Microplastic Ingestion by Wild Fish Across an Urbanization Gradient in a Megacity River: A Case Study of the Suzhou River, Shanghai

Microplastic (MP) pollution in highly urbanized rivers poses increasing ecological risks; however, the ingestion characteristics of wild fish across urbanization gradients remain insufficiently understood. In this study, gastrointestinal suspected microplastic ingestion was investigated in a diverse fish assemblage (n = 310, 10 species) collected from core urban, intermediate urban, and suburban reaches of the Suzhou River, Shanghai. A total of 496 visually identified suspected microplastics were detected, with an overall detection rate of 71.29% and a mean abundance of 1.60 ± 1.67 items/individual. Statistical analyses revealed significant spatial variation in suspected microplastic abundance (p 0.05). In the indicator species Coilia nasus, gastrointestinal suspected microplastic abundance showed a significant positive correlation with both body length (R2 = 0.205, p < 0.001) and body weight (R2 = 0.153, p < 0.005). Morphological characterization indicated that small-sized (≤1 mm, 79.47%), transparent fibres predominated among the detected particles. Overall, these findings suggest an association between urbanization and fish suspected microplastic exposure, although downstream accumulation likely co-drives this spatial pattern. Furthermore, differences associated with specific ecological traits were not statistically significant and require further targeted investigation. This study provides baseline ecological evidence for understanding microplastic exposure in urban river ecosystems and highlights the importance of integrated long-term monitoring for ecological risk assessment and management of emerging pollutants.