Seasonal variability of microplastic transport modulated by tides: A mass-based assessment in a turbidity maximum zone

Estuaries are key pathways for microplastics entering the ocean, with tides modulating in their transport. This study assessed microplastic abundance and mass within the turbidity maximum zone (TMZ) of a tidal estuary across two tidal cycles, using high-resolution sampling and mass-based quantification. The results show that microplastic abundance decreased and particle size diminished seaward, reflecting processes of fragmentation and settling within the estuary. In the TMZ, elevated summer temperatures, strong turbulence, and eddy-shear covariance (ESCO) circulation promoted the bottom enrichment of heavier microplastics, leading to higher mass-particle number flux ratios, larger instantaneous transport, and greater bottom concentrations compared with surface waters. Microplastic dynamics were modulated by tidal fluctuations and ESCO, with bottom-layer abundances varying out of phase with tidal height. Surface responses exhibited a time lag relative to the bottom, which was shorter in summer (6 h) than in winter (9 h), likely due to stronger winter stratification (Brunt-Vaisala frequency > 10 s⁻²) that lengthened the vertical exchange. Tidal asymmetry and pumping further regulated fluxes, as vertical compensation between surface export and bottom import reduced particle-number export by over 90 %. The tidally integrated flux across the TMZ interface indicated a net seaward flux of 19.41 kg day in summer versus 1.33 kg day in winter when seaward residual circulation was weaker. The tidally integrated fluxes characterize local exchange that may be further attenuated downstream by residual circulation, tidal asymmetry, and ESCO, thus potentially reducing effective export to the bay.