Influence of glacial meltwater on summer biogeochemical cycles in Scoresby Sund, East Greenland

Greenland fjordsreceive considerable amounts of glacial meltwaterdischarge from the Greenland Ice Sheetdue to present climate warming. This impacts the hydrographyvia freshening of the fjordwaters and biological processes due to altered nutrient input and the addition of silts. We present the first comprehensive analysis of the summer carbon cycle in the world's largest fjordsystem situated in southeastern Greenland. During a cruise onboard RV Maria S. Merian in summer 2016, we visited Scoresby Sund and its northernmost branch, Nordvestfjord. In addition to direct measurements of hydrography, biogeochemical parameters and sediment trapfluxes, we derived net community production (NCP) and full water columnparticulate organic carbon (POC) fluxes, and estimated carbon remineralization from vertical flux attenuation. While the narrow Nordvestfjord is influenced by subglacial and surface meltwaterdischarge, these meltwatereffects on the outer fjordpart of Scoresby Sund are weakened due to its enormous width. We found that subglacial and surface meltwaterdischarge to Nordvestfjord significantly limited NCP to 32 – 36 mmol C m-2 d-1 compared to the outer fjordpart of Scoresby Sund (58 – 82 mmol C m-2 d-1) by inhibiting the resupply of nutrients to the surface and by shadowing of siltscontained in the meltwater. The POC flux close to the glacier fronts was elevated due to silt-ballasting of settling particles that increases the sinking velocity and thereby reduces the time for remineralization processes within the water column. By contrast, the outer fjordpart of Scoresby Sund showed stronger attenuation of particles due to horizontal advection and, hence, more intense remineralization within the water column. Our results imply that glacially influenced parts of Greenland's fjordscan be considered as hotspots of carbon export to depth. In a warming climate, this export is likely to be enhanced during glacial melting. Additionally, entrainment of increasingly warmer Atlantic Water might support a higher productivity fjordsystems. It therefore seems that future ice-free fjordsystems with high input of glacial meltwatermay become increasingly important for Arctic carbon sequestration.