Governing Vertical Distribution of Microplastics in Freshwater Ecosystems: a Comprehensive Review

Abstract Microplastics (MPs) are ubiquitous in freshwater environments owing to their widespread use and persistence. Understanding the factors governing the vertical sedimentation of MPs is crucial for assessing their ecological risk and developing effective pollution control strategies. This review systematically examines the intrinsic particle characteristics, environmental drivers, and biological mediators that influence the settling behavior of MPs in freshwater systems. The density, shape, and size of MPs are critical determinants of their buoyancy, hydrodynamic properties, and aggregation potential. Environmental conditions, including hydrodynamics, water physicochemical properties, and surrounding media, regulate MP transport and deposition through complex interactions. Biological processes such as biofouling, ingestion and egestion by aquatic organisms, and bioturbation further modify the vertical distribution of MPs. Microbial colonization and extracellular polymeric substance (EPS) production alter MP density and surface properties, facilitating aggregation and sedimentation. Aquatic plants, zooplankton, and fish influence MP redistribution via interception, uptake, and excretion. Benthic macroinvertebrate bioturbation affects MP burial and resuspension at the sediment-water interface. Despite recent progress, knowledge gaps persist in understanding the synergistic effects of multiple factors under realistic environmental conditions. Future research should focus on developing integrated aggregation models that incorporate biofilm growth, environmental factors, and hydrodynamics, as well as creating high-precision vertical-flux models. Addressing these challenges will enhance the accuracy of MP fate predictions and inform targeted risk assessments and pollution control strategies for freshwater ecosystems.