Reliable River Microplastic Monitoring Using Innovative Fluorescence Dyes—A Case Study

Microplastic (MP) contamination in riverine systems poses a growing environmental challenge, and their spatial and temporal variability complicates proper assessments. This study investigated MP concentrations (≥10 µm) across three German rivers using fluorescent staining-based detection. The results reveal highly heterogeneous distributions ranging from 4 to 1761 MP/L. The Rehbach displayed the highest mean MP concentration (540 ± 476 MP/L), whereas the Alb had the lowest (98 ± 54 MP/L). Long-term monitoring underscored pronounced temporal fluctuations linked to changing inputs, weather events, and hydrodynamics. To capture these fluctuations, monitoring campaigns must consider an appropriate temporal sampling framework. Further, to address detection challenges, the study compared 0.5 L grab sampling with 100 L pump sampling (PSU) and observed that the PSU yielded 4.7 times higher MP concentrations with improved reproducibility (27 ± 25% vs. 49 ± 33%). These results highlight the critical need for standardized protocols and scalable, cost-effective methods for reliable MP quantification and hotspot identification in freshwater environments.