Benthic metabolism and macrofauna bioturbation along a glacier-driven gradient in Kongsfjorden

Climate change is warming the Arctic at a rate four times faster than the global average. Due to a unique pattern in ocean currents, the Kongsfjorden fjord in Svalbard (Norway) is experiencing even more rapid warming causing tidewater glaciers to retreat. Increasing freshwater and terrestrial inputs are expected to determine deep impacts at fjord head more than at fjord mouth, affecting both benthic communities and biogeochemistry, selecting highly tolerant pioneer taxa, such as polychaetes with a high bioturbation and nutrient recycling potential. In this work, 16 intact sediment cores were collected from the head and the mouth of Kongsfjorden and incubated in the dark at in situ temperature. Benthic fluxes of nutrients and gas, and denitrification rates were assessed during the incubations and macrofauna abundance and community composition were analyzed. Benthic metabolism and macrofauna bioturbation were then contrasted and interpreted with respect to the glacier disturbance. At the glacier front (fjord head), oxygen and nitrate respiration, methane efflux and nutrient regeneration were higher than at the fjord mouth. At the fjord head, the macrofauna community was more abundant and dominated by deep burrowing polychaetes, tolerant to glacier disturbance. Through bioturbation, they played a crucial role in stimulating coupled nitrification and denitrification, ammonium, and methane release. Our models indicate that polychaetes contribute significantly (32 %) to the total flux variability confirming their role in nutrient cycling. In this Arctic fjord, warming, glacial retreat and disturbance favor pioneer species of macrofauna that promote the recycle of nutrients and the evasion of methane.