Physical Controls on Microplastic Retention in Estuarine Systems: Interactions Between Hydrodynamics, Stratification, Bathymetry, and Particle Properties

Microplastics, introduced into estuaries through riverine discharge, wastewater, and surface runoff, have become a growing environmental concern due to their persistence and potential impacts on aquatic ecosystems. Understanding the mechanisms governing their transport and retention is important for predicting their accumulation and ecological effects. While estuaries are recognized as key zones for microplastic retention and export, existing studies often focus on individual processes or sites, leaving a limited, integrated understanding of how estuarine physical dynamics control microplastic fate. This review aims to synthesize current research to address this gap by examining how estuarine hydrodynamics, stratification, bathymetry, and particle characteristics interact to govern microplastic retention and export. Tidal asymmetry drives net landward transport under flood-dominated conditions, while ebb-dominated tides increase seaward export; residual circulation concentrates sinking particles near the bed and transports buoyant plastics at the surface. Stratification decreases vertical mixing, creating discrete layers that trap particles and promote estuarine turbidity maxima, with stronger freshwater inflows increasing the retention of dense particles and marginally buoyant microplastics more likely to escape during high discharge. Bathymetric complexity, including channels, shoals, and constrictions, localizes particle convergence and creates lateral segregation, with narrower estuaries being more prone to retention and wider systems promoting export. Particle density and biofouling modulate these effects: spherical sinking plastics often track sediment dynamics, while fouling can increase retention of previously buoyant particles. Altogether, microplastic fate is highly systemdependent, influenced by interactions between estuarine morphology, flow dynamics, and particle properties, as many transport processes are highly location-specific.