Effects of a large run-of-river dam closure on the functional diversity of subtropical freshwater fish assemblages

Run-of-river hydropower plants are increasingly promoted as a low-impact alternative for energy generation because of their reduced water storage capacity. However, their effects on fish functional structure remain poorly understood. We evaluated changes in fish functional diversity following dam closure at the Foz do Chapecó run-of-river hydropower plant. We quantified 23 functional traits and calculated four indices: functional richness (FRic), functional evenness (FEve), functional divergence (FDiv), and functional dispersion (FDis). These indices were weighted by fish abundance and biomass using catch per unit effort (CPUE) data and compared between pre- and post-damming periods using bias-corrected and accelerated bootstrap confidence intervals (BCa). Functional diversity declined across most river stretches following dam closure. At impoundment sites, FRic and FDis decreased by 69% and 38%, respectively, while downstream tributaries exhibited a 39% reduction in FRic. Low-order upstream tributaries showed reductions greater than 20% in FEve and FDis, indicating loss of functional complementarity and simplification of ecological strategies. In contrast, upstream tributaries displayed increases in FRic (32%) and FDis (19%), suggesting functional reorganization after reservoir formation. Community-weighted means revealed declines in traits associated with detritivory and parental care, likely reflecting reductions in benthic habitats and reproductive environments following impoundment, whereas traits related to piscivory, herbivory, and migratory behavior increased in upstream sections of the reservoir. Our findings indicate that the closure and operation of a large run-of-river hydropower plant was associated with substantial changes in fish functional structure, likely driven by habitat and ecohydrological alterations. Responses differed among reservoir, tributary, and downstream environments, revealing contrasting functional trajectories.