Influence of land uses on microplastic abundance and composition in surface waters and sediments of Sefidroud River, Iran.

Microplastics (MPs) are increasingly recognized as critical contaminants in riverine environments, yet the influence of land use sources on their occurrence and characteristics remains poorly quantified. This study assessed MP abundance, size distribution, color, morphology, and polymer composition in surface water and sediments of the Sefidroud River, Iran the largest tributary to the Caspian Sea across forest, agriculture, tourism, residential, sewage-impacted, and estuarine zones. Triplicate samples (n = 33 per matrix) were collected from 11 stations on October 2024. MP concentrations in water ranged from 1,600 ± 300 to 7,300 ± 1,300 items·m⁻3, while sediments contained up to 600 ± 99 particles/kg (dry weight). Fibers dominated both matrices (75.78% in water; 67.63% in sediments). The most frequent particle size was 1000-2000 µm in sediments (40.69%) and water (39.75%), and black and white were the prevailing colors. Detected polymers included polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), and polystyrene (PS), with significant differences among the land-use types, indicating distinct source signatures. These findings provide the first comprehensive baseline of MP pollution in this ecologically important basin and underscore the need for targeted mitigation strategies, including improved wastewater management and regulation of agricultural plastics to reduce MP fluxes to the Caspian Sea.