Longitudinal variation in macroinvertebrate assemblages below a large-scale hydroelectric dam

Abstract

Worldwide, dams have had pervasive impacts on their associated river systems. In this study, we examined the effects of a large-scale dam on the downstream macroinvertebrate assemblages of a 65-km stretch of the Chattahoochee River (Georgia, USA) over an 11-year period. We quantified differences among six sites within the study reach using NMS, ANOSIM, invertebrate metrics (Shannon–Weiner diversity, number of EPT taxa, and number of sensitive taxa), and indicator analysis. We compared results from this study to the nearby unregulated Middle Oconee River. There were significant differences among all sites and increases in all macroinvertebrate metrics as distance from the dam increased (and diel flow variation decreased). Indicator analysis supported this pattern: no EPT indicator taxa existed directly below the dam, whereas 17 were indicators at the final site. Similarity between the Middle Oconee and the Chattahoochee was moderate (52%). Our results suggest that impacts of the dam are far-reaching and long-term, and that the macroinvertebrate assemblage does not return to a fully representative state even 65 km downstream. This study contributes to the small but growing field of research on effects of dams over long temporal and spatial scales.