Catchment-Specific Drivers of Microplastic Contamination in Two Central European Rivers: A Citizen Science Monitoring Campaign Along the Neckar and the Lusatian Neisse

Understanding microplastic (MP) transport dynamics in river systems requires spatially and temporally resolved monitoring to disentangle localized point sources from diffuse catchment inputs. This study investigated MP contamination across two contrasting Central European catchments: the heavily impounded, industrialized Neckar (n = 136; June 2026) and the transboundary Lusatian Neisse (n = 102; March and May 2026). A standardized monitoring campaign collected 0.5 L grab samples across both rivers and at the Neckar–Rhine confluence in Mannheim, including an inland port area, where 12 sites were sampled on four dates between January and April 2026. Samples underwent hydrogen peroxide digestion, fluorescent staining with a MP-specific dye mixture (particles > 10 µm), automated fluorescence microscopy, and geospatial analysis incorporating land cover and wastewater infrastructure metrics. Microplastic concentrations were significantly higher in the Neisse than in the Neckar (median 22.5 vs. 5.0 MP L−1; Cliff’s delta = 0.44). Because the two rivers were sampled in different months and at different water levels, seasonal and hydrological contributions to this difference cannot be excluded. In the Mannheim port area, the highest site mean was recorded upstream of a lock structure (544 MP L−1) with pronounced temporal variability at this site (5 to 1591 MP L−1). Neither local land use nor wastewater treatment plant (WWTP) metrics showed a detectable association with MP abundances in either river under the present design, which could reliably detect only moderate to strong associations. These results indicate that immediate land cover and WWTP proximity left no detectable signal in surface-water MP concentrations, possibly because point-source signals are masked by diffuse inputs, hydrological dynamics and temporal variation. Concentrations in the Neisse decreased from the upper to the lower reaches; whether this reflects the secondary WWTPs of the upper catchment or other longitudinal differences (e.g., flow conditions) could not be resolved.