Lisa Roscher

• Microplastics (MP) have received increased scientific, political and societal attention due to their environmental omnipresence. This thesis aims to provide comprehensive data on aquatic MP pollution through the application of state-of-the-art analytical methods, and compares data outputs from two data pipelines. Within the River Weser–North Sea transitional system, small MP (<500 µm) predominated, with a notable abundance of suspected paint particles. Large MP (>500 µm) exhibited low abundances, mainly composed of the common plastic polymers polyethylene and polypropylene. The estuary’s turbidity maximum zone showed the highest MP concentrations, then declining towards the North Sea, possibly influenced by increased vertical and horizontal export or dilution in the larger marine water body. Additionally, this thesis evaluated two wastewater treatment plants as potential riverine MP point sources. Interference by post-processing residual material required an adaptation of the FTIR reference database by the inclusion of new reference material. Results showed that polyolefins were prevalent in the effluent, and that observed temporal patterns in MP concentrations could be partially explained by technical and environmental parameters. Input of MP into the River Weser via effluent is likely, necessitating more research to understand the full dynamics of MP pollution within this river system. Furthermore, the MP analysis pipeline comparison study showed discrepancies for certain polymer types, possibly due to different polymer grouping methods, or overestimation effects. By excluding these polymer types, both datasets generally were in accordance, suggesting a harmonization of both pipelines should be undertaken to improve comparability of MP data. In summary, this thesis provides a detailed foundation for understanding MP dynamics in the River Weser–North Sea system and highlights methodological challenges inherent in the field of MP pollution research.