Turbulent mixing through aggregation-driven bioconvection in <i>Effrenium voratum</i>

Microalgae play a crucial role in aquatic ecosystems by contributing to global primary production and nutrient cycling. At high cell densities, these microorganisms trigger a phenomenon known as bioconvection, where their collective swimming behavior generates self-organized flow patterns within the surrounding fluid. In this study, we investigate the bioconvection mixing in the marine algae Effrenium voratum through controlled experiments and numerical simulations based on a continuum model. We employ particle image velocimetry to quantify both cell migration and the dynamics of fluid flow. Our results demonstrate that bioconvection enhances vertical transport and mixing, particularly when cell density exceeds normal population levels. Our findings reveal two significant phenomena: First, the energy spectra of velocity in a laminar bioconvection system exhibit an exponential decay of −5/3, reminiscent of Kolmogorov scaling and indicative of turbulence. Second, we directly capture the temporal switching of upward and downward fluxes, highlighting their role in promoting nutrient exchange between water layers. Cells accumulate at the air–water interface, leading to rapid sinking of the surrounding fluid and the formation of plumes, resembling Rayleigh–Taylor instability. Additionally, our minimal model effectively reproduces these dynamics, confirming that aggregation-driven bioconvection enhances vertical mixing. These findings provide new insights into how aggregation-induced bioconvection affects nutrient distribution and light transmission in aquatic systems.