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Hydrodynamic controls on microplastic transport in tidal estuaries: A process-based review and conceptual framework
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This review pulls together existing research to show that tides act like a pump for microplastic pollution in estuaries — some tide patterns flush plastic particles out to the ocean, while others trap and recirculate them near coastlines where seafood is harvested. Understanding these patterns matters because it helps scientists predict where microplastics build up in the water we fish from, and could improve how we monitor and eventually reduce human exposure through contaminated seafood.
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.
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Researchers tracked how tidal cycles influence the movement and distribution of microplastics within estuaries, finding that tidal dynamics play a major role in concentrating or dispersing particles across intertidal zones. The study provides a framework for understanding microplastic transport in tidal environments.
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Researchers investigated how tidal cycles influence the abundance and characteristics of microplastics in an urban estuary, finding that tidal dynamics significantly affect MP transport dynamics and that estuaries serve as a major conduit for MP transfer from rivers to the ocean.
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Estuaries — where rivers meet the ocean — act like traps for microplastics, with tides, sediment, and shoreline interactions causing much of this plastic pollution to get stuck rather than flushed out to sea. This matters because these are often the same waters used for fishing and shellfish harvesting, meaning trapped microplastics may build up in local food sources over time rather than dispersing and diluting in the open ocean.
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Scientists found that tiny plastic particles in river water get trapped and concentrated near the bottom of estuaries (where rivers meet the ocean) by tides and saltwater layers. Over time, these microplastics get pushed out to sea, meaning rivers are continuously dumping plastic pollution into our oceans. This matters because microplastics can enter the food chain through seafood we eat, and understanding how they move helps us figure out where the pollution is coming from and how to reduce it.
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Researchers tracked microplastics in the Shenzhen River estuary during tidal cycles, measuring how particles moved with incoming and outgoing tides. Surface and underwater samples showed common plastics like PET, PE, and PP shifted position based on tides and water layering, with heavier PET particles stirred up during mid-to-high tides while lighter plastics stayed near the surface. The study suggests plastic pollution in this estuary cycles around rather than steadily building up.
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