Boreal river impoundments caused nearshore fish community assemblage shifts but little change in diversity: a multiscale analysis

River flow regulation, fragmentation and changes in water quality caused by dams have varying effects on aquatic biodiversity and ecosystem functions, but are not clearly resolved in boreal ecosystems. We adopted a multi-scale approach to quantify fish community trajectories over 20 years using a network of sites spread across four reservoirs in two hydroelectricity complexes in northern Quebec, where other anthropogenic factors have been negligible. Across three spatial scales, we found little evidence of directional temporal trends in diversity relative to reference sites. Using beta-diversity analyses, we also detected a high degree of stability in fish composition over time and space at the complex and reservoir scales. However, changes in species assemblage following impoundment were detected at the scale of the sampling station. At this scale, we found that some species consistently benefited (coregonids and pikes) from impoundment whereas others were detrimentally affected (suckers and one salmonid...