Hydrological Period, Drainage and Local Environmental Conditions Influence Fish Assemblages in Upland Streams in the Eastern Amazon, Brazil

Amazon streams are home to a great richness and diversity of fish, having an essential role in maintaining the aquatic ecosystem multifunctionality and global biodiversity. Here, we investigated the structure of the ichthyofauna of upland streams of the Lower Tapajós River and analyzed ecological descriptors of fish assemblages in different drainages in the rainy and dry seasons. A total of 3715 individuals from 110 species were collected. Species richness was higher during the dry season (99 species) than in the rainy season (66 species). Local environmental variables were measured or obtained from publicly accessible databases. Our results showed that ichthyofauna responds to hydrological changes in upland streams in the eastern Amazon. Abundance and richness were greatest during the dry season, with important contributions from representatives of the order Characiformes. Stream structural variables explained most of the variance in assemblage composition (adjusted R 2 = 0.102, p = 0.004), with channel width, depth, and canopy cover as key factors. The findings underscore the importance of assessing drainage and seasonality effects not only to understand ichthyofaunal biodiversity but also to adequately design research efforts, conservation strategies, and monitoring programs for aquatic environments in the eastern Amazon.