through their feeding and burrowing activities - strongly involved in the degradation of organic matter, it can be expected that the gradient in development of benthic communities in Potter Cove will be somehow reflected in the local patterns in carbon and nitrogen cycling.In 2015-2016, we had the unique opportunity to measure carbon and nitrogen cycling in situ over a seasonal cycle (summer, winter under ice measurements, and spring). To this aim, skilled divers deployed a set of benthic chambers on the sediment and measured fluxes of oxygen, inorganic carbon and dissolved inorganic nitrogen species at the sediment-water interface. At the same time, the sediment was sampled to assess environmental variables and benthic assemblage structure. Preliminary results show that despite the low water temperatures (0-2°C), benthic carbon and nitrogen cycling rates were similar to those of temperate regions, which highlights the productivity of the area. Carbon cycling in winter was remarkably lower than in spring and summer, which probably relates to a lower activity and/or biomass of the benthos. The most recent ice-free site, also most frequently disturbed by the glacier calving, was characterized by the least developed communities and lowest carbon cycling rates.This seasonal set of carbon and nitrogen cycling measurements along a gradient of benthicassemblage statuses in Potter Cove will provide an example dataset for direct and indirect effects of glacier retreat on benthic ecosystem functioning, representing a unique study in the Western Antarctic Peninsula region.