Spatiotemporal Patterns and Assembly Processes of Benthic Macroinvertebrate Communities in River Reaches of Different Geomorphic Types in Sichuan Province, China

Geomorphic setting and seasonal variation shape benthic macroinvertebrate communities, but their effects on community patterns and assembly processes remain unclear. We surveyed 49 sites across seven Sichuan rivers during spring and autumn 2025. Reaches were classified as Mountain, Hilly, or Plain. Principal coordinate analysis (PCoA), random forest models, a neutral community model (NCM), and the modified stochasticity ratio (MST) examined community patterns, predictors, and assembly processes. Diptera had high relative abundances in Mountain and Hilly reaches in spring, whereas the proportion of Ephemeroptera increased across reach types in autumn. Taxa richness and Shannon and Margalef indices were higher in Hilly and Plain reaches. Community composition differed significantly among reach types, with reach type explaining 10.3% and 11.4% of variation in spring and autumn, respectively. For Ephemeroptera, Plecoptera, and Trichoptera (EPT) richness, random forest models ranked dissolved oxygen, channel width, elevation, and ammonia nitrogen highly in spring, and the permanganate index, dissolved oxygen, and flow velocity highly in autumn. NCM fit and Nm were higher in spring and Plain reaches. Mean MST values below 0.5 suggested deterministic dominance, with the highest estimated deterministic contribution in Mountain reaches (80.97%). These findings highlight seasonal differences in benthic responses to environmental gradients and potential stressors and may inform river bioassessment that accounts for geomorphic and seasonal context.