Abstract:By coupling advection-diffusion equation and buoyancy regulation, a coupled model for simulating diurnal changes of vertical concentration distribution of Microcystis was developed. Different from existing models, the present model incorporated the buoyancy regulation of the Microcystis colonies with different diameters. The numerical simulations showed a good agreement with the field data acquired in Lake Vinkeveen, the Netherlands. It was found that the highest concentration showed up below rather than on the water surface and turbulent diffusion had a significant impact on the vertical distribution. The colonies with smaller sizes (20~50μm in diameter) distributed more uniformly than those with larger sizes (100~200μm in diameter). The densities of colonies decreased in the evening, and thus Microcystis float upwards. However, the densities increased and the Microcystis sank in the daytime. Hence, the surface concentration of the Microcystis reached the highest value at the sunrise and reached the lowest value at the sunset.