Hydrodynamic controls on microplastic transport in tidal estuaries: A process-based review and conceptual framework

The abundance of microplastics in estuarine waters can vary significantly throughout tidal cycles. However, the hydrodynamic mechanisms controlling the transport of these particles remain poorly integrated across available studies. Reported responses often differ according to tidal phases, tidal amplitude, estuarine hydrodynamics, and methodological approaches, limiting comparisons among studies and broader conceptual understanding. This review critically analyzes current evidence on the influence of tidal dynamics on the abundance and morphological characteristics of microplastics in estuarine waters, with emphasis on the hydrodynamic processes regulating the transport, retention, and redistribution throughout daily and lunar tidal cycles. The analysis reveals that microplastic responses to tidal forcing are highly variable and strongly influenced by tidal asymmetry and estuarine hydrodynamic conditions, including river discharge, current velocity, tidal pumping, mixing intensity, residence time, and estuarine front dynamics. Generally, ebb-dominated regimes favor microplastic export to the ocean due to the dominant role of river discharge, while flood-dominated regimes promote greater retention and recirculation associated with resuspension induced by tidal currents and landward transport. Spring tides intensify microplastic transport and resuspension due to increased hydrodynamic energy, while neap tides favor particle deposition. Tidal dynamics exert less influence on microplastic characteristics, although buoyancy particles may be transported more efficiently landward during flood tides. To synthesize current knowledge, we propose a conceptual framework integrating the main hydrodynamic controls and transport regimes governing microplastic dynamics in estuarine waters. We also propose an actionable framework with recommendations for sampling design in tidal estuaries to improve methodological standardization and comparability among studies.