Phytoplankton responses to the dynamics of large cascading neotropical reservoirs

Cascade reservoirs consist of a series of dams built along a watercourse or within a watershed. Successive reservoirs, which regulate the flow of matter, form hydrological gradients. Consequently, they create limnological gradients resulting from altered biogeochemical cycles along the longitudinal axis of the river. Our objective was to examine the dynamics of cascade reservoirs in a large neotropical river, considering their hydrological and limnological characteristics, and the responses to biomass, richness and functional diversity, as well as species composition and functional traits. In this study, we assessed seven hydropower reservoirs installed in a cascade along a stretch of 1,500 km in a large neotropical river. Our results demonstrated that water residence time and distances upstream were important hydrological variables in determining limnological and hydrological longitudinal gradients, leading to a decrease in conductivity and total phosphorus and an increase in water temperature. We observed the selection of species and taxonomic groups of Cyanobacteria such as Raphidiopsis raciborskii upstream and Microcystis sp. downstream, and species with unicellular and mucilaginous characteristics throughout the cascade. Our work demonstrated that the dynamics between cascade reservoirs interfered with the spatial distribution of species and the selection of functional traits of phytoplankton.