Typical migration patterns and fates of microplastics with varying properties in bays and their impacts on coastal ecologically sensitive areas

Rapid urbanization has intensified microplastic pollution in many global bays, yet the mechanisms driving microplastic behavior in these environments remain unclear. This study utilized field surveys, statistical analysis, and modeling methods to address this issue. The findings revealed three typical migration patterns of microplastics in bays. Microplastics less dense than seawater were easily transported by currents, drifting extensively throughout the bay, with about 37.2 % temporarily hovering in tidal channels and low-lying areas, ultimately leaving with ebb tides. Spherical microplastics denser than seawater were predominantly concentrated near the shore, primarily lingering in the subsurface layers. In contrast, fibrous microplastics, which are denser than seawater and the most prevalent type in human-impacted bays, displayed a unique behavior. The combined effects of their density and shape resulted in over 80 % being trapped in intertidal ecologically sensitive areas (ESAs). As a result, local ESAs, such as mangroves, bird habitats, and aquaculture, faced significant threats from fibrous microplastics and their sorption complexes associated with Cu, Pb, Cd, and Hg. Additionally, bay sediments acted as a source-sink community for microplastics. In Shenzhen Bay, China, approximately 27.1 × 1011 microplastic items were buried annually in sediments under normal hydrological conditions. However, if annual rainfall exceeded a threshold, these microplastics could be washed into ocean, serving as a source. Coastal raft aquaculture emerged as a significant contributor to marine microplastics, releasing about 3 %-8 % of terrestrial sources in Shenzhen Bay. This study enhances our understanding of microplastic behaviors and risks in bays.