Spatial variation of meso- and macroplastic’s in the Lower Rhine : implications for an effective monitoring framework

Mesoplastics (0.5–2.5cm) and macroplastics (>2.5cm) form a growing concern in the Lower Rhine, where they persist, fragment into smaller particles, and are transported downstream toward the North Sea, contributing to regional and marine plastic pollution. To address these challenges, the Dutch and German organisations launched The Rhine Connects initiative, which aims to identify plastic sources and develop targeted mitigation strategies. Plastic items are classified using a modified River OSPAR protocol, linking identifiable items to likely sources. Effective monitoring requires understanding where plastics accumulate to select representative sampling locations. This study addresses this need through long-distance sampling at eight sites along the Lower Rhine. Sampling took place during low discharge to minimise hydrological variability. At each site, plastics were collected across riverbanks, bend types, and depths, and analysed for concentration and composition. Results showed no significant differences in plastic concentration between sites, but clear downstream trends in composition and size: smaller and more degraded plastics became increasingly dominant and shifts in OSPAR categories indicated inputs along the transect. Which means that monitoring shouldn't reduce the used sampling locations to avoid missing sources because of the compositional changes and more distance between source means the plastic loses identifiable features.

Hydromorphological factors created heterogeneous patterns in abundance and composition, underscoring the need to monitor across all morphological factors to avoid bias. Overall, this study shows that meso- and macroplastic patterns in the Lower Rhine are shaped by local inputs, hydrodynamic conditions and material properties, providing essential insights for designing long-term monitoring strategies.