Modelling the vertical migration of different-sized Microcystis colonies: coupling turbulent mixing and buoyancy...

Exceptional vertical migration ability provides the cyanobacterium Microcystis with competitive advantages in bloom formation and dominance establishment. Studies have been conducted on the vertical migration patterns of Microcystis since the 1970s; however, bloom simulations remain limited. Here, we used a simple model based on the viscous drag force of turbulence and analysed the motion characteristics of Microcystis colonies. The vertical distribution of turbulent kinetic energy (K_Z), cell concentration and colony size profiles in Lake Taihu (Meiliang Bay and Gonghu Bay) and the critical vertical turbulent kinetic energy of colonies (TK_Z, i.e. the anti-turbulence ability of colonies) were tested. The results showed that, under steady K_Z profiles, colonies had relative rest positions (RRPs) where K_Z ≈ TK_Z and colonies of the same size gathered together. The vertical migration patterns were affected more by turbulence than by density (colony mass density) if the average K_Z of the water column (MK_Z) was not equal to TK_Z. If MK_Z ≈ TK_Z, the colonies could exhibit a diurnal pattern of sinking at noon and floating upwards before dawn without steady RRPs. Our results suggest that studies on RRPs may offer optimizations for bloom forecast and control in the future due to easier simulation of K_Z profiles than that of flow fields.