Large amounts of the greenhouse gas methane are released from the seabed to the water column1, where it may be consumed by aerobic methanotrophic bacteria2. 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 redox dynamics3,4,5,6,7. Here, we report repeated measurements of methanotrophic activity and community size at methane seeps west of Svalbard, and relate them to physical water mass 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 with short-term variations in the meandering West Spitsbergen Current. We conclude that the shift from an offshore to a nearshore position of the current can rapidly and severely reduce methanotrophic activity in the water column. Strong fluctuating currents are common at many methane seep systems globally, and we suggest that they affect methane oxidation in the water column at other sites, too.
The authors thank the captains, crews and shipboard scientific parties of R/V Poseidon and R/V Maria S. Merian for their excellent help at sea. We greatly acknowledge K. Hissmann and J. Schauer for operating the submersible Jago. Model simulations were performed at the North-German Supercomputing Alliance (HLRN). This work received financial support through a D-A-CH project funded by the Swiss National Science Foundation and German Research Foundation (grant no. 200021L_138057). Further support was provided through the EU COST Action PERGAMON (ESSEM 0902), a Postgraduate Scholarship of the National Research Council of Canada, the Centre of Excellence ‘CAGE’ funded by the Norwegian Research Council (grant no. 223259), the cooperative Projects ‘RACE—Regional Atlantic Circulation and Global Change’ funded by the German Federal Ministry for Education and Research (BMBF) and the Cluster of Excellence ‘The Future Ocean’ funded by the German Research Foundation.