Microplastics (10 µm-5 mm) in European Atlantic Coastal Waters

• Microplastics (MPs) between 10 and 300 µm are ubiquitous in Atlantic European coastal waters. • The concentration of MPs (10- 5000 µm) ranged from 5 to 1603 MPs m -3 . • Estuaries (e.g, Scheldt and Ramsø estuaries) and wastewater effluents (e.g, in Ria de Vigo) are hotspots of coastal microplastic pollution. • Median MP abundance in European Atlantic waters is almost 3 times lower than median level in coastal regions globally. • Detected MPs levels are below harmful acute effects on the pelagic food web. Microplastics (MPs) are pervasive pollutants in coastal waters, raising significant ecological concerns. This study assessed the abundance and characteristics of small-sized MPs (down to 10 µm) across European Atlantic coastal sites using harmonized sampling and analytical methods. A filter-pump microplastic sampler, the "Universal Filtering Object" (UFO), was employed at all sites, with Manta net (300-µm mesh) sampling conducted at selected locations. Microplastic concentrations ranged from <10 MPs m -3 to >1600 MPs m -3 , with the Gulf of Finland showing the lowest concentration (5 MPs m -3 ) and the Scheldt estuary in Belgium the highest (1603 MPs m -3 ). Most MPs (80%) were <300 µm, primarily consisting of polyester, polypropylene, and polyethylene fragments. Manta net sampling consistently underestimated both total microplastic concentrations and microplastics larger than 300 µm compared to UFO sampling. Estuaries and wastewater effluents were identified as pollution hotspots, strongly influencing local MP distributions. The median microplastic concentration found in European Atlantic waters in this study was lower than the global median for coastal waters measured using pump-based sampling devices. Although current MP levels are unlikely to pose an immediate risk to the marine pelagic food web, the projected increase in plastic production, combined with its low degradability and chemical leaching, underscores the urgency of implementing mitigation measures to prevent future environmental impacts.