Coastal waters freshening and extreme seasonality affect organic matter sources, quality, and transfers in a High Arctic fjord (Young Sound, Greenland)

International audience; Arctic benthic ecosystems are expected to experience strong modifications in the dynamics of primary producers and/or benthic-pelagic coupling under climate change. However, lack of knowledge about the influence of physical constraints (e.g. ice-melting associated gradients) on organic matter sources, quality, and transfers in systems such as fjords can impede predictions of the evolution of benthic-pelagic coupling in response to global warming. Here, sources and quality of particulate organic matter (POM) and sedimentary organic matter (SOM) were characterized along an inner-outer gradient in a High Artic fjord (Young Sound, NE Greenland) exposed to extreme seasonal and physical constraints (ice-melting associated gradients). The influence of the seasonal variability of food sources on 2 dominant filter-feeding bivalves (Astarte moerchi and Mya truncata) was also investigated. Results revealed the critical impact of long sea ice/snow cover conditions prevailing in Young Sound corresponding to a period of extremely poor and degraded POM and SOM. Freshwater inputs had a very local impact during summer, with relatively more degraded POM at the surface compared to bottom waters that were less nutritionally depleted but more heterogeneous among the sampled stations. Terrestrial inputs contributed to the SOM composition but showed a large variability along the fjord. Finally, diet analyses underlined the contrasted nutritional conditions, showing much higher lipid reserves in A. moerchi than in M. truncata during winter. Under a scenario with increased freshwater input, such results suggest a decline in organic matter quality and production in Young Sound, with subsequent impacts on benthic food webs.