Implications of Low-Head Dams: From Habitat Quality to Population Genetics

Low-head dams are prevalent throughout the United States and have been shown to impact riverine systems in multiple ways. Fragmentation of river systems can lead to changes in habitat that consequently influence functional community composition and genetic structuring of fishes. We assessed how low-head dams on two Illinois rivers affect habitat quality and fish functional guilds by monitoring below-dam, above-dam (pool), and upriver sites on each river. Fishes were aggregated into habitat and reproductive guilds to functionally analyze dams’ impacts on fish communities. We analyzed microsatellite loci of Longear Sunfish (Lepomis megalotis) and Bluntnose Minnow (Pimephales notatus) to investigate dams’ impacts on genetic differentiation and diversity in these species. Analyses of habitat and fish communities indicated significant differences in habitat quality and functional composition between degraded pool sites and high quality upriver sites. Genetic analyses revealed little differentiation in Longear Sunfish among sites but two genetically distinct populations of Bluntnose Minnow separated by one of the two dams. Data suggest that the presence of these low-head dams strongly influences habitat quality and fish functional composition. Furthermore, the impacts of dams on fish genetic structuring seem to depend on the focal species, river system, and the individual dam itself.