The control of physical and biological drivers on pelagic methane fluxes in a Patagonian fjord (Golfo Almirante Montt, Chile)

Methane fluxes from coastal waters such as fjords and the underlying control mechanisms are poorly understood. During the austral summer, we investigated a fjord in the Chilean part of Patagonia, the Golfo Almirante Montt. The study is based on measurements of methane concentration, stable carbon isotopes and the distribution and activity of methane-oxidizing bacteria in the water column, as well as oceanographic and geological observations. Our results indicate that methane is of biogenic origin and released from gas-rich sediments at the entrance of the fjord, characterized by pockmarks and gas flares. Tidal currents and turbulent mixing at the sill cause a near-surface methane plume to spread into the main fjord basin and mix with the methane- and oxygen-depleted deep water. Wind-induced mixing at the sea surface controls the methane flux from the plume into the atmosphere. The methane plume is consumed by methanotrophic bacteria of the Methylomonadaceae and Ga0077536 families, which are differently distributed along the water column. An enrichment of the characteristic gene methane monooxygenase (pmoA) in the methane-poor deep water, and a conspicuously high δ13C-CH4 signature suggest that methane-rich intrusions regularly enter the deep water, where the methane is microbially oxidized. Our interdisciplinary study offers a comprehensive insight into the complex physical and biological processes that modulate methane dynamics in fjords and thus help to better assess how methane emissions from these systems will change under anthropogenic influence.