A theoretical modeling framework for motile and colonial harmful algae

1. Harmful algal blooms are increasing in both severity and frequency across the globe. Many bloom-forming species are capable of vertical motility and colony formation. The cyanobacterium Microcystis aeruginosa is a common example of such a species, yet current models poorly predict vertical distributions of M. aeruginosa . 2. To couple the hydrodynamics, buoyancy, and the colony dynamics of Microcystis , we present a system of one-dimensional advection-diffusion-aggregation equations with Smoluchowski aggregation terms. 3. Results indicate Smoluchowski aggregation accurately describes the colony dynamics of M. aeruginosa . Further, transport dynamics are strongly dependent on colony size, and aggregation processes are highly sensitive to algal concentration and wind-induced mixing. Both of these findings have direct consequences to harmful algal bloom formation. 4. While the theoretical framework outlined in this manuscript was derived for M. aeruginosa , both motility and colony formation are common among bloom-forming algae. As such, this coupling of vertical transport and colony dynamics is a useful step for improving forecasts of surface harmful algal blooms.