Assessing Benthic Ecological Integrity Under Cascade Hydropower Regulation: Seasonal Pathways in a Mountainous Headwater Basin

With the continuous expansion of hydropower development in China, basin-scale river regulation has gradually evolved from the operation of single diversion dams to coordinated cascade regulation of multiple dams. Nevertheless, the mechanisms underlying cumulative disturbance induced by cascade diversion dams in riverine benthic ecosystems remain poorly understood. Taking the Baoxing River Basin, a mountainous headwater basin in Southwest China, as the study area, this study analyzed the coupled response characteristics among hydrology, water quality and benthic index of biotic integrity (B-IBI), based on spring-autumn field monitoring data from 15 sampling sites and discharge records of upstream diversion dams during 2020–2024, so as to evaluate the comprehensive benthic ecological impacts of cascade diversion dams. The results showed that: (1) Cascade regulation was associated with spatially heterogeneous hydrological conditions. The mean cumulative number of upstream hydropower stations was 0.8 in the upstream section and 11 downstream, while mean nearest upstream releases in the downstream section were 12.49 m3·s−1 in spring and 9.49 m3·s−1 in autumn. (2) Driven by such hydrological gradients, the structure of benthic macroinvertebrate communities shifted: the proportion of sensitive EPT (Ephemeroptera, Plecoptera and Trichoptera) taxa decreased, while Diptera and other tolerant taxa became dominant. Correspondingly, the mean B-IBI declined by 28.6% from 4.39 to 3.13. (3) Random forest analysis identified turbidity as the strongest predictor of B-IBI, followed by hydrological regulation and other environmental variables. (4) Covariance-based structural equation modeling (CB-SEM) revealed season-specific patterns of association in separately fitted spring and autumn models. The spring model highlighted associations involving turbidity, dissolved oxygen, and biomass, whereas the autumn model emphasized physicochemical conditions and tolerance-related community attributes. Overall, this study provides insights into the associations among hydropower release, water-quality conditions, and seasonal benthic macroinvertebrate responses under cascade hydropower regulation. These findings provide scientific support for ecological flow management and adaptive ecological assessment of mountain rivers subjected to cascade hydropower impacts.