Aggregation and breakage of cyanobacterial colonies

Harmful cyanobacterial blooms are a frequent nuisance in many freshwater bodies around the world. Microcystis is a common genus of colonial cyanobacteria which relies on its large colony size to achieve high flotation velocities. The colony size and morphology are strongly influenced by the local turbulence in the suspension, induced either by wind or artificial mixers. We investigate both numerically and experimentally the dynamics of colony formation under laminar and turbulent shear flows. Special attention is given to balance between cell division, aggregation and breakage. The experimental setup consists of the simultaneous use of rotational disks and microscopy. Colony shape, size distribution and moment kinetics are characterized by the laboratory culture of M. aeruginosa. The governing regimes are identified as a function of the main non-dimensional physical parameters. The results provide valuable criteria for the design of artificial mixing systems for bloom control.