Claudia Lorenz

• Microplastics (MP) in the marine environment are of global concern. The European Marine Strategy Framework Directive (MSFD), targeting marine pollution, provides a legal framework for MP research in the North Sea. Utilizing state-of-the-art methods, this thesis presents a comprehensive dataset of concentrations, polymer types, size distribution, weathering status, and spatial distribution of MP (11–5000 µm) in sediments and surface waters of the southern North Sea. In surface waters, low-density polymers like polyethylene (PE) and polypropylene (PP) dominated, with the highest MP concentration (245 particles m-3) detected close to the English Channel and the Rhine delta. In sediments, PP was omnipresent but less abundant, accompanied by varnishes and rubbers, and the highest concentration (1189 particles kg-1) found in the central southern North Sea. Weathering status, assessed through carbonyl indices for PE and PP, differed significantly for the compartments and was on average lower for large MP (500–5000 µm) than for small MP (10–500 µm), potentially indicating a weathering dependent fragmentation. Surface water samples were also analyzed for paraffin waxes of 500 to 5000 µm size, which were detected at 14 of 24 stations. Their synthetic origin was confirmed by spectroscopic complemented with thermoanalytical techniques. Sediments of an urban fjord in Norway were analyzed with harmonized methods. The fjord’s MP concentrations ranged from 12×103 to 205×103 particles kg-1, exceeding the highest concentration in the southern North Sea 10 to 100 times. This thesis confirmes the significant influence of regional input processes and highlights that harmonized methods providing comprehensive data are indispensable for inter-study comparisons. It emphasizes that future studies and monitoring efforts should analyze small MP, which showed much higher concentrations and more diverse polymer compositions than large MP, to allow for reliable environmental risk assessment data