Modeling seasonal salinity variations in a large West African lagoon (Nokoué, Benin): Major drivers and mechanisms

Nokoué lagoon is a shallow water body connected to the Gulf of Guinea through the long and narrow Cotonou channel. The salinity dynamics in the lagoon is investigated using the 3D SYMPHONIE numerical model. We first validate the model using salinity and water level data. Simulated and observed salinity and water level variations compare well demonstrating that the reference model simulation correctly reproduces the dynamics of the lagoon. By performing several simulations with varying external forcings and freshwater fluxes, the main drivers of salinity variability in the lagoon are identified. We first focus on the salinization phase of the lagoon at the end of the tropical wet season (between November and February), and we investigate the total change in salinity and the salt fluxes involved in these variations. The high frequency salt fluxes associated with the ocean tide import salt from the ocean via the Cotonou channel to the south of the lagoon. Baroclinic fluxes, associated with the influence of salt on density and associated pressure gradient, increase this local salinity input in the south, but also play a major role in the dispersion of salt throughout the lagoon. Two rivers provide a permanent freshwater inflow in the north/ northeast of the lagoon that limits the salinization of the whole lagoon. Finally, wind-generated high frequency recirculation, partially prevents the salinisation of the North-East of the lagoon. During the desalinization period (between May and September), the lagoon salinity variations are highly sensitive to the magnitude of river inflow. At a constant river flux rate, the lagoon attains an equilibrium state in salinity. As this equilibrium is reached, both the overall salinity level and the time needed to achieve it decrease notably with higher fluxes. Beyond 100 m3 s-1, only a small area near the Cotonou channel retains higher salinity, while surpassing 500 m3 s-1 results in complete desalination of the entire Nokoué lagoon.