Direct numerical simulation of algae migration in a lake

The spatial distribution of algae in lakes inter alia depends on ambient mixing conditions in the water, which, in turn, are driven by atmospheric conditions at the boundary. Also, their density can be actively regulated by a physiological response to light, as such to adjust their buoyancy. The aim of the present study is to investigate the combined effect of turbulent mixing and buoyancy regulation on the vertical migration of algae under natural conditions. Given the typical size of algae, small scale turbulence will play an important role in this migration. Therefore, in this research we propose coupling Direct Numerical Simulations of turbulent flow with particle tracking of biologically active organisms. The intrinsic gap between biological and flow time-scales makes such an endeavor a non-trivial exercise. Therefore, in order to conceal time scales of flow and biology we propose a scaling procedure. The proposed methodology is applied to study the vertical migration of a particular cyanobacteria species: Microcystsis Aeruginosa. Our results show that our model is effective in representing the vertical daily migration of Microcystis Aeruginosa, in a sense that realistic characteristics are simulated as compared to nature.