The seasonal variability of inorganic carbon in the surface waters of the Scotian Shelf region of the Canadian northwestern Atlantic Ocean was investigated. Seasonal variability was assessed using hourly measurements, covering a full annual cycle, of the partial pressure of CO₂, (pCO₂), and hydrographic variables obtained by an autonomous moored instrument (44.3°N and 63.3°W). These measurements were complemented by seasonal shipboard sampling of dissolved inorganic carbon (DIC), total alkalinity (TA), and pCO₂, at the mooring site, and over the larger spatial scale. Biological processes were found to be the dominant control on mixed-layer DIC, with the delivery of carbon-rich subsurface waters also playing an important role. Annual mixed-layer net community production was 2.4 mol C m^− 2 yr^− 1. The air–sea fluxes of CO₂ were computed using observed hourly wind speeds from the Sable Island Meteorological Station (43.9°N and 60.3°W). The region acts as a net source of CO₂ to the atmosphere on the annual scale (F = − 1.4 mol C m^− 2 yr^− 1), with a reversal of this trend occurring only during the spring phytoplankton bloom, when a pronounced undersaturation of the surface waters is reached for a short period. Outside of the spring bloom period, the competing effects of temperature and biology influence surface pCO₂ in roughly equal magnitude.