Seasonal effects of hydrodynamics and biofouling on the vertical transport of microplastics in the Vietnam coastal region, South China Sea

Microplastic (MP) pollution poses a significant threat to marine ecosystems, eliciting widespread concern. In the coastal upwelling region, both hydrodynamic and biological processes play crucial roles in effecting MP transport. How they impact the sinking characteristics of MPs in the coastal region remains largely unknown. In this study, a hydrodynamic model coupled with a biofouling module was employed to simulate the seasonal vertical transport and distribution of MPs, considering four polymer types and six particle size classes. Results showed that the vertical displacement of expanded polystyrene, polypropylene, and low-density polyethylene MPs (radii ≥ 1.0 mm) was initially driven by their buoyancy, but gradually became dominated by sinking velocity as biofouling increased their density. In contrast, the vertical displacement of rigid polyamide (RPA) MPs with the same sizes were primarily governed by vertical advection during all seasons except winter. In summer and spring, over 95 % of the MPs remained within the upper 0-5 m surface layer. In late seasons, especially winter, a substantial portion of MPs sank to deeper layers, with over 52 % of RPA MPs (radii ≥ 1.0 mm) accumulating at depths of 50-100 m. These findings provide a scientific basis for assessing the environmental impacts of MPs in marine systems and developing policies for the mitigation of MPs.