Numerous experimental studies showed that the phytoplankton Chla-to-Carbon ratio (Chla:C) is highly variable, whereas most of the marine ecosystem models use a constant ratio. In this work, we tested three different formulations for computing the modelled Chla in a 3D coupled hydrodynamical-biogeochemical model of the Southwest lagoon of New Caledonia. The first formulation considers a constant Chla:C ratio. In the second one, Chla is a diagnostic variable related to the variable phytoplankton nitrogen-to-carbon ratio. In the last formulation, Chla is a state variable of the model, which is dynamically simulated. Results showed important differences between the formulations, the first leading to overestimate the Chla concentration in low nutrients conditions. Thus, this study strengthens the importance of the Chla modelling in a coupled model in order to better estimate a crucial variable for validation of ecosystem models.