Seasonal and spatial variability in the CO2 system on the Scotian Shelf (Northwest Atlantic)

As part of the Atlantic Zone Monitoring Program, dissolved inorganic carbon (DIC), total alkalinity, and nitrate measurements were made throughout the Scotian Shelf in 2007. A shelf-wide assessment of the spatio-temporal variability of the inorganic carbon system was made relying on observations in spring (April) and autumn (October). Over the 6-month period, a combination of biological production, surface dilution, and air-sea CO2 exchange resulted in seasonal decreases in surface DIC of up to 70 μmol kg−1 and subsurface (between 50 and 100 m) increases of DIC on the order of 50 μmol kg−1 on the inner shelf. The regional mean surface water pH was roughly 7.8 in spring and increased to greater than 8.0 in autumn; subsurface pH was approximately 7.6 throughout the region and a seasonal decrease, attributed in part to the respiration of organic matter at depth, was observed. The surface aragonite saturation state increased from less than 2.0 to a maximum of 3.2 between spring and autumn; the region as a whole exhibited relatively low saturation states, however values approaching 1.0 were only observed in the Cabot Strait at depths below 100 m. Winter-to-spring and winter-to-autumn deficits in surface inorganic carbon and nitrate were used to estimate net community production (NCP) throughout the region. The nitrate-based estimates of NCP using the autumn observations were significantly lower (0.1 to 0.3 mol C m−2 month−1) than the carbon-based estimates (0.1 to 0.8 mol C m−2 month−1) at most stations. The cumulative autumn NCP based on nitrate (0.4 to 1.9 mol C m−2 over 6 months) was up to 50% lower than the cumulative NCP based on inorganic carbon deficits (0.5 to 4.7 mol C m−2 over 6 months), suggesting that continued biological production through the summer season occurs in nitrate-depleted waters.