Spatio-temporal heterogeneity of microplastic occurrence characteristics in surface waters of straight and meandering river channels

Although rivers are recognized as major pathways for microplastic transport from land to sea, the regulatory role of river morphology in this process remains poorly quantified. Here, we investigated the spatial characteristics and seasonal variations of microplastics in surface water from straight/meandering channels within the same river. Microplastic abundances in straight channels ranged from 46.44 to 108.54 (2022 rainy), 17.00-42.57 (2023 dry), to 15.15-27.91 n/m (2023 rainy). In contrast, meandering channels exhibited broader concentration ranges of 7.67-71.85, 19.70-81.71, and 17.46-64.93 n/m for the same periods, respectively. Straight channels, characterized by simplified hydraulic architecture and laminar flow regimes, largely preserve source-dominant discharge patterns. Meandering channels, however, are governed by morphology-driven hydrodynamics that drive complex redistribution processes. Larger (330-5000 μm) and denser microplastics (e.g., PVC, PMMA) tend to deposit in straight channels, but may remain suspended in meandering channels. While most microplastic shapes exhibited distinct seasonal signatures in straight channels, their distributions became randomized in meandering reaches. Critically, as particle density approaches that of water, local hydrodynamic forces override intrinsic microplastic properties as the primary factor governing microplastic distribution in meandering systems. These findings collectively support a morpho-hydrodynamic sorting framework, redefining rivers from passive conduits to active filters that fundamentally restructure microplastic assemblages.