Microplastic flux simulation and ecological risk assessment currently face two major challenges. First, existing models cannot simulate microplastic flux with high precision at arbitrary river sections, especially when concentrations are low or data is scarce. Second, conventional risk assessment methods overlook the spatial heterogeneity of receptor species and ecosystems. To solve these problems, this study developed a Microplastic Flux Simulation (MFS) model that couples hydrological processes with microplastic emission and transport mechanisms, enabling high-precision dynamic simulation. Furthermore, a Species Sensitivity Distribution-based Ecological Risk Assessment (SSD-ERA) method was proposed, which dynamically adjusts risk thresholds according to local species richness, significantly enhancing assessment accuracy. Applying the MFS model and SSD-ERA method to the Jinsha River and the Yellow River Source Region demonstrated their strong performance. The results reveal that the Qinghai-Tibet Plateau transports about 276.90 tons of microplastics annually downstream via these two rivers, and 25-30% of the river segments are at medium-to-high risk. These medium-to-high risk segments are primarily located near provincial boundaries and upstream of dams, a pattern closely linked to insufficient cross-regional collaborative governance and the interception effects of dams. The MFS model and SSD-ERA method can provide crucial technical support for microplastic flux simulation and ecological risk assessment in different regions.

Microplastic flux,Ecological risk assessment,Species sensitivity distribution,Qinghai-Tibet Plateau