Suspended sediment flocculation and riverine-lacustrine hydrodynamics jointly shape the vertical transport and fate of buoyant microplastics.

Understanding the vertical transport and fate of microplastics (MPs) in freshwater systems is critical for clarifying their redistribution and downstream transfer. However, in river-connected lakes, the mechanisms controlling the downward transport of buoyant MPs remain poorly quantified. Here, we developed a three-dimensional hydrodynamic-particle tracking model for Lake Poyang to investigate the vertical transport of buoyant MPs under contrasting river-lake exchange regimes, incorporating sediment flocculation as a dynamic process modifying the size and effective density. The simulations revealed size-dependent and spatially heterogeneous settling behavior: 69, 210, and 750μm MPs initiated settling after 4.5, 35.4, and 756.8 h on average, with maximum spatial differences exceeding 60% for the same size. Sediment flocculation shortened the settling timescale of small MPs from weeks to hours, whereas hydrodynamic mixing advanced the downward transport of larger particles. Riverine conditions favored export and deposition, lacustrine conditions enhanced surface retention; during backflow, Yangtze River input increased in-lake MP abundance, allowing suspended-sediment-induced densification to promote rapid deposition of small buoyant MPs. These findings highlight the importance of coupling particle-size-dependent responses with sediment flocculation and river-lake hydrodynamics to understand buoyant MP transport and fate in river-connected lakes, providing process-based insights for prediction and risk assessment.