Abundant small microplastics hidden in water columns of the Yellow Sea and East China Sea: Distribution, transportation and potential risk.

Microplastics (MPs) pose significant concerns for marine ecological security due to their minuteness and ubiquity. However, comprehensive knowledge on their distribution and fate in seawater columns remains limited. This study investigated the abundances and characteristics of MPs across 3-6 water layers in the South Yellow Sea and East China Sea. Results indicate that high-abundance small MPs (< 100 µm) (average 6567 items/m³) were hidden beneath the sea-surface, predominantly fine-grained particles (< 20 µm) and high-density polymers (> 1.03 g/cm³). The total suspended MPs (5.0-834.2 µm) are estimated at 2.9-3.1 × 10¹⁷ particles, with most of them occurring in upper layers. In profiles, their distribution varied by physical properties with depth; fragment-shaped and high-density MPs increased in proportion at greater depths, contrasting with fibrous MPs. These MPs originated primarily from the Yangtze River and their winter transport was driven by the Yangtze River Dilution Water, East China Sea Coastal Current, and Yellow Sea Warm Current, resulting in their accumulation in coastal and estuarine regions. Consequently, the Yangtze River Estuary ecosystem faces substantial risks from MP pollution throughout the water column. This work unveils the prevalence of small MPs in coastal water columns and intricate interaction between their fate and hydrodynamic conditions.