Implementation and assessment of a model including mixotrophs and the carbonate cycle (Eco3M_MIX-CarbOx v1.0) in a highly dynamic Mediterranean coastal environment (Bay of Marseille, France) (Part. II): Towards a better representation of total alkalinity when modelling the carbonate system and air-sea CO₂ fluxes

Abstract. The Bay of Marseille (BoM), located in the north-western Mediterranean Sea, is affected by various hydrodynamic processes (e.g., Rhône River intrusion and upwelling events) that result in a highly complex local carbonate system. In any complex environment, the use of models is advantageous since it allows to identify the different environmental forcings, thereby facilitating a better understanding. By combining approaches from two biogeochemical ocean models and improving the formulation of total alkalinity, we develop a more realistic representation of the carbonate system variables at high temporal resolution which enables us study air-sea CO₂ fluxes and seawater pCO₂ variations more reliably. We apply this new formulation to two particular scenarios, typical for the BoM: (i) summer upwelling and (ii) Rhône River intrusion events. In both scenarios, our model was able to correctly reproduce the observed patterns of pCO₂ variability. Summer upwelling events are typically associated with pCO₂ decrease that mainly results from decreasing near-surface temperatures. Furthermore, Rhône River intrusion events are typically associated with pCO₂ decrease, although in this case the pCO₂ decrease results from a decrease in salinity and an overall increase in total alkalinity. While our model was able to correctly represent the daily range of air-sea CO₂ fluxes, we were unable to correctly estimate the yearly total air-sea CO₂ flux. Although the model consistent with observations, predicted the BoM to be a sink of CO₂ on a yearly basis, the magnitude of this CO₂ sink was underestimated which may be an indication of the limitations inherent in dimensionless models for representing air-sea CO₂ fluxes.