Time-series CH4 measurements from Saanich Inlet, BC, a seasonally anoxic fjord

Abstract

We present 7 years of water column methane (CH4) measurements from Saanich Inlet, a seasonally anoxic fjord in British Columbia, Canada. Our time-series observations show that CH4 concentrations and sea-air fluxes vary significantly over seasonal cycles, driven primarily by changes in O2-availability throughout the water column. Methane concentrations were always supersaturated in near-surface waters (mean 1160 ± 590% saturation at 10 m depth) and remained high between 10 and 85 m depth. We observed a persistent mid-depth CH4 minimum between 85 and 110 m associated with a turbidity maximum near the base of the oxycline. CH4 accumulated steadily below 150 m depth from the onset of anoxia until the subsequent annual renewal, with maximum bottom water values indicative of sedimentary sources. Beyond the strong seasonal cycle in CH4 concentrations, we also observed significant inter-annual variability. Most notably, the moderate 2009–2010 El Niño resulted in a weak deep basin renewal, and anomalously high CH4 concentrations in both the deep basin and upper 85 m. Over the time-series, we observed statistically significant shoaling of the hypoxic boundary, associated with declining CH4 concentrations in the upper water column (10–85 m depth). Our results provide insight into the potential effects of expanding hypoxia on CH4 cycling in coastal waters.