A Close Look at Dissolved Silica Dynamics in Disko Bay, West Greenland

Discharge of calved ice, runoff and mixing driven by subglacial discharge plumes likely have consequences for marine biogeochemistry in Disko Bay, which hosts the largest glacier in the northern hemisphere, Sermeq Kujalleq. Glacier retreat and increasing runoff may impact the marine silica cycle because glaciers deliver elevated concentrations of dissolved silica (dSi) compared to other macronutrients. However, the annual flux of dSi delivered to the ocean from the Greenland Ice Sheet is poorly constrained because of difficulties distinguishing the overlapping influence of different dSi sources. Here we constrain silica dynamics around Disko Bay, including the Ilulissat Icefjord and four other regions receiving glacier runoff with contrasting levels of productivity and turbidity. Both dissolved silica and Si* ([dSi]-[NO_x ^-]) concentrations indicated conservative dynamics in two fjords with runoff from land-terminating glaciers, consistent with the results of mixing experiments. In three fjords with marine-terminating glaciers, macronutrient-salinity distributions were strongly affected by entrainment of nutrients in subglacial discharge plumes. Entrainment of dSi from saline waters explained 93 ± 51% of the dSi enrichment in the outflowing plume from Ilulissat Icefjord, whereas the direct contribution of freshwater to dSi in the plume was likely 0%-3%. Whilst not distinguished herein, other minor regional dSi sources include icebergs and dissolution of amorphous silica (aSi) in either pelagic or benthic environments. Our results suggest that runoff around Greenland is supplemented as a dSi source by minor fluxes of 0.25 ± 0.67 Gmol yr^-1 dSi from icebergs and ∼1.9 Gmol year^-1 from pelagic aSi dissolution.