Interplay of Morphology and Polymer Composition Drives Estuarine Fate and Food Web Distribution of Nanoplastics: Evidence from a Multitrophic Mesocosm Using Upconversion Nanoparticle Tracers

Abstract Current ecological risk assessments often rely on spherical reference particles, potentially misrepresenting the behavior of abundant fibrous nanoplastics. Using upconversion nanoparticle (UCNP) labeling and inductively coupled plasma mass spectrometry (ICP-MS), we quantified the fate of size-matched spherical polystyrene (PS) and fibrous polyacrylonitrile (PAN) nanoplastics in an estuarine mesocosm. Distinct particle attributes─driven by the synergistic interplay of geometric morphology and polymer intrinsic density─altered environmental partitioning: spherical PS exhibited repeated settling–resuspension cycling, whereas fibrous PAN rapidly accumulated in surface sediments (94% aqueous removal within 12 h). This physical divergence shifted bioaccumulation patterns across trophic levels. Lower trophic organisms (Zostera asiatica, Crassostrea gigas, Rapana venosa) accumulated PS in proportion to water-column availability. Conversely, the demersal fish Sebastes schlegelii exhibited higher burdens of fibrous PAN despite lower aqueous concentrations. Time-gated imaging revealed stronger PAN-associated signals in hepatic tissues, consistent with morphology-enhanced tissue retention in benthic vertebrates. These results indicate that spherical models may underestimate microfiber risks in benthic food webs, highlighting the need to incorporate shape-specific partitioning and retention factors into nanoplastics risk assessments.