Biofilm growth on buoyant microplastics leads to changes in settling rates: Implications for microplastic retention in the Great Lakes

Buoyant microplastic pollution disperses widely from sources via strong wind-driven water currents in lakes and oceans. This ability for dispersal depends critically upon the particle's density, which can change over time due to microbial growth (biofilm). This study quantifies biofilm-induced sinking rates of irregularly-shaped polypropylene granules (~125-2000 μm) via ex-situ experiments emulating a Great Lakes freshwater environment. Biofilm development increases particle density and lowers microplastic rise velocities, eventually causing sinking. We observed sinking for 100% of small and intermediate microplastics, and 95% of large microplastics. Under constant environmental conditions, sinking onset was observed sooner for smaller particles (~125-212 μm, 18 days) than for larger particles (~1000-2000 μm, 50 days). Differences in settling onset would lead to size-fractionation of particle sedimentation, whereby smaller particles are deposited closer to their sources relative to larger particles. Our study demonstrates a novel mechanism by which buoyant microplastics can selectively sink from the lake surface.