Impacts of Sea-ice Dynamics and Snow Cover on Arctic Algal Biomass and Production during the N-ICE2015 Drift Expedition.

The Arctic icescape is rapidly transforming from a thick multi-year ice cover to a thinner and largely seasonal first-year ice cover with significant consequences for Arctic primary production inside the sea ice (ice algae) and in the underlying water column (phytoplankton). We studied the effects of changing sea-ice and snow conditions on the seasonality of phytoplankton and sea-ice algae in the marginal ice zone in the Arctic Ocean (AO) north of Svalbard from January to June 2015 during the Norwegian Young Sea Ice cruise (N-ICE2015). This drift expedition provided a unique seasonal data set during the winter-spring transition in the high Arctic pack-ice ecosystem contributing to a realistic forecast of the evolution of the AO marine ecosystem. Phytoplankton productivity stayed low throughout winter and early spring. By late May, a massive under ice bloom (>300 mg Chl a m^-2) dominated by Phaeocystis pouchetii developed underneath the snow-covered pack-ice. Although the initial biomass probably was advected from the ice margin, the bloom continued growing below the ice, consuming nutrients, until its culmination in late June. Sea-ice algal productivity was generally low due to thick snow cover (up to 0.5 m) on the ice and was mainly confined to new ice formed on refrozen leads and to first-year ice ridges. Interestingly, distinct sea-ice algae assemblages populated different parts of the ridge ledges. In addition, due to the thin ice and deep snow cover, we observed infiltration communities, mainly composed of phytoplankton taxa such as Phaeocystis pouchetii and Thalassiosira spp., to accumulate significant algal biomass at the snow-ice interface alongside cracks in the sea ice. Although rarely observed in Arctic environments due to a typical low snow depth to ice thickness ratio, we suggest that with the thinning ice cover infiltration communities may increase their occurrence into the future.