Comparison of microplastic intake in two fish species from different functional feeding groups in Europe's second-largest river

This study assessed microplastic (MP) contamination in fishes along a 1716 km transect of the Danube River, from the Austrian-German border to the Bulgarian-Romanian border, using standardized sampling during the Joint Danube Survey 4. MPs were detected in the digestive tracts of two fish species: bleak (Alburnus alburnus; a pelagic species, N = 1192) and round goby (Neogobius melanostomus; a benthic species, N = 761). MPs occurred in 37.6 % of bleak compared to 22.7 % of round goby, with higher mean particle counts and fiber lengths in bleak. In both species, blue fibers predominated. Size-related patterns were evident: larger goby and smaller bleak carried higher MP burdens. Compared to round goby, bleak showed significantly higher frequencies and quantities of MPs in tributaries than in the Danube, which probably reflects differences in habitat use and exposure. A downstream increase of MPs was observed in both species, indicating a longitudinal contamination gradient. MP presence appeared not to have a measurable effect on fish condition. The interplay of ecological traits (feeding behavior, habitat preference, mobility) and environmental factors (vertical MP distribution, urban and wastewater inputs) play a major role in MP ingestion. This first large-scale assessment of MPs in Danubian fish provides robust basic data and underscores the need for coordinated research, including spectroscopic validation, to better understand ingestion patterns and ecological implications.