Ecological risk assessment of sediment microplastics in the upper Yellow River: Index applicability and implications...

The Yellow River is the second longest river in China and an important water source. In recent years, microplastic (MP) pollution in sediments and its ecological risks have received increasing attention. However, the risk characteristics of sediment MPs in different functional zones of the upper Yellow River remain unclear, and the applicability of different assessment indices and their implications for risk reference refinement require further clarification. In this study, we investigated the pollution characteristics of MPs, physicochemical properties, and microbial community structure in sediments from different functional zones of the upper Yellow River. The pollution load index (PLI), polymer hazard index (PHI), and potential ecological risk index (PERI) were further used to evaluate differences in ecological risk characterization among the three indices. The results showed that MP abundance in sediments ranged from 183.33 to 1016.67 n/kg, with most particles falling within the 0-50 μm size range. The dominant polymers were polyurethane (PU) and polyvinyl chloride (PVC), both of which have relatively high hazard scores. MP abundance and polymer composition differed markedly among functional zones. Risk assessment results showed that the abundance-based PLI indicated low risk at all sites, whereas PHI and PERI, which incorporate polymer hazard information, indicated relatively high risk. PERI was more closely aligned with PHI than with PLI. Correlation analyses showed that PU and PVC were significantly associated with several physicochemical factors and dominant microbial taxa, suggesting potential links between the composition of high-hazard-score polymers and shifts in sediment microbial communities. These results indicate that, in the study area, the spatial differentiation of ecological risks associated with sediment MPs was mainly related to the composition of high-hazard-score polymers, and that abundance-based assessment alone may underestimate the potential risks arising from hazardous polymer composition. This study provides a reference for evaluating the applicability of existing ecological risk assessment indices for MPs and for refining the current risk assessment framework.