The effect of small reservoirs on chironomid diversity and trait composition in Savanna streams: evidence for Serial Discontinuity Concept

Habitat discontinuity is one of the main causes of diversity reduction in lotic ecosystems. We tested the predictions of the Serial Discontinuity Concept (SDC) caused by small reservoirs on the functional diversity (FD) of Chironomidae assemblages in Neotropical Savanna streams. We obtained taxonomic information from segments upstream and downstream of small reservoirs. In addition, abiotic variables, such as stream segment width, flow velocity, dissolved oxygen, pH, conductivity, water temperature, and organic matter were measured. We analyzed the Chironomidae assemblage FD using the functional richness (FRic), functional dispersion (FDis), trait relevance, as well as the species richness metrics. We used a non-parametric paired tests to compare differences in the FD indices, species richness, and the abiotic variables between the upstream and downstream segments. The results suggest that there were reductions in the FRic, FDis, species richness, and organic matter percentage below reservoirs. Moreover, depth, width, and dissolved oxygen increased in the downstream segments. The discontinuity length presented a negative influence in the FD indexes toward downstream segments as proposed by the SDC theory. We concluded that the discontinuity length affected the dispersal ability of some Chironomidae taxa, causing richness reduction and dissimilarity in the assemblages' functional traits toward downstream stretches.