Implementation and assessment of a carbonate system model (Eco3M-CarbOxv1.1) in a highly-dynamic Mediterranean coastal site (Bay of Marseille, France)

Abstract. The Bay of Marseille (BoM, France) is impacted by the urbanized and industrialized Aix-Marseille Metropolis, which is subject to significant increases in anthropogenic emissions of CO2. A carbonate chemistry balance module has been implemented into a biogeochemical model of the planktonic food web. The resulting model, named Eco3M-CarbOx includes 22 states variables that are dispatched into 5 compartments: phytoplankton, heterotrophic bacteria, detritus, dissolved organic and inorganic matter. The model suggests that the variability of the dissolved inorganic carbon system is mainly driven by the seawater temperature dynamics. A seasonal trend is identified by the model and it shows that, during the mixed water column period, the BoM is a sink for atmospheric CO2 and a net autotroph ecosystem, while during stratified water column period, the BoM is a source of CO2 to the atmosphere and a net heterotroph ecosystem. External forcings have an important impact on the carbonate equilibrium. Wind events change seawater temperature quickly, as during upwelling, for which the BoM waters change within a few days from a source of CO2 to the atmosphere to a sink into the ocean. Moreover, the higher the wind speed is, the higher the air-sea CO2 gas exchange fluxes are. The river intrusions with nitrate and alkalinity supplies lead to a decrease in the pCO2 value, favoring the conditions of a sink of atmospheric CO2 into the BoM. The nearby highly urbanized environment of the Aix-Marseille metropolis produces strong atmospheric values of CO2, also favoring the conditions of a sink of atmospheric CO2 into the waters of the BoM.