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Studies on beta diversity across different spatial scales can yield insight into the ecological factors that shape communities and the mechanisms of maintenance and loss of diversity. The coastal rivers in southeast Brazil have different lengths and salinity gradients because of variation in coastal plain width and slope. These differences in environmental characteristics promote natural gradients and patterns of macrophyte assemblage variation within and among rivers. To investigate the drivers of macrophyte beta diversity, we recorded the occurrence of species and life forms and collected limnological variables at 100 sampling sites over 8 unconnected rivers that run directly to the sea. We estimated macrophyte beta diversity, and its nestedness and turnover components, within (local scale) and among (regional scale) coastal rivers using traditional metrics (Jaccard dissimilarity matrix) and a metric that accounts for sampling effect (permutational analysis of multivariate dispersions). At each sampling site, we collected variables from water (nutrients, dissolved oxygen, and pH), sediment (nutrients, salinity, and grain type), and channel (width, littoral declivity, distance from river's mouth, and coastline). For assessment of regional-scale drivers of macrophyte beta diversity, we measured river length and calculated salinity gradient. We evaluated the potential of environmental variables to explain local and regional beta diversity using distance-based redundancy analysis and multiple linear regression. As expected, at the local scale, high species turnover and life-form nestedness were predominantly explained by the rivers' longitudinal gradients (mainly salinity, P, and channel width). At the regional scale, we found that river length explained life-form nestedness among rivers (small rivers are subsets of the largest rivers); however, neither river length nor salinity gradient explained the high species turnover among rivers. We suggest that dispersal limitation among rivers and historical colonization shaped the species pool in each river resulting in almost completely distinct assemblage compositions. This high turnover at the regional scale suggests that conservation strategies for macrophytes must consider multiple rivers and not only the largest ones. Overall, our results suggest that environmental variation strongly influences macrophyte beta diversity within and among coastal tropical rivers. [ABSTRACT FROM AUTHOR]