Anthropogenic carbon dioxide (CO2) emissions have led to a global increase in atmospheric CO2 concentrations since the pre-industrial era, with the concentration of CO2 in the surface ocean increasing in response. Physical and biogeochemical processes cause oceanic CO2 distributions to be regionally variable, differing with temperature and salinity. Because of this, regional CO2 flux determination is necessary to account for this variability and better constrain global carbon flux estimates. The distinctly cold temperatures and relatively fresh waters of the Arctic Ocean, including Southern Hudson Bay and James Bay (SHB-JB), contribute to additional oceanic CO2 uptake capacity relative to other regions, influencing their potential to be globally important atmospheric CO2 sinks. Despite this, SHB-JB remain understudied with respect to the carbonate system.

To address this gap, we measured SHB-JB surface water CO2 partial pressure (pCO2), salinity, and temperature for three-week periods in August from 2021-2023. Surface water pCO¬2 varied within and between years. Mean pCO¬2 (456, 454, 477 μatm) exceeded atmospheric CO2 concentrations (~416 μatm) in each year sampled. A strong correlation between surface water pCO2 and temperature was observed resulting in heightened pCO2 concentrations near areas where seawater is overlain by warm, fresh river inflow. The mean calculated CO2 flux from the ocean to the atmosphere were 2.75 ±3.78, 1.48 ±3.33, and 3.67 ±3.74 mmol CO2 m-2 day-1 for 2021-2023, respectively. The surface water of SHB-JB acts as a weak source of CO2 to the atmosphere during the late summer, contrary to what is observed in open arctic waters.