Understanding the drivers of fish beta diversity from beaches on a reduced flow stretch in an Amazonian River

In this study, we aimed to identify the factors that drive the patterns in beta diversity of fish assemblages on the beaches of the Volta Grande do Xingu, a river stretch under the influence of Belo Monte dam. Fishes were collected using a trawl net in 28 sandy beaches during the low water period of 2021 (each beach was sampled once during this period). Measurements of environmental variables were carried out on-site, while spatial variables were obtained using Asymmetric Eigenvector Maps (AEM). Using the Bray–Curtis dissimilarity index, we calculated beta diversity and conducted a distance-based redundancy analysis (db-RDA) with the dissimilarity matrix to assess its relationship between beta diversity and environmental and spatial variables. Additionally, we performed a variance partition to quantify the pure and shared contributions of the explanatory matrices to our response matrix. The results revealed low beta diversity among the sampled beaches, and that this beta diversity was explained in part by the spatial variables and pH. Our findings underscore the influence of spatial structure on the assembly of local communities, indicating that dispersion processes and spatial connectivity are relevant mechanisms that contribute to the beta diversity of beach fish communities under influence of a reduced flow by a Hydropower Plant.