Methane and nitrous oxide distributions in coastal and open ocean waters of the Northeast Subarctic Pacific during 2015-2016

We measured depth profiles of CH4 and N2O concentrations at six stations along a coastal - oceanic transect in the Northeastern Subarctic Pacific Ocean in winter, spring and summer of 2015 and 2016. In the offshore region, we observed mixed layer CH4 concentrations that were close to atmospheric equilibrium with no evidence of a near-surface CH4 maximum. Sub-surface waters of the oceanic region exhibited persistent CH4 under-saturation, suggesting in situ biological consumption below the mixed layer. Methane concentrations in coastal waters were significantly more variable, with maximum super-saturation of up to 600% on the Vancouver Island shelf and slope, likely due to supply from seeps and gas hydrates. In both coastal and open ocean waters, upper water column concentrations of N2O were close to atmospheric equilibrium, while subsurface waters exhibited a strong N2O maximum coincident with the depth of the oxygen minimum at ~1000 m. The relationship between ∆N2O, N* and apparent oxygen utilization (AOU) revealed likely signatures of both nitrification and denitrification as N2O sources, suggesting a possible contribution of lateral advective supply. Sea-air fluxes for both N2O and CH4 were low in the open ocean and increased towards the coast, with the highest values for N2O observed during the summer upwelling season. Our results provide new information on the distribution of CH4 and N2O in the Northeast Subarctic Pacific, and the potential underlying mechanisms driving the production and consumption of these gases. Continued time-series measurements will provide valuable information on climate-driven changes in the biogeochemical cycling of CH4 and N2O.