Combined contamination of microplastics and heavy metals in a riverine ecosystem: insights from fish and amphibian species

Microplastics (MPs) and heavy metals (HMs) are increasingly recognized as emerging pollutants that pose serious risks to aquatic ecosystems and human health. However, comparative assessments across different vertebrate groups remain limited. This study presents the first simultaneous evaluation of MPs and HMs in freshwater fish (Barbus luteus, Capoeta damascina, Barbus barbulus) and amphibians (Pelophylax bedriagae, Bufotes sitibundus) from the Seimareh River, western Iran. A total of 71 specimens were analyzed for MP abundance and characteristics, as well as tissue concentrations of Pb, Cd, Cr, Cu, Zn, Ni, and As. MPs were detected in all species, with amphibians ingesting significantly more particles than fish (≈9 % higher). Fibers dominated the MP fraction (≈68 %), primarily <0.25 mm in size, with polyester (PES) and polyethylene terephthalate (PET) as the most common polymers. HMs concentrations varied significantly among species: fish accumulated higher levels of Zn and Cu, whereas amphibians showed elevated Pb and Cr. Multivariate analyses confirmed these interspecific differences, independent of body size. Strong positive correlations between MP abundance and selected metals (notably Pb, Cd, and As) suggest that MPs may act as vectors, enhancing metal bioavailability. These findings identify amphibians as particularly sensitive indicators of combined pollution and highlight the Seimareh River as being under a dual threat from MPs and HMs. The results underscore the need for integrated monitoring, risk assessment, and management strategies to address emerging contaminants in freshwater ecosystems.