Distribution patterns and influence of environmental variables on the fish assemblages in a neotropical Amazonian river

Based on the river continuum and flood pulse concepts, this study investigated the seasonal, spatial and limnological effects on fish assemblage distribution along the São Miguel River, Amazon basin, Brazil. Fish were sampled and environmental variables measured during the high- and low-water periods in upper, middle and lower sections of the river. The results revealed significant spatial and seasonal effects, as well as interactions between spatial and temporal gradients, which bring about distinct differences in assemblage composition. These patterns were associated with variations in limnological parameters along a longitudinal gradient. Species distribution and turnover were shaped by both the flood pulse and the spatial gradient, which are processes that drive environmental changes and influence trophic resource dynamics – key factors for species occurrence. Overall, our findings indicate that the structure of the fish assemblages in the São Miguel River is shaped by hydrological variation, with the flood pulse being the primary regulating factor of the taxonomic composition of the ichthyofauna.