Characterizing vertical migration of Microcystis aeruginosa and conditions for algal bloom development based on a light-driven migration model

Light-driven vertical migration is critical in dense aggregation of Microcystis aeruginosa during algal blooms under reduced turbulence conditions in natural water bodies. This study examined the vertical migration characteristics of Microcystis aeruginosa in calm water based on a colony migration model with consideration of cell density change, and demonstrated the effects of mucilage fraction, colony size, irradiance intensity, and water turbidity. The results suggest that colonies with larger radii and under higher irradiance usually had a larger daily averaged retention time at the water surface (DRT). In addition to colony size, mucilage was found to be important in changing the vertical migration behavior of Microcystis colonies, in which increasing mucilage volume fraction can increase the migrating velocity of the colony as well as the length of time it remains at the water surface. Increase of light extinction also favors the aggregation of colonies at the surface. An approximate critical value of 2400 µmol photons m² s⁻¹ for maximum irradiance was found for persistent algal bloom development under the given simulation conditions. Extremely small colonies exposed to irradiance below the critical value were not likely to migrate to the water surface to form algal blooms. According to the DRT values, three regions with different ranges of irradiance and colony size were proposed as the critical conditions for algal bloom development. Rough comparisons to field observations suggested that these results were reasonable and meaningful, and have the potential to be applied in real cases following further validation by more detailed investigations.