In Greenland, tidewater glaciers discharge turbid subglacial freshwater into fjords, forming a plume near the calving front. To elucidate the effects of this discharge on nutrient and dissolved inorganic carbon transport to the surface in these fjords, we conducted observational studies on Bowdoin Glacier and in its fjord in northwestern Greenland during the summer of 2016. Our results provide evidence of macronutrient and dissolved inorganic carbon transport from deep in the fjord to the surface in front of the glacier. This transport is driven by plume formation resulting from subglacial freshwater discharge and subsequent upwelling along the glacier calving front. The plume water is a mixture of subglacial freshwater and entrained fjord water. The fraction of glacial meltwater in the plume water is ~14% when it reaches the surface. The plume water is highly turbid because it contains substantial amounts of sediment derived from subglacial weathering. After reaching the surface, the plume water submerges and forms a turbid subsurface layer below fresher surface water at densities of 25.0 to 26.5 σθ. Phytoplankton blooms (~6.5 μg/L chlorophyll a) were observed near the boundary between the fresher surface and turbid subsurface layers. The bloom was associated with a strong upward NO3− + NO2− flux, which was caused by the subduction of plume water. Our study demonstrated that the subglacial discharge and plume formation at the front of Bowdoin Glacier play a key role in the availability of nutrients and the subsequent growth of phytoplankton in the glaciated fjord.

Glacial fjords in Greenland show high productivity owing to the runoff of meltwater from the glaciers. Macronutrient dynamics (of nitrate, phosphate, and silicate) associated with subglacial discharge plumes in front of marine-terminating glaciers are widely cited as important drivers of summer phytoplankton blooms in the fjords. However, the dynamics of iron (Fe), an essential micronutrient for primary production, remain largely unstudied in glacial fjords. To investigate the role of subglacial discharge plumes in Fe supply processes in glacial fjords, a comprehensive survey of Bowdoin Fjord, adjacent to the marine-terminating Bowdoin Glacier in northwestern Greenland, was conducted. The subglacial discharge of Fe-rich meltwater induces a buoyancy-driven upwelling plume in front of the glacier that entrains nutrient-rich Arctic-and Atlantic-origin waters. The plume water potentially carried 4.5-8.7 × 10 7 g day −1 of total dissolvable Fe out of Bowdoin Fjord in summer. The concentration of dissolved Fe (dFe) in the plume water (~15.6 nmol kg −1) was 4 times higher than that in the water in the outer part of the fjord (~3.8 nmol kg −1). The dFe:nitrate + nitrite ratio (mmol mol −1) in the plume water varied between 0.58 and 3.2, several orders of magnitude higher than the phytoplankton cellular Fe:nitrate ratio estimated using the hypothetical Fe:C ratio and observed particulate organic carbon:nitrate ratio of the fjord. Hence, the plume water is replete with Fe with respect to phytoplankton demands. Subglacial discharge drives the upwelling of Fe and macronutrients toward the euphotic zone, which is vital for the generation of summer phytoplankton growth in glacial fjords.

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Antarctic sea ice is an important temporal reservoir of iron which can boost primary production in the marginal ice zone during the seasonal melt. While studies have reported that Antarctic fast ice bears high concentrations of iron due to the proximity to coastal sources, less clear are the biogeochemical changes this iron pool undergoes during late spring. Here we describe a 3-week time series of physical and biogeochemical data, including iron, from first-year coastal fast ice sampled near Davis Station (Prydz Bay, East Antarctica) during late austral spring 2015. Our study shows that dissolved and particulate iron concentrations in sea ice were up to two orders of magnitude higher than in under-ice seawater. Furthermore, our results indicate a significant contribution of lithogenic iron from the Vestfold Hills (as deduced from the comparison with crustal element ratios) to the particulate iron pool in fast ice after a blizzard event halfway through the time series. Windblown dust represented approximately 75% of the particulate iron found in the ice and is a potential candidate for keeping concentrations of soluble iron stable during our observations. These results suggest that iron entrapped during ice formation, likely from sediments, as well as local input of coastal dust, supports primary productivity in Davis fast ice. As ice-free land areas are likely to expand over the course of the century, this work highlights the need to quantify iron inputs from continental Antarctic dust and its bioavailability for ice algae and phytoplankton. Plain Language SummaryOceanic single-celled algae are the base of the ocean food web and play an important role in the Earth climate. In the Southern Ocean, the growth of these microorganisms is limited by the naturally low concentration of iron in the seawater. Microalgae benefit from the presence of the Antarctic sea ice since iron is highly concentrated in sea ice relative to the seawater. Less clear though is the contribution of the potential sources of iron to the sea ice. We collected and analyzed sea ice cores for a series of parameters, including iron, from first-year coastal sea ice sampled near Davis Station (Prydz Bay, East Antarctica) during late austral spring 2015. Our results suggest that iron entrapped during ice formation, likely from seafloor sediments, as well as dust blown by winds from the neighboring Vestfold Hills, are the main sources of iron to Davis coastal sea ice. Since we can expect the expansion of ice-free areas and exposed grounds over the course of this century, our results highlight the need to quantify the amount of iron coming from continental Antarctic dust and to access if microalgae can use this form of iron for their basic physiological needs.