Temporal methane variability in the water column of an area of seasonal coastal upwelling: A study based on a 12 year time series

Abstract Temporal distribution of dissolved CH4 was analysed in a zone of strong seasonal coastal upwelling off central Chile (36.5°S 73°W). Observations were taken from a twelve-year time series that included monthly sampling of the water column at eight depths. CH4 concentration fluctuated between 1.75 and 100.9 nmol L-1 (or 67.11% and 3965% of saturation), with the highest levels at bottom waters, which increase as upwelling evolved. Three kind of CH4 profiles were identified; a classical diffusion-advection distribution, with bottom/surface CH4 concentration ratio > 2, was predominantly observed in ∼54% of the all profiles and attributed to high CH4 production in the sediments during coastal upwelling season (austral spring-summer); a period of higher biological productivity, as well as in hypoxic/anoxic condition. In contrast, relatively homogeneous profiles (CH4 level ratio between bottom and surface depth