How fluid flow influences colony formation and migration of cyanobacteria*

Cyanobacterial colonies frequently cause toxic blooms in many freshwater bodies around the world. These suspended bacterial aggregates rely on their large size to achieve fast, density-regulated migration. Yet, the interaction between colony migration and hydrodynamics remains elusive. This study investigates how colony size and shape affect their movement and how fluid flow may induce fragmentation and aggregation of these colonies. We performed flotation velocity experiments on field samples of cyanobacterial colonies and showed that the colony settling factor and density are correlated with their size and morphology. Furthermore, cone-and-plate shear experiments were conducted on colonies to measure variations in size distribution under various flow stresses. Experiments are complemented by a theoretical formulation, where a two-category population model captured well the measured size distributions. Our results provide useful guidelines for prediction models and control techniques for toxic cyanobacterial blooms.