Dam reverse flow events influence limnological variables and fish assemblages of a downstream tributary in a Neotropical floodplain

Abstract We used a dam‐free tributary (the Baía River) in the upper Paraná River floodplain downstream of a major dam in the Paraná River, Brazil to investigate the effects of dam‐regulated reverse flow on limnological variables and fish. We tested the hypotheses that limnological variables in tributaries change based on flow direction and that fish assemblages respond to this variation. Sampling sites were determined considering flow direction (normal or reverse) and position (near or far from the river mouth). Limnological variables showed higher values for transparency, oxygen, pH, and electrical conductivity at sites near the mouth of the Baía River during reverse flow. Species richness and evenness differed significantly in relation to position, with higher values closer to the Paraná River. The average standard length of fish species was higher near the mouth of the Baía River and during the reverse flow period. No significant differences in species abundance were found. Reverse flow into the Baía River brought nutrient‐poor water from the dammed Paraná River, thereby altering the limnological variables. This flow condition impelled the entry of species with higher average standard lengths. However, increased species richness and low evenness were due to the increase in species dispersal rates under all flow conditions. Our results emphasize that the effects of dams can extend several kilometres into the floodplain, provide basic knowledge on the effects of major dams on downstream pristine tributaries, and highlight the need for further studies to understand the wider influences.