Spatiotemporal variability of pH and carbonate parameters on the Canadian Atlantic Continental Shelf between 2014 and 2020

Abstract. The Atlantic Zone Monitoring Program (AZMP) was established by Fisheries and Oceans Canada (DFO) in 1998 with the aim of monitoring physical and biological ocean conditions in Atlantic Canada in support of fisheries management. Since 2014, at least two of the carbonate parameters (pH, Total Alkalinity - TA, Dissolved Inorganic Carbon - DIC) have also been systematically measured as part of the AZMP, enabling the calculation of derived parameters (e.g., carbonate saturation states - Ω, partial pressure of CO₂ - pCO₂, etc.). The present study gives an overview of the spatiotemporal variability of these parameters between 2014 and 2020. Results show that the variability of carbonate parameters reflects changes in both physical (e.g., temperature, salinity) and biological (e.g., plankton photosynthesis and respiration) parameters. For example, most of the region undergoes a seasonal warming and freshening. While the former will tend to increase Ω, the latter will decrease both TA and Ω. Spring and summer plankton blooms decrease DIC near the surface and then remineralize and increase DIC at depth in the fall. The lowest pCO₂ values are located in the cold Coastal Labrador Current and the highest in the fresh waters of the Gulf of St. Lawrence and the St. Lawrence Estuary. The latter is also the host of the lowest pH values of the zone. Finally, most of the bottom waters of the Gulf of St. Lawrence are undersaturated with respect to aragonite (Ω_arg<1). In addition to providing a baseline of carbonate parameters of the Atlantic Zone as a whole, this comprehensive overview is a necessary and useful contribution for the modeling community and for more in-depth studies. The full data set of measured and derived parameters is available in the Federated Research Data Repository at https://doi.org/10.20383/102.0673.