Abstract
Abstract. Subglacial upwelling of nutrient-rich bottom water is known to sustain elevated summer primary production in tidewater-glacier-influenced fjord systems. However, the importance of subglacial upwelling during the early spring season has not been considered yet. We hypothesized that subglacial discharge under sea ice is present in early spring and that its flux is sufficient to increase phytoplankton primary productivity. We evaluated the effects of the submarine discharge on primary production in a seasonally fast-ice covered Svalbard fjord (Billefjorden) influenced by a tidewater outlet glacier in April and May 2019. We found clear evidence for subglacial discharge and upwelling. Although the estimated bottom-water entrainment factor (1.6) and total fluxes were lower than in summer studies, we still observed substantial impact on the fjord ecosystem and primary production at this time of the year. The subglacial discharge leads to a salinity-stratified surface water layer and sea ice formation with low bulk salinity and permeability. The combination of the stratified surface layer, a 2-fold higher under-ice irradiance due to thinner snow cover, and higher N and Si concentrations at the glacier front supported phytoplankton primary production 2 orders of magnitude higher (42.6 mg C m−2 d−1) compared to a marine reference site at the fast-ice edge. Reciprocal transplant experiments showed that nutrient supply increased phytoplankton 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 brine channel space 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 in early spring could lead to decreased under-ice phytoplankton primary production. In contrast, sea ice algae production and biomass may become increasingly important, unless sea ice disappears first, in which case spring phytoplankton primary production may increase.
Highlights
Tidewater glacier fronts have recently been recognized as hotspots for marine production including top trophic levels, such as marine mammals, birds, and piscivorous fish (Lydersen et al, 2014; Meire et al, 2016b), and primary producers (Meire et al, 2016b; Hopwood et al, 2020)
Our first hypothesis that submarine discharge persists in early spring, supplying nutrient-rich glacial meltwater and upwelling of bottom fjord water to the surface, has been confirmed as discussed in detail below
With air temperatures below freezing point during the sampling periods, surface runoff based on snowmelt was not possible and no melting was observed during fieldwork
Summary
Tidewater glacier fronts have recently been recognized as hotspots for marine production including top trophic levels, such as marine mammals, birds, and piscivorous fish (Lydersen et al, 2014; Meire et al, 2016b), and primary producers (Meire et al, 2016b; Hopwood et al, 2020). Large amounts of freshwater are released below the glacier and entrap nutrient-rich bottom water, sediments, and zooplankton during the rise to the surface (Lydersen et al, 2014; Meire et al, 2016a). Together with katabatic winds pushing the surface water out of the fjords, subglacial discharge creates a strong upwelling effect (Meire et al, 2016a). Vonnahme et al.: Subglacial discharge fuels under-ice primary production
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