| Summary: | From the seabed to the water column, where it may be consumed by aerobic methanotrophic bacteria. The size and activity of methanotrophic communities, which determine the amount of methane consumed in the water column,are thought to be mainly controlled by nutrient and redoxdynamics3–7. Here, we report repeated measurements of methanotrophic activity and community size at methaneseeps west of Svalbard, and relate them to physical watermass properties and modelled ocean currents. We show that cold bottom water, which contained a large number of aerobic methanotrophs, was displaced by warmer water with a considerably smaller methanotrophic community within days.Ocean current simulations using a global ocean/sea-ice model suggest that this water mass exchange is consistent withshort-term variations in the meandering West Spitsbergen Current. We conclude that the shift from an oshore to a nearshore position of the current can rapidly and severelyr educe methanotrophic activity in the water column. Strong fluctuating currents are common at many methane seep systems globally, and we suggest that they aect methaneoxidation in the water column at other sites, too.
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