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

Abstract

Abstract We measured depth profiles of CH 4 and N 2 O 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 CH 4 concentrations that were close to atmospheric equilibrium with no evidence of a near-surface CH 4 maximum. Sub-surface waters of the oceanic region exhibited persistent CH 4 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 N 2 O were close to atmospheric equilibrium, while subsurface waters exhibited a strong N 2 O maximum coincident with the depth of the oxygen minimum at ~1000 m. The relationship between ∆N 2 O, N* and apparent oxygen utilization (AOU) revealed likely signatures of both nitrification and denitrification as N 2 O sources, suggesting a possible contribution of lateral advective supply. Sea-air fluxes for both N 2 O and CH 4 were low in the open ocean and increased towards the coast, with the highest values for N 2 O observed during the summer upwelling season. Our results provide new information on the distribution of CH 4 and N 2 O 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 CH 4 and N 2 O.