Subglacial upwelling in winter/spring increases under-ice primary production

Abstract

Abstract. Subglacial upwelling of nutrient rich bottom water is known to support high summer primary production in Arctic fjord systems. However, during the winter/spring season, the importance of subglacial upwelling has not been considered yet. We hypothesized that subglacial upwelling under sea ice is present in winter/spring and sufficient to increase phytoplankton primary productivity. We evaluated the effects of the subglacial upwelling on primary production in a seasonally fast ice covered Svalbard fjord (Billefjorden) influenced by a tidewater outlet glacier in April/May 2019. We found clear evidence for subglacial upwelling. Although the estimated entrainment factor (1.6) and total fluxes were lower than in summer studies, we observed substantial impact on the fjord ecosystem and primary production. The subglacial meltwater leads to a salinity stratified surface layer and sea ice formation with low bulk salinity and permeability. The combination of the stratified surface layer, a two-fold higher under-ice irradiance, and higher N and Si concentrations at the glacier front supported two orders of magnitude higher primary production (42.6 mg C m−2 d−1) compared to a marine reference site at the fast ice edge. The nutrient supply increased primary production by approximately 30 %. The brackish water sea ice at the glacier front with its low bulk salinity contained a reduced brine volume, limiting the inhabitable place and nutrient exchange with the underlying seawater compared to full marine sea ice. Microbial and algal communities were substantially different in subglacial influenced water and sea ice compared to the marine reference site, sharing taxa with the subglacial outflow water. We suggest that with climate change, the retreat of tidewater glaciers could lead to decreased under-ice phytoplankton primary production, while sea ice algae production and biomass may become increasingly important.

A first constraint on basal melt-water production of the Greenland ice sheet

“…Thus, the basal melt introduced and quantified here is a primary source of winter subglacial discharge, and this influx of winter basal water is poorly understood and sparsely measured 38 . Biological productivity is affected by subglacial discharge that modifies mixing in the fjords 14 , 39 , but the impact of increasing winter freshwater on Arctic fjord environments is as-yet unknown. Studies suggest that winter basal melt discharge may drive year-round submarine meltwater plumes leading to persistent ice-front melting, and that basal melt discharge may pull in warm water from the Atlantic further enhancing frontal melt rates 40 .…”