Seafloor vents augment riverine contributions to methane emissions from a fjord-type estuary: Puget Sound

This study provides the first comprehensive assessment of methane dynamics in Puget Sound, a deep glacially carved fjord where newly discovered seafloor gas plumes add complexity to traditional estuarine methane cycling. Our objectives were to quantify primary methane sources and sinks, evaluate the relative importance of seafloor venting, and assess the role of microbial oxidation in controlling emissions. We combined multiple sampling approaches, including surface water surveys, deep water profiles, river sampling, and acoustic quantification of seafloor vents, to estimate methane fluxes. Our results show seafloor plumes enhance river-sourced methane concentrations and provide a novel source of volatile, non-conservative, and bioactive chemicals. Unlike typical estuaries where rapid export of riverine freshwater and dissolved solutes dominates, advective export in Puget Sound is reduced due to mixing over the shallow seaward sill, leading to partial recycling of methane within the system. The seafloor plumes contribute significantly, adding approximately 27% to methane emissions beyond river inputs. Microbial oxidation emerges as the principal sink, transforming methane into carbon dioxide and organic material while reducing emissions by 17-81% across sub-basins. This study reveals that our current understanding captures only about 45% of total methane inputs, highlighting the need for a more comprehensive investigation of this dynamic urban estuary.