Microplastic distribution and potential ecological risk index in a South American sparsely urbanized river basin: Focus on abiotic matrices and the native fish Jenynsia lineata

• Widespread MP contamination in river matrices, even in sparsely urbanized areas. • First report of polymers ingested by a South American native fish species. • The two most commonly identified polymers in all of the matrices were C and PET. • Microplastic aging, particle detachment and metal adsorption/transport have been confirmed • High microplastic bioavailability and polymer risk was identified in the studied area. This study represents the first comprehensive assessment of microplastics (MPs) in water, sediment, and the native South American fish species Jenynsia lineata within the Suquía River basin, Argentina, across varying levels of anthropization. MPs were detected in 100% of water and sediment samples and at least 80% of fish, with no significant concentration differences between sites. Notably, MPs in fish (660.3 ± 50.5 μm) were significantly smaller than those in water (804.1 ± 59.6 μm) and sediment (955.9 ± 94.8 μm). Fibers comprised 65% of MPs in abiotic matrices and 96% in fish, with blue particles predominating (∼40% in water and sediment; 80% in fish). The most abundant polymers were cellulose and polyethylene terephthalate (PET). Our findings confirm that MP interactions with environmental elements, including metals, are influenced by surface modifications, enhancing their role as contaminant and pathogen vectors. Titanium (Ti) and silicon (Si) in plastic particles suggest intrinsic sources, while iron (Fe) and aluminum (Al) are likely environmentally derived. MP bioavailability (PLI) increased from less urbanized upper catchments to highly urbanized middle catchments. A significant polymer risk (PHI ≥1000) was linked to epoxy resin (ER) and polyacrylonitrile (PAN) in water and PAN in fish. Water showed the highest potential ecological risk index (PERI). Understanding MP dynamics in freshwater systems is crucial for assessing their broader ecological impacts due to the interconnectedness of aquatic environments. Further research is needed to clarify MP ingestion drivers, toxicological effects, and their role in contaminant transfer within these ecosystems. Such insights are crucial for developing effective strategies to mitigate plastic pollution in both freshwater and marine environments.