Simulation of vertical position of buoyancy regulating Microcystis aeruginosa in a shallow eutrophic lake

Vertical distributions of the cyanobacterium Microcystis aeruginosa are exa mined in a shallow lake in relation to mixing and thermal stratification ov er three days. A model of buoyancy regulation by Microcystis aeruginosa, ap plicable for turbulent environments, is coupled with a one-dimensional hydr odynamic model. The coupled model is applied to Thomsons Lake in Western Au stralia to examine the relationship between buoyancy regulation and the dai ly stratification/destratification cycle. The vertical distribution of Micr ocystis aeruginosa in Thomsons Lake depends on the carbohydrate ballast dyn amics and the colony size. When thermal stratification occurs, all the simu lations show a similar general pattern of diurnal vertical migration of the Microcystis aeruginosa colonies. The colonies accumulate at the surface du ring the night and in the morning the colonies lose buoyancy, which leads t o a reduction by similar to 50% in colony concentration in the top 0.2-0.3 m of the water column. Afternoon winds redistribute the population over the entire water column. When the lake is fully mixed, the vertical migration pattern of the Microcystis aeruginosa colonies may be affected, depending o n the colony size and the intensity of the mixing.