Implementation and assessment of a carbonate system model (Eco3M-CarbOx v1.1) in a highly dynamic Mediterranean coastal site (Bay of Marseille, France)

A carbonate chemistry balance module was implemented into a biogeochemical model of the planktonic food web. The model, named Eco3M-CarbOx, includes 22 state variables that are dispatched into 5 compartments: phytoplankton, heterotrophic bacteria, detrital particulate organic matter, labile dissolved organic, and inorganic matter. This model is applied to and evaluated in the Bay of Marseille (BoM, France), which is a coastal zone impacted by the urbanized and industrialized Aix-Marseille Metropolis, and subject to significant increases in anthropogenic emissions of CO2. The model was evaluated over the year 2017, for which in situ data of the carbonate system are available in the study site. The biogeochemical state variables of the model only change with time, to represent the time evolution of a sea surface water cell in response to the implemented realistic forcing conditions. The model correctly simulates the value ranges and seasonal dynamics of most of the variables of the carbonate system except for the total alkalinity. Several numerical experiments were conducted to test the response of carbonate system to (i) a seawater temperature increase, (ii) wind events, (iii) Rhone River plume intrusions, and (iv) different levels of atmospheric CO2 contents. This set of numerical experiments shows that the Eco3M-CarbOx model provides expected responses in the alteration of the marine carbonate balance regarding each of the considered pertur- bation. When the seawater temperature changes quickly, the behavior of the BoM waters alters 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 supplies lead to a decrease in the pCO(2) value, favoring the conditions of a sink for atmospheric CO2 into the BoM. A scenario of high atmospheric concentrations of CO2 also favors the conditions of a sink for atmospheric CO2 into the waters of the BoM. Thus the model results suggest that external forcings have an important impact on the carbonate equilibrium in this coastal area.