Antarctic landfast sea ice: autotrophy vs heterotrophy, sink vs source of CO2

Sea ice is a biome actively participating in the regional cycling of CO2 as both a source and a sink at different times of the year depending on its trophic status (autotrophic vs heterotrophic). In the frame of the YROSIAE project (Year-Round survey of Ocean-Sea-Ice-Atmosphere Exchanges), carried out at Cape Evans in McMurdo Sound (Antarctica) from Nov. 2011 to Dec. 2012, ice cores, seawater, and brines were collected at regular time intervals. We used dissolved inorganic carbon (DIC) and chlorophyll-a (chl-a) as proxies of net community production and autotrophic biomass, respectively. From spring, very high chl-a concentrations (>2400𝜇𝑔.𝐿!!) were observed at the bottom of the ice. This suggests high primary production. Strikingly, at the same time, nutrients increased significantly indicating strong remineralization at the bottom. In the ice interior, evolution of DIC was marked by a succession of autotrophic and heterotrophic phases. The overall increase of DIC suggests that the ice interior was rather heterotroph. Such sea ice system should expel CO2. Yet, strong under-saturation in CO2 and DIC depletion appeared at the ice surface, suggesting that sea ice should take up CO2 from the atmosphere. On the whole, land fast sea ice in McMurdo Sound appears as a puzzling ecosystem. High primary production and remineralization develop simultaneously at the bottom while the top of the ice is rather heterotrophic but still able to pump CO2 from the atmosphere.