Dam cascade in run‐of‐river systems promotes homogenisation of fish functional traits in a plateau river

Abstract

Impoundments caused by dams are often responsible for intense changes in physical, chemical, geomorphological and hydrological characteristics of rivers, and are one of the main causes in global freshwater biodiversity declines. When these changes derive from multiple cascading dams, the cumulative effect can amplify, causing more pronounced consequences than each individual reservoir, which may affect, among other factors, the biotic and functional conditions of fish fauna. We aim to evaluate the impact of dam cascades on fish functional diversity by applying trait‐based analysis. We evaluate the functional attributes (functional indices and community‐weighted means of trait values) and environmental variables at 11 sites, which encompassed different segments affected by the hydroelectric plants (transition, reservoir, reduced discharge section and tributary) in the Antas River Basin dam cascade system located in southern Brazil. The spatial differences of the functional indices (richness, evenness, divergence, dispersion and diversity) as well as the weighted averages of each functional trait were tested through analysis of variance. The most affected functional characteristics were those related to habitat exploration by detritivorous species, especially the use of food resources and body shape, as well as the type of swimming movement. These traits were more representative at the most downstream site of the complex. Thus, our results suggest that the cascade of dams acts in the functional homogenisation of the ichthyofauna in the intermediate stretches, mainly reflecting physical changes in habitat.

Effects of small hydropower dams on macroinvertebrate and fish assemblages in southern Brazil

“…River ecologists have theorised that the spatial position of dams along the longitudinal gradient may result in distinct ecological effects, and that some of these effects can even accumulate downstream (Ward & Stanford, 1983). These ideas are now gaining traction as a greater body of empirical evidence has accumulated (dos Santos et al, 2017; Loures & Pompeu, 2018; Miranda & Dembkowski, 2016; Ticiani et al, 2023). The degree of flow regulation and water residence time accumulate in the downstream direction in dam cascades (Lehner et al, 2011; Swanson et al, 2021), with observed patterns in macroinvertebrate assemblages being a potential response to serial disturbances in flow regimes by the SHP cascades of the Chapecó Basin.…”

“…River ecologists have theorised that the spatial position of dams along the longitudinal gradient may result in distinct ecological effects, and that some of these effects can even accumulate downstream(Ward & Stanford, 1983). These ideas are now gaining traction as a greater body of empirical evidence has accumulated(dos Santos et al, 2017;Loures & Pompeu, 2018;Miranda & Dembkowski, 2016;Ticiani et al, 2023). The degree of flow regulation and water residence time accumulate in the downstream direction in dam cascades(Lehner et al, 2011;Swanson et al, 2021), with observed patterns in macroinvertebrate assemblages being a potential response to serial disturbances in flow regimes by the SHP cascades of the Chapecó Basin.One of the primary environmental concerns regarding the proliferation of SHPs is their potential for causing ecological changes that can manifest cumulatively across the riverscape(Couto et al, 2021; F I G U R E 5 Polar plot depicting taxonomic shifts (Δ) in fish assemblages between the least-impacted upstream site and the dam-affected sites under the influence of the 12 SHPs (a-c).…”