Responses of Macroinvertebrate Communities to Heterogeneity Among Typical Reaches in the Upper Yangtze River.

Benthic macroinvertebrates are important indicators of river ecosystem health, and their spatial distribution is highly sensitive to environmental heterogeneity. In large mountainous rivers characterized by complex geomorphology and strong hydrodynamics, habitat structural diversity may influence community structure through multi-pathway mechanisms. This study focused on three representative reaches of the upper Yangtze River-the Canyon, Anabranching, and Reef-Tuo reaches. By integrating two-dimensional hydrodynamic simulations, in situ measurements of water and sediment conditions, and benthic community surveys, we systematically analyzed the effects of environmental factors on community composition. Non-metric multidimensional scaling (NMDS) and redundancy analysis (RDA) revealed significant differences in community richness, diversity, and functional group composition among reaches. Partial least squares path modeling (PLS-PM) further demonstrated a "dual-pathway" ecological mechanism: flow velocity not only directly promoted the abundance of rheophilic taxa but also indirectly influenced the distribution of sediment-preferring taxa by altering sediment condition. Hydrodynamic simulation results further validated the spatial basis of this mechanism. Flow velocity gradients were strongly shaped by geomorphic features, and notably, stable low-velocity zones persisted in the Anabranching and Reef-Tuo reaches. These areas served as important refugia for benthic communities, reinforcing the ecological applicability of the "dual-pathway" mechanism in complex river segments.