Characterizing the Carbon Isotopic Composition of Dissolved Inorganic Carbon in Sea Ice Pore Water as a Carbon Source for Sea Ice Algae in the Arctic

Sea ice algae in the Arctic are an important food source for benthic and pelagic organisms. Ice algae utilize the dissolved inorganic carbon (DIC) in brine channels of the sea ice matrix as carbon source during photosynthesis. The relatively closed system within the brine channels has been hypothesized to cause carbon limitation as a means of accounting for the high δ¹³C values of ice algae that have been recorded in the Arctic, relative to open ocean phytoplankton. We provide the first empirical data documenting the stable carbon isotope composition of DIC from pore water taken from sea ice in the Arctic. We collected DIC and ice algal samples during the SUBICE project to the Chukchi Sea. The mean and max δ¹³C values of DIC from sea ice pore water were higher, ~2‰ and ~5‰ respectively, compared with DIC in open water collected from directly underneath the ice. δ¹³C DIC values generally decreased throughout the ice cores sampled, with increasing distance to the seawater interface, likely due to reduced ice algal production higher in the core. Corresponding ice algal samples were also measured for their δ ¹³C values. Over the course of 15 days (Jun 3 – Jun 18) at the same ice floe, the δ¹³C values of DIC and core temperature decreased for all ice core depths, likely linked to the opening of the brine channels to the surrounding ocean, allowing exchange with ocean water. Calculated brine salinity also decreased during the study period. Our research provides insight into the mechanisms leading to the relatively high δ¹³C values in sea ice algae in the Arctic, which can be used to follow the fate of ice algae through the arctic food web.