Iron Supply by Subglacial Discharge Into a Fjord Near the Front of a Marine‐Terminating Glacier in Northwestern Greenland

Abstract

AbstractGlacial 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 × 107 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.