2025-10-092025-10-092025-04-01PINEDA, Alfonso et al. A run-of-river mega-dam in the largest Amazon tributary changed the diversity pattern of planktonic communities and caused the loss of species. Freshwater Biology, [s. l.], v. 70, n. 4, e70025, 2025. DOI: 10.1111/fwb.70025. Disponível em: https://onlinelibrary.wiley.com/doi/10.1111/fwb.70025?af=R. Acesso em: 6 out. 2025.e- 1365-24270046-5070https://onlinelibrary.wiley.com/doi/10.1111/fwb.70025?af=RRun-of-river dams (RORs) are often considered less environmentally damaging than storage dams, but information on their impact on regional biodiversity is scarce. In particular, the effects of RORs on species commonness are largely unknown. In this study, we assessed the impacts of ROR on the biodiversity of phytoplankton and zooplankton in the largest tributary of the Amazon River. We investigated the effects of the ROR on changes in environmental heterogeneity, species richness and beta diversity and examined whether these changes varied with species commonness. We examined the shift in species composition and environmental heterogeneity between damming phases and assessed changes in species richness for the whole community (q0), more abundant species (q2) and typical species (q1—a community without rare and more abundant species). We also decomposed the total diversity (γ) registered at each phase into its local diversity (α) and its spatial (β1—variation among sites) and temporal (β2—variation between samplings and β3—variation between seasons) components. We then evaluated whether the contributions of these components varied with damming for q0, q1 and q2. After damming, the environmental heterogeneity and the richness of both communities decreased (considering the whole community). Beta diversity decreased for phytoplankton and increased for zooplankton. Changes in the contribution of γ components after damming were more evident for typical and more abundant species. For phytoplankton, β1 and β3 decreased while the importance of α increased. For zooplankton, while β1 and β3 increased, α decreased. We showed that analysing the commonness of species could be useful for understanding the impact of ROR. On the one hand, if the interest is in evaluating the response of communities in terms of species number, researchers could consider the whole community. On the other hand, if the interest is in evaluating changes in spatial and temporal patterns, typical and more abundant species appear to be more sensitive than the whole community. Our findings showed changes in diversity patterns and species loss after damming, suggesting that RORs are not as friendly as expected, at least for plankton. Our findings contribute to the evaluation and monitoring strategies for existing dams and those that will be built in the Amazon region in coming years.engAcesso RestritoArtigo10.1111/fwb.70025