Assessment of ecological health and macroinvertebrate community characteristics in the mainstream of Yalong River

To assess the ecological health status of large alpine canyon rivers, this study systematically investigated benthic macroinvertebrate communities at 29 sampling sites along the mainstream of the Yalong River during the wet season (May 2023) and the dry season (November 2023). The spatial and seasonal differentiation characteristics of the communities were analyzed, and a Benthic Index of Biotic Integrity (B-IBI) was constructed. The results indicated significant spatial heterogeneity across the basin: the upstream natural sections were relatively stable, dominated by sensitive cold-water taxa such as Ephemeroptera, Plecoptera, and Trichoptera (EPT). Conversely, the middle and lower reaches underwent a marked transition with a significant increase in disturbance-tolerant taxa due to cascade regulation. Seasonally, community differentiation during the dry season was higher than in the wet season, and β-diversity was primarily driven by the species turnover component (74.83%~84.26%). The B-IBI assessment showed that the upstream natural sections generally maintained a "Good" or better status, whereas the lacustrine sections of the reservoirs and water-reduced sections in the middle and lower reaches exhibited relatively lower ecological health levels. The macroinvertebrate-based B-IBI effectively reflects the spatial pattern of ecological health in the Yalong River mainstream.