Pollution Load and Transport Dynamics Govern Microplastic Export from Subtropical Estuaries

Estuaries are dynamic but understudied nodes in the global plastic cycle where hydrodynamics and particle interactions govern microplastic fate. We applied high-sensitivity pyrolysis gas chromatography tandem mass spectrometry and elutriation-based settling velocity experiments to quantify polymer-specific concentrations and settling behavior across Gulf of America estuaries. Site-averaged concentrations were highest in Galveston Bay (0.82–19.8 μg L–1), particularly along the Houston Ship Channel, followed by Matagorda Bay (0.09–3.7 μg L–1), whereas other Gulf estuaries had lower concentrations (0.12–3.4 μg L–1). Polymer compositions differed among estuaries likely driven by specific point sources. Settling experiments revealed a continuum of velocities, from buoyant to >38 m d–1, with frequent deviations from density-based expectations due to aggregation, biofouling, and resuspension. Time-series observations at estuarine mouths showed ebb-dominant export, indicating temporary retention within estuaries followed by tide-modulated export to the coastal ocean. Comparisons with transport modeling confirmed that retention and export depended on both polymer properties and hydrodynamic forcing. These results demonstrated that estuaries act as semipermeable filters, intermittently trapping and remobilizing microplastics before exporting them to the shelf, with important implications for regional and global plastic budgets.