Expanded size distribution of microplastics and ecological risk in a semi-enclosed coastal estuary

Microplastics (MPs) are ubiquitous emerging pollutants in aquatic environments, however, most studies have focused on large MPs (≥50 μm), potentially underestimating total contamination and associated ecological risk. This study quantified MPs ranging from 1 μm to 5 mm in the surface water of Tolo Harbour, a semi-enclosed urban estuary in Hong Kong, using Nile Red staining combined with stereomicroscopy for large MPs (≥50 μm) and flow cytometry for small MPs (1-50 μm). Average large and small MPs abundances ranged from 18 to 105 particles L^-1 and 30,713-447,943 particles L^-1, respectively. Small MPs abundance differed significantly among regions, decreasing from the Shing Mun River to Inner Tolo Harbour and the Lam Tsuen River, whereas large MPs abundance showed no significant spatial variation. Anthropogenic activities in densely populated catchments and hydrodynamic conditions, particularly low velocities in Inner Tolo Harbour, were identified as potential factors contributing to the spatial distribution of small MPs. Expanded size distribution patterns suggested that large MPs are potentially influenced by buoyancy-driven surface transport, whereas small MPs showed more uniform dispersion, potentially reflecting Brownian motion and complex water mixing. A species sensitivity distribution-based risk assessment indicated that large MPs concentrations were generally below the derived predicted no-effect concentration (PNEC), whereas small MPs concentrations exceeded the corresponding PNEC at most locations, indicating a potential ecological concern. This study provides an expanded size profile and screening-level ecological risk assessment for Tolo Harbour and highlights the importance of including small MPs in the monitoring and management of urban coastal waters.