Factors controlling methane and nitrous-oxide variability in the southern British Columbia coastal upwelling system

• Seasonal differences in CH4 and N2O distributions and fluxes are linked to upwelling.
• Seeps appear to be important sources of water column CH4.
• Nitrification is dominant pathway of in situ N2O production.
• Upwelling increases advective supply and in situ production of N2O in shelf waters.
• N2O yields from nitrification increase under low O2 availability.

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 significant seasonal differences in CH4 and N2O distributions and fluxes driven predominantly by upwelling. 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 to the study site. Shelf waters receive N2O from a deep, off-shelf N2O maximum in the OMZ core, and from nitrification in the water column and possibly sediments. Both the physical transport of N2O and its apparent in situ production are enhanced under upwelling conditions. N2O yields from nitrification, estimated from changes in N2O and nitrate + nitrite (NO3− + NO2−) along isopycnals, ranged from 0.04–0.49%, with the highest values observed under low ambient O2 concentrations. Sea–air fluxes ranged from − 4.5–21.9 μmol m− 2 day− 1 for N2O and 2.5–34.1 μmol m− 2 day− 1 for CH4, with the highest surface fluxes observed following summer upwelling over the broad continental shelf of southern 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 in understanding the longer-term impacts of climate-driven variability on marine biogeochemical cycles in these dynamic near-shore waters.