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

The methane flux from coastal water areas such as fjords and the underlying control mechanisms have been little studied to date. Fjords are characterized by a complex hydrography that is shaped by marine and limnic interactions and leads to a pronounced stratification of the water column. The resulting low ventilation of the deep water together with high primary production rates in the surface water and the subsequent transport of the organic material to the seabed often lead to high methane releases from the seabed. In our study, we analyzed a fjord system in the Chilean part of Patagonia, the Golfo Almirante Montt. The investigation is based on studies of water column methane concentration and stable carbon isotopes, the distribution and activity of methane-oxidizing bacteria, and oceanographic and geological observations. Our results indicate that methane is of biogenic origin is released from gas-rich sediments at the entrance of the main fjord basin, which is characterized by pockmarks and gas flares. Tidal currents and turbulent mixing at the sill cause a methane plume near the surface to spread into the main fjord basin and mix with the methane- and oxygen-depleted deep water. The wind-induced mixing at the sea surface controls the methane flux from the methane plume into the atmosphere. The methane plume is consumed mainly by methanotrophic bacteria. An enrichment of the signature gene particulate methane monooxygenase (pmoA) in the methane-poor deep water, and a conspicuously high δ¹³C-CH₄ signature of the methane suggest that methane-rich intrusions are periodically introduced into the deep water, which are subsequently converted microbially. Additionally, a δ¹³C-CH₄ anomaly in deep water that correlates with a zooplankton accumulation in this depth during daytime is considered to be a product of zooplankton-associated methane production. 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.