Beyond abundance: Integrating fragmentation, toxicity, and vulnerability into coastal microplastic risk assessment

Microplastic (MP, <5 mm) pollution is a growing threat to both marine ecosystems and human health; however, the ecological risks associated with it remain poorly understood in complex coastal environments. We developed a three-tier coastal microplastic risk framework that couples species sensitivity distribution (SSD)-based short-term thresholds with a fragmentation-adjusted, polymer-specific chronic index and functional-zone vulnerability weighting. In Shenzhen's nearshore waters, summer riverine concentrations peaked at 47.43 items/L and exceeded SSD thresholds at Yantian Harbor. However, long-term risks were re-ordered by polymer persistence, sediment retention, and the potential to absorb pollutants and release additives and associated metals, shifting hotspots eastward in autumn toward semi-enclosed fjords dominated by aging polystyrene (PS) foams and coating-derived or other additive-rich polymers. Across zones, aquaculture, ports, and tourism hubs systematically overlapped with areas of high vulnerability, revealing governance conflicts invisible to metrics based solely on abundance. Our framework quantitatively reconciles laboratory thresholds with in situ dynamics, providing a transferable basis for aligning microplastic risk zoning with 2035 coastal conservation targets. This study advances ecological risk assessment by linking laboratory-derived toxicity thresholds, polymer degradation, and chronic toxicity potential with in situ dynamics, providing a transferable model for managing coastal ecosystems under accelerating urbanization and plastic pollution.