Hopwood, Mark J. , Carroll, Dustin, Gu, Yuanyuan, Huang, Xin, Krause, Jana, Cozzi, Stefano, Cantoni, Carolina, Gastelu Barcena, María Fernanda, Carroll, Shandy and Körtzinger, Arne (2025) A Close Look at Dissolved Silica Dynamics in Disko Bay, West Greenland. Global Biogeochemical Cycles, 39 (1). Art.Nr.: e2023GB008080. DOI 10.1029/2023GB008080.

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Abstract

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.

Plain Language Summary

Silica is one nutrient required by marine phytoplankton, specifically siliceous microalgae such as diatoms. Glacier runoff delivers higher concentrations of silica into the ocean compared to other nutrients such as nitrate or phosphate. Changes in the cryosphere, such as glacier retreat and increasing ice discharge or runoff, may therefore have downstream ecological effects due to shifts in the availability, and ratios, of nutrients. However, the magnitude of dissolved silica fluxes into the ocean from the present day Greenland Ice Sheet has proven challenging to determine, with two existing estimates varying by an order of magnitude. This is because of uncertainties in how to disentangle the overlapping influence of different dSi sources. Here, we conducted a detailed survey of the Disko Bay region in west Greenland, including inshore and offshore dSi measurements, to assess how the chemistry of dSi changes in estuaries. In order to further understand estuarine dSi dynamics, we conducted incubation experiments at sea and in the laboratory. Our results reduce uncertainty in the magnitude of dSi supplied to the ocean from Greenland's glaciers and suggest a modest dSi flux from runoff with small additional fluxes from melting icebergs and dSi release from suspended particles.

Key Points

Both runoff and entrainment in subglacial discharge plumes influence estuarine nutrient dynamics around Disko Bay
Entrainment from saline waters explains 93 ± 51% of dSi enrichment in the Ilulissat Icefjord outflow, freshwater <3%
We estimate 0.25 and ∼1.9 Gmol yr −1 dSi fluxes from Greenland's icebergs and pelagic aSi dissolution, respectively

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