Microplastic pollution in an anthropogenically modified river: sediment occurrence, associated microbiota, and environmental correlates.

River sediments integrate heterogeneous anthropogenic inputs and hydrological and sedimentary processes, making it difficult to identify the factors associated with microplastic (MP) accumulation. We investigated sediment MPs at 19 sites along the urban-agricultural gradient of the Shihe River by combining particle characterization, bacterial community analysis, human mitochondrial DNA (Hmt-DNA) quantification, redundancy analysis, and partial least squares structural equation modeling. MP abundance ranged from 40 to 16,460 particles/kg, with a mean of 3368 particles/kg. Particles smaller than 0.5 mm accounted for 77.41% of the total, while pellets and fragments dominated the shape composition. Although mean MP abundance did not differ significantly between the urban and agricultural reaches, Hmt-DNA showed the largest positive path coefficient to MP abundance in the PLS-SEM model. This contrast suggests that site-specific human pressure provides information that is not captured by broad land-use classifications. MP-associated bacterial communities were dominated by Proteobacteria and had a BugBase-predicted pathogenic potential of 0.523. These findings identify site-specific human pressure as the strongest measured correlate of sediment MP abundance in this watershed, highlighting the value of source-oriented strategies for MP mitigation in anthropogenically modified rivers.