Evidence and ecological implications of subglacial discharge under sea-ice at a Svalbard tidewater glacier

Subglacial discharge of meltwater at tidewater glaciers has been recognized as important drivers of Arctic fjord circulation, facilitating high primary production via subglacial upwelling in summer. The presence of subglacial discharge in early spring has been described for both land- and marine terminating glaciers. However, its magnitude and importance on the microbial carbon cycle and diversity in sea ice-covered fjords has not been considered. We hypothesized that subglacial discharge and upwelling is also happening in spring with pronounced impacts on the microbial food web. At a shallow tidewater glacier on Svalbard, we found evidence for submarine discharge in CTD, Nutrients, and turbidity profiles, which was absent from a land-terminating and a marine reference site. The meltwater input lead to a strongly stratified 2 to 4 m thick brackish surface layer and sea ice with very low bulk salinity. Nutrient concentrations were enriched in both the brackish surface layer and sea ice indicating a direct or indirect freshwater origin. In the brackish surface layer, we found three orders of magnitude higher primary production (5.3 mg C m^-3 d^-1) leading to higher phytoplankton biomass compared to the marine reference site. We attributed higher nutrient concentrations (subglacial upwelling and direct silicate inputs with glacial meltwater), the shallow mixed layer depth, and two times more light penetration (less snow cover, less ice algae biomass) to cause the formation of this moderate under ice phytoplankton bloom. In sea ice at the tidewater glacier, algae biomass was three orders of magnitude lower and algal communities significantly different compared to the full marine sites, which we attribute to the very low brine volume fraction leading to limited nutrient exchange with the underlying water column and limited inhabitable space. With retreating tidewater glaciers in a warming climate this submarine discharge may disappear which leading to lower under ice phytoplankton production and higher sea ice algae biomass. However, sea ice is also disappearing, which may have the opposite effect.