Aydat lake is a dimictic eutrophic lake where Cyanophycean blooms occur. In order to study with accuracy this lake functioning, a one-dimensional vertical model (Licorne 2 model) was developed. This model takes into account physical, chemical and biological interactions and simulates the algal succession of the five main phytoplankton algae for two different years (1984 and 1985). It appears that the growth rate, the sedimentation rate, or the death rate do not explain by themselves the algal succession, and more specially the blue-green blooms. However, a good correlation between field data and Licorne 2 results are obtained by introducing specific advantages of blue-green algae such as buoyancy regulation mechanisms, capacity to fix atmospheric nitrogen and to grow at high pH value and toxin production. In the view of these results, a new understanding of the Aydat lake algal succession is then possible, where competition for light, food and limitation of the losses (sedimentation, grazing) take an important place in a thermal and chemical vertical stratified environment.