METHANE AND NITROUS-OXIDE IN SEASONALLY UPWELLED, HYPOXIC CONTINENTAL SHELF WATERS WITH METHANE SEEPS: MULTI-YEAR DATA FROM COASTAL BRITISH COLUMBIA

Coastal upwelling systems are important marine sources of methane (CH₄) and nitrous-oxide (N₂O). Current understanding of the controls on CH₄ and N₂O distributions in these coastal waters is restricted by limited data availability. We present the first multi-year measurements of CH₄ and N₂O distributions from the seasonally upwelling shelf waters of British Columbia, Canada, a coastal end-member of the north Pacific oxygen minimum zone (OMZ). Our data show that upwelling is the dominant control on surface water concentrations and sea-air fluxes of CH₄ and N₂O in the study area, with surface gas concentrations significantly correlated with the mean upwelling state. Methane is supplied to the water column primarily from sediments (especially near methane seeps), and is transported to the surface mixed layer by upwelling. A positive correlation between CH₄ concentrations and salinity indicates limited inputs from Fraser River estuary waters. Shelf waters receive N₂O from a deep, off-shelf N₂O maximum (in the OMZ core), and from nitrification in the water column. Both the physical transport of N₂O and its in situ production are enhanced under upwelling conditions. N₂O yields from nitrification, estimated from changes in N₂O and nitrate + nitrite (NO_{3 +} NO₂) along isopycnals, ranged from 0.04 – 0.45%, with the highest values observed under low ambient O₂ concentrations. Sea-air fluxes ranged from 1.4 – 26.9 µmol m^-2 day^-1 for N₂O and 1.6 – 34.7 µmol m^-2 day^-1 for CH₄, with the highest values observed following upwelling over the broad continental shelf along the southern portion of Vancouver Island. Our results provide new insight into the factors driving spatial and inter-annual variability in marine CH₄ and N₂O in high productivity coastal upwelling regions. Continued time-series measurements will be invaluable to understanding the longer-term impacts of climate-driven variability on marine biogeochemical cycles in these dynamic near-shore waters.