Drivers of net community production in the bottom of Arctic pack ice

Abstract. The availability of nutrients to sea-ice microbial communities varies as drifting pack ice traverses distinct hydrographic regimes and experiences episodic mixing events. Vertical turbulent fluxes generated by processes such as ice movement and wind can enhance nutrient resupply at the ice–ocean interface, potentially influencing community composition and net community production. In this study, we evaluate the relationships between bottom sea-ice algal communities and oceanic conditions during a sea-ice drift study north of Svalbard, which crossed different branches of the Atlantic Water inflow to the Arctic over the Yermak Plateau and the Sofia Deep. Despite elevated nitrate concentrations in under-ice waters associated with the Svalbard Branch of Atlantic Water inflow, the largest nutrient and heat fluxes to the bottom ice occurred during a storm event. These fluxes coincided with increased biomass-specific net community productivity but also enhanced algal biomass loss through melt. At the same time, snow redistribution during the storm increased snow depths and reduced light availability at the bottom ice, affecting net community production. Independent of nutrient conditions, the transition from net heterotrophy (oxygen consumption) to net autotrophy (oxygen production) occurred when under-ice transmitted photosynthetically active radiation exceeded a daily range of 1.1 to 3.5 µmol m^–2 s^–1. Although nutrient supply remains important, our results suggest that light availability may exert a stronger control on sea-ice net community production than nutrient supply when nutrients are not critically depleted. We further highlight the importance of episodic storm events in modifying nutrient and heat vertical fluxes, and light availability at the ice–ocean interface, thereby contributing to regional variability in ice algal bloom dynamics.