Vertical Stratification and Driving Factors of Microplastics in the South China Sea: Distributions, Mechanisms, and Ecological Risks

The South China Sea, a semienclosed marginal sea and global hotspot for plastic pollution, presents complex hydrodynamics and pronounced water column stratification, offering a unique setting to investigate the vertical behavior of microplastics. This study examines vertical distribution, environmental drivers, and ecological risks of microplastics using water samples collected from 15 stations across 5 depth layers. Microplastic abundance significantly decreased with depth, accompanied by a compositional shift from low-density, small-sized, fibrous polyethylene and polypropylene at surface to higher-density, larger-sized, granular polyvinyl chloride and polyester in deeper layers. Principal component analysis and stratification index analysis revealed that gradients in temperature, salinity, density, and pressure jointly drive microplastic vertical differentiation. Notably, water column stability─primarily controlled by density gradients─plays a pivotal role in limiting microplastic vertical transport, as evidenced by a strong positive correlation between stratification index and vertical abundance gradient. A species sensitivity distribution model based on representative Chinese coastal species yielded a hazardous concentration for 5% of species of 12.3 items/L and a predicted no-effect concentration of 6.15 items/L. Moderate ecological risks were observed in surface waters, particularly for bivalves and planktonic crustaceans. These findings highlight the critical role of stratification in modulating microplastic exposure risks in marginal seas.