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Delta-FrontCities as Dynamic Receptors of MicroplasticFluxes: Seasonal Source Switching across Multioutlet Deltas
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Coastal cities near river deltas—like Macao, studied here—collect much higher levels of microplastic pollution in their waters than the open ocean nearby, and researchers found the plastic's source shifts with the seasons depending on winds and water currents. This means efforts to clean up or reduce microplastic pollution in these coastal waters (which people swim in, fish from, and get seafood from) need to target different river branches at different times of year rather than using a one-size-fits-all approach.
Delta-front coastal cities are terminal receptors of riverine microplastics (MPs), yet transport pathways linking multioutlet deltas to nearshore exposure under seasonal hydrodynamic variability remain poorly resolved. Using Macao in the Pearl River Delta (PRD) as a case study, we integrated four-season observations, high-resolution ROMS backward particle tracking, and literature-based fingerprinting. Mean MP abundance in Macao Bay (1.05 ± 1.18 n/m3) was 2.7 times higher than offshore (0.39 ± 0.57 n/m3). Pump sampling revealed subsurface peaks in autumn (up to 15.92 ± 39.58 n/m3). Piecewise structural equation modeling indicated that thermohaline variability regulates vertical sorting. Buoyant PP/PE assemblages (∼45–60%) dominate in summer, while denser PET/PS particles increase in winter. Particle tracking identified a seasonal source-switching pattern: western PRD outlets dominate in winter, whereas the Modaomen outlet contributes most in spring and summer, driven by plume orientation and wind forcing. Literature synthesis corroborates the consistency between modeled pathways and outlet-specific fingerprints. These results suggest that MP delivery is governed by seasonally reconfigured hydrodynamic connectivity rather than static sources. This framework shifts MP attribution from static bulk flux estimation toward dynamically constrained source-pathway-receptor linkages, providing a basis for seasonally targeted MP mitigation in complex deltaic systems.
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