Sources and hotspots of microplastics of the rivers ending to the southern Caspian Sea

The occurrence of microplastics (MPs) in beach sediments of the southern Caspian Sea was well documented, however, there are still many unknowns about the abundances and distributions of MPs in the rivers ending to the Caspian Sea. Here, bank sediments of 26 sites in the thirteen rivers were surveyed in two seasons. However, there was not any significant difference (p > 0.05) between the concentrations of MPs during the two seasons. MPs were detected in all samples with mean concentrations of 214.08 ± 14.35 MPs/kg. The most common size, shape, color, and polymer types of MPs were L < 300 μm, fragment/film, white/transparent, and polystyrene (PS), respectively. In all rivers, positive MP gradients from upstream to downstream were observed. Maximum concentrations of MPs were found in the downstream parts of Chalus, Haraz, and Safarud rivers. Recreational-tourism and fishing activities had significant positive relationships (p < 0.05) with concentration of MPs in the rivers.