Estimates of carbon sequestration potential in an expanding Arctic fjord (Hornsund, Svalbard) affected by dark plumes of glacial meltwater

In polar regions, glaciers are retreating onto land, gradually widening ice-free coastal waters, which are known to act as new sinks of atmospheric carbon. However, the increasing delivery of inorganic suspended particulate matter (iSPM) with meltwater might significantly impact their capacity to contribute to carbon sequestration. Here, we present an analysis of satellite, meteorological, and SPM data as well as results of a coupled physical-biogeochemical model (1D GOTM-ECOSMO-E2E-Polar) with a newly implemented iSPM group to show the impact of iSPM on the ecosystem dynamics in a warming polar fjord (Hornsund, European Arctic) with numerous shallow-grounded marine-terminating glaciers. Our results indicate that with a longer melt season (9 d per decade, 1979-2022), the loss of sea ice cover (44 d per decade, 1982-2021) and the formation of new marine habitats after the retreat of marine-terminating glaciers (around 100 km² in 1976-2022, a 38 % increase in the total area), glacial meltwater has transported increasing loads of iSPM from land (3.7 g m^-3 per decade, reconstructed for 1979-2022). The simulated light limitation induced by the iSPM input delayed and decreased the peaks in phytoplankton, zooplankton, and macrobenthos. The newly ice-free areas still markedly contributed to plankton primary and secondary production and carbon burial in sediments (5.1, 2.0, and 0.9 Gg C yr^-1, respectively, on average for 2005-2009 in the iSPM scenario). However, these values would have been 5.0, 2.1, and 0.1 Gg C yr^-1 higher, respectively, without the iSPM input. Since carbon burial was the least affected by iSPM (a decrease of around 16 %, in comparison to 50 % for plankton primary and secondary production), the impact of marine ice loss and enhanced land-ocean connectivity should be investigated further in the context of carbon fluxes in expanding polar fjords.