Influence of regional climate forcing on surface water pCO2, ΔO2/Ar and dimethylsulfide (DMS) along the southern British Columbia coast

Abstract

Oceanographic surveys of surface water hydrography, pCO2, biological O2 saturation (ΔO2/Ar) and dimethylsulfide (DMS) were conducted around Vancouver Island, B.C. during late spring of 2007 and 2010. Winter El Niño conditions during early 2010 resulted in strong downwelling and decreased sub-surface nutrient inventories relative to 2007, while reduced Fraser River input, high wind speeds and greater cloud cover led to relatively weak vertical stratification and decreased sea surface irradiance during the 2010 survey. Phytoplankton biomass (Chla) was significantly higher along the west coast of Vancouver Island (WCVI) during late May 2007, and we observed a strong biological imprint on surface gas distributions during this survey, with biologically induced O2 saturation in excess of 40%. Minimum pCO2 along the WCVI was 100ppm during the 2007 survey, and this region served as a moderate CO2 sink at the time of our sampling (mean sea-air flux of -3.2mmolCO2m−2d−1). By comparison, both pCO2 and ΔO2/Ar were much closer to atmospheric equilibrium during our 2010 survey. Despite these lower sea-air gradients, high wind speeds led to a significantly larger CO2 sink (13.4mmolm−2d−1) during the 2010 survey, and a greater uncoupling of ΔO2/Ar and pCO2 through differential gas exchange. Relative to the WCVI, pCO2 and ΔO2/Ar distributions showed smaller differences among cruises along the east coast of Vancouver Island (ECVI), where surface water properties were largely driven by physical circulation. Nonetheless, we did observe lower pCO2 and higher ΔO2/Ar in the Strait of Georgia during the 2010 survey, possibly due to a wind-driven mixing event which may have stimulated local primary productivity. DMS concentrations exhibited large ranges and significant small-scale spatial variability for both cruises, but mean DMS concentrations were more than three-fold higher along the WCVI during late May 2007. Higher wind speeds in late May 2010 led to greater DMS fluxes during our 2010 survey (10 vs. 5μmolm−2d−1 for 2010 and 2007, respectively) despite lower sea-air concentration gradients. For both cruises, about 40% of the variability in surface water DMS concentrations was explained by Chla/mixed layer depth ratios when data were binned to a spatial resolution of 0.25°. Based on our results, we discuss the potential linkages between regional climate variability and surface gases in coastal B.C. waters.