Summer Hydrodynamics as a Dual Driver of Microplastic Retention and Settling in Shallow Water Columns

Abstract In shallow lakes, wind-driven turbulence and thermally induced convection control water mixing. Together, and depending on depth, they interact with microplastic (MP) settling to determine how particles are distributed throughout the water column. To investigate these processes, two MP addition experiments were conducted in a 112 m 3 aquatic mesocosm during summer using 1–5 µm microspheres. High resolution data on MP concentrations, water velocities, wind speeds, and water and air temperatures were collected. Additionally, using OpenFOAM, a three-dimensional CFD model incorporating fluid–particle interactions was configurated to quantitatively interpret the experimental data of MP transport. The results indicated that although Stokes’ settling velocity predicted MP would take up to 182 days to reach the mesocosm bottom, MP of all sizes was detected just above the bed (3 m) within only 3 days. The vertical distribution of MP, characterized using the Péclet number (Pé, settling velocity/turbulent diffusion), increased with depth but remained < 1. In the near-surface layer (< 0.25 m), approximately 10% of MP remained in suspension by forced convection (wind-induced turbulence) and particle–fluid interactions. Free convection dominated MP transport between 0.25 and 3 m depth. Pé values < 1 indicate that, despite increasing gravitational settling with depth, free convection remains the dominant process. These results demonstrate that MP transport in shallow water columns is not governed by gravitational settling alone but is fundamentally controlled by the interplay of wind-driven turbulence and thermally induced convection. This generates depth-dependent mixing regimes and redistributes MP across the water column, increasing the likelihood of exposure to aquatic organisms at all depths.