The role of floodplain vegetation in filtering microplastics during a major Rhine flood event

Microplastics in rivers originate from various sources and can be transported by river water. During their time in the river, the properties of microplastics may change leading to a temporary deposition and accumulation in the riverbed. In particular, during floodings, stronger flow velocities occur and can remobilize microplastics and sediments, transporting them further downstream and to adjacent floodplains. Floodplains represent dynamic and vegetation-rich environments, where the vegetation increases the floodplains’ roughness, resulting in slower flow velocities during floods and potentially enhance deposition of sediments and microplastics. While previous research has shown that factors such as local topography and flood frequency influence microplastic distribution in floodplains, however, the role of vegetation in microplastic filtering during floods has not been studied.  This study investigates the retention of microplastics and natural sediments by floodplain grassland vegetation during a major river flood. Directly after the flood event in July 2021, we sampled vegetation from a formerly flooded Rhine floodplain north of Cologne, Germany. For comparison we sampled vegetation from an adjacent non-flooded grassland, which was only affected by atmospheric microplastic deposition. After rinsing the deposits from the vegetation, we used ZnCl₂ density separation to extract microplastics, followed by enzymatic-oxidative purification to remove organic material and µ-FPA-FTIR imaging for microplastic analysis.   Our findings show that microplastics from fluvial and atmospheric origin differ in terms of their numbers, shapes, sizes, and polymer types. Concerning the samples from flooded vegetation, higher vegetation biomass was associated with increased deposition of both natural sediments and small microplastics. However, we observed distinct deposition patterns for natural sediments and microplastics. Our results provide valuable insights into the role of floodplain vegetation in the retention, accumulation, and distribution of microplastics at the interface between aquatic and terrestrial ecosystems.