Ecological succession of fishes in a large Amazonian off river reservoir.

Converting river flows into reservoirs is a major disturbance that triggers ecological succession, altering the functional and phylogenetic structure of aquatic communities. However, this process in off-river reservoirs remains understudied, despite its potential implications for biodiversity conservation. Here, we investigated fish assemblage colonization and establishment in a recently created off-river intermediate reservoir (IR) within the Belo Monte Hydroelectric Complex in the Amazon. Fish were sampled over seven years in the IR and compared to the main reservoir (MR) and a more pristine sector upstream (UP). Biodiversity in the IR increased over time, alongside shifts in species composition, traits, and phylogenetic structure. Dominance of large and benthic-rheophilic species rose, and the proportion of adults increased, indicating improved reproductive potential. These changes were likely influenced by environmental filtering, dispersal limitations, and resource scarcity during the initial years post-formation. Over time, the IR became more similar to the MR than the UP, reflecting species decay similarity and environmental resemblance between reservoirs. Dissimilarity between the IR and river sectors diminished as succession progressed. Our results suggest IR may serve as a refuge for endemic and rare rheophilic fish, supporting fish stocks for riverine communities and offering a conservation tool amid hydropower development and climate change. The true test of the IR's value for local biodiversity will unfold as it ages, making consistent monitoring of the IR essential to assess its long-term ecological impact.