The potentially harmful phytoplankton bloom is a world wide problem that became frequent in the last decade. Microcystis aeruginosa is a toxic cyanobacterium that proliferates frequently in reservoirs deteriorating their water quality. Hence, a monitoring and warning system to forecast the risk of cyanobacteria proliferation became essential for lake managing. For that purpose, a one dimensional hydrodynamic (DYRESM) model coupled to an ecological model (CAEDYM) has been applied to simulate the growth of cyanobacteria colonies found in a French reservoir. As primary results the model was able to follow the trend of the cyanobacteria dynamics in the reservoir.