The Salinity Difference and Clay Mineral Types Affect the Distribution of Microplastics in the Seabed: New Evidence from the Western North Yellow Sea

Abstract

Salinity and clay mineral types have been shown to influence the migration and settlement efficiency of microplastics (MPs) under restrictive experimental conditions. However, current research is limited to deep trenches or laboratory conditions, and studies in the semi-enclosed sea area of the continental shelf are still lacking. We investigated the effects of bottom seawater salinity and clay mineral types on MPs distribution in surface sediments using the western part of the North Yellow Sea as an example, where current conditions are complex and salinity changes rapidly over short distances. Under detection conditions with a minimum detection limit of 10 μm, the abundance range of MPs in the investigated sea area reached 24–1134 items/(g dry weight). The distribution of MPs was in good agreement with the isohaline of the bottom seawater, and MPs tended to converge in the high salinity area. However, there is an exceptional case in which the temperature and salinity difference caused by the cold water mass can create a frontal flow that blocks the transport of terrigenous materials to the middle of the cold water mass. This phenomenon causes MPs to settle at the edge of the cold water mass. A significant positive correlation exists between montmorillonite with expansive properties and fragment MPs and MPs with particle size > 100 μm, which have a larger surface area (p < 0.05). The negative charges on the surface of MPs and clay minerals are neutralized, promoting the heterogeneous aggregation between clay minerals and MPs and accelerating the sedimentation process of MPs in the ocean. This is another important reason for the accumulation of MPs in the high-salinity region. This study provides a basis for pollution prevention and control of MPs in the shallow sea, supplying new insights into the effects of bottom seawater salinity and clay mineral type on the distribution of MPs.