Floodplain vegetation filters microplastics during a major Rhine flood event

Abstract Microplastics enter rivers via various point and diffuse sources and are transported with the water. However, as their properties change over time in the aquatic environment, they may settle and accumulate at least temporarily in the riverbed. Increasing flow velocity during flood events facilitates remobilisation of microplastics and sediments, enabling their transport and redeposition further downstream and in adjacent floodplains. Floodplains are highly dynamic and vegetation-rich environments. Vegetation cover in floodplains generally increases both surface area and surface roughness, which slows down water flow during flooding and can enhance sediment deposition. While recent studies suggest that microplastic distribution in floodplains is influenced by factors such as local topography and flood frequency, the potential role of vegetation in microplastic deposition during flooding has yet to be analysed. Thus, this study investigates the retention of microplastics and natural sediments by floodplain grassland vegetation during a single major river flood. We sampled grassland vegetation in a Rhine floodplain in Northern Cologne (Germany) after a summer flooding in 2021 in a flooded area and compared it to a close non-flooded grassland that was only affected by atmospheric microplastic deposition. The deposits from both sampling sites were rinsed off from the plants and subjected to ZnCl ₂ density separation. Organic matter was removed via enzymatic-oxidative purification, and remaining microplastics were analysed using μ-FPA-FTIR spectroscopy. Our study shows that (I) more vegetation biomass resulted in increased deposition of natural sediments and small microplastics (6–1000 μm). (II) Our results also indicate that microplastics and natural sediments follow different deposition dynamics. (III) Additionally, fluvial and atmospheric deposition on vegetation strongly differ in microplastic numbers, shapes, size and polymer type. Our study provides insights on the role of vegetation for microplastics retention and thus accumulation and distribution in floodplains at the interface of fluvial and terrestrial ecosystems.