Controlling phosphorus is fundamental to limit the risk of eutrophication of continental aquatic ecosystems. Integrated modelling of its concentration in the aquatic continuum requires specific tools for water bodies. However, although simple static empirical models and complex biogeochemical models are numerous, there are few relatively simple and flexible models able to simulate seasonal variations in phosphorus concentrations in water bodies and particularly in reservoirs. In this study, the two-layer dynamic model, LakeMab, simulating phosphorus variations in water bodies, was enhanced to consider some tributary characteristics and reservoir specificities. It was then applied to the case of a reservoir in western France, Lake Bultiere. Without any calibration, the modified model reproduced reasonably well seasonal variations in phosphorus concentration in the lake. A sensitivity analysis showed the importance of improvements related to reservoir functioning (outlet depth, water level fluctuations) and the smaller importance of those related to tributaries (variable concentrations, depth of riverine inputs). The model can easily be applied to diverse lentic systems and could be coupled to stream models, thereby making it a useful tool for managing water quality in lakes and reservoirs.