Waterfalls affect stream fish species richness and abundance, but not species composition or beta diversity in Southern Pantanal

Local and regional factors at different scales influence fish biodiversity in streams. Similarly, waterfalls can contribute to the distribution of species richness and composition of assemblages. The Upper Paraguay River basin is crucial for conserving various fish species, but few large-scale studies on fish assemblages can be found in the literature. To address this knowledge gap, we investigated the hydrological anomalies of waterfalls from two perspectives: their effects on the richness and composition of fish assemblages and their contribution to beta diversity in the Rio Negro basin, the largest tributary of the Paraguay River on its left side. We specifically considered the effects of regional (e.g., sub-basins and altitude) and local factors (e.g., mean flow, water conductivity, water pH, concentration of dissolved oxygen) on these metrics. To do this, we sampled fifty-seven stretches of streams in the headwaters of the Rio Negro basin from May to August 2020 (dry period), using a different sampling method with rectangular sieve, trawl net and electrofishing. The areas below the waterfalls exhibited greater fish richness explained by regional (e.g., altitude) and local (e.g., oxygen concentration and conductivity) factors. Species composition, however, did not differ relative to these factors. The greatest contributions to Local Contribution to Beta Diversity (LCBD) came from sites with the lowest altitude and lowest oxygen concentration. Our results demonstrate the effect of waterfalls on fish assemblages and help to estimate the large-scale effect of local and regional factors on fish species richness and composition in a relatively pristine ecosystem important to the conservation of Neotropical fish species.