Systematic CFD-based evaluation of physical factors influencing the spatiotemporal distribution patterns of microplastic particles in lakes.

Spatiotemporal distribution patterns of microplastic (MP) particles in lakes hinge on both the physical conditions in the lake and particle properties. Using numerical simulations, we systematically investigated the influence of lake depth and bathymetry, wind and temperature conditions, MP particle release location and timing, as well as particle diameter (10, 20, and 50 μm). Our results indicate that maximum lake depth had the greatest effect on the residence time in the water column, as it determines the settling timescale and occurrence of hydrodynamic complexity such as density-driven flows in the lake. Increasing particle size from 10 to 20 and 50 μm also significantly reduced the residence time making particle size the factor with the second strongest effect on the residence time and, in turn, on the availability of MP particles for uptake by organisms. Changing bathymetry from a uniform to a non-uniform had a less pronounced effect on particle residence time compared to maximum depth and particle size. Release location, wind conditions, and release time had comparably little effect on particle behavior but became more important as MP particle size decreased. The release of the 10 μm MP particles in the deeper lakes with uniform bathymetry during summer with stable thermal stratification, resulted in a nearly month-long turnover phase in the fall in which both settling and rising of particles occurred simultaneously. This was caused by convective heat and water transport during this period. In these scenarios about 2.6 to 5.4 % of the released MP particles were held in or returned to the water layers near the lake surface. While acknowledging the dominant role of lake depth and MP particle size on the particle residence time, this study further emphasizes that it is ultimately a particular combination of different factors and their interactions that shape MP distribution patterns in lakes.