Oxygen fluxes beneath Arctic land‑fast ice and pack ice: towards estimates of ice productivity

Sea-ice ecosystems are among the most extensive of Earth’s habitats; yet its autotrophic and heterotrophic activities remain poorly constrained. We employed the in situ aquatic eddy-covariance (AEC) O₂ flux method and laboratory incubation techniques (H¹⁴CO₃−, [³H] thymidine and [³H] leucine) to assess productivity in Arctic sea-ice using different methods, in conditions ranging from land-fast ice during winter, to pack ice within the central Arctic Ocean during summer. Laboratory tracer measurements resolved rates of bacterial C demand of 0.003–0.166 mmol C m−² day−¹ and primary productivity rates of 0.008–0.125 mmol C m−² day−¹ for the different ice floes. Pack ice in the central Arctic Ocean was overall net autotrophic (0.002–0.063 mmol C m−² day−¹), whereas winter land-fast ice was net heterotrophic (− 0.155 mmol C m−² day−¹). AEC measurements resolved an uptake of O₂ by the bottom-ice environment, from ~ − 2 mmol O₂ m−² day−¹ under winter land-fast ice to~ − 6 mmol O₂ m−² day−¹ under summer pack ice. Flux of O₂-deplete meltwater and changes in water flow velocity masked potential biological-mediated activity. AEC estimates of primary productivity were only possible at one study location. Here, productivity rates of 1.3 ± 0.9 mmol O₂ m−² day−¹, much larger than concurrent laboratory tracer estimates (0.03 mmol C m−² day−¹), indicate that ice algal production and its importance within the marine Arctic could be underestimated using traditional approaches. Given careful flux interpretation and with further development, the AEC technique represents a promising new tool for assessing oxygen dynamics and sea-ice productivity in ice-covered regions.