Effect of grain size on microplastic occurrence and spatial distribution in the Cávado River

The exponential growth of plastic production over recent decades has raised increasing concern regarding microplastic (MP) pollution in aquatic ecosystems. Rivers act as major conduits transporting microplastics from inland sources towards marine environments, yet their spatial distribution and accumulation within riverine sediments remain incompletely understood. In particular, the combined influence of riverine structures, sedi- ment characteristics, and anthropogenic pressures on microplastic occurrence is still unclear. This study in- vestigates the spatial distribution of sediment-associated microplastics along the C´avado River (Portugal), with a focus on the potential role of sediment grain size and environmental context. Sediment samples were collected from 14 sites spanning diverse environmental conditions, including urbanised, industrial, and agricultural areas. A subset of samples was subjected to granulometric analysis to assess potential relationships between grain size and microplastic abundance. Samples were processed using density separation and analysed visually under optical microscopy following a less invasive methodological approach. Results indicate a statistically supported downstream increase in microplastic abundance, with localised hotspots observed near wastewater treatment plants, industrial complexes, and urban areas. Although no significant linear relationship was identified between sediment grain size and microplastic abundance, finer sediments, characterised by higher silt and clay fractions, exhibited trends towards greater microplastic retention. These findings highlight the role of riverbed sediments as important accumulation zones for microplastics and underscore the need for spatially targeted mitigation strategies. The study contributes to improving the understanding of microplastic distribution patterns in fresh- water systems and provides a basis for future investigations integrating refined sampling designs and comple- mentary analytical approaches.