An Evaluation of the Relative Influence of Habitat Complexity and Habitat Stability on Fish Assemblage Structure in Unregulated and Regulated Reaches of a Large Southeastern Warmwater Stream

Abstract

AbstractRiver regulation and development are the foremost problems threatening lotic fishes and other aquatic biota in the United States. The operation of hydroelectric facilities can influence both habitat availability and environmental stability in downstream reaches. We evaluated the relative influence of habitat complexity and environmental stability on fish assemblage structure at unregulated and hydropower‐regulated reaches of the Flint River in southwestern Georgia. The availability of different habitat types was highly variable at the regulated reach owing to large, daily fluctuations in discharge. Habitat‐specific fish assemblages also differed between reaches, as a greater number of species occupied identical habitat types at the unregulated reach, most notably in shallow, slow‐flowing habitats. Differences in fish assemblage structure between study reaches in comparable habitat types were explained equally well by patterns of habitat structure and variability. Within‐reach patterns of fish assemblage structure were best explained by patterns of habitat structure at both study reaches. However, the relative influence of habitat complexity and habitat stability differed within each study reach, habitat variability influencing fish assemblage structure to a greater extent at the regulated reach than at the unregulated reach. These differences suggest that both habitat structure and variability influence Flint River fish assemblages and that flow regulation associated with hydropower operation primarily affects riverine fish communities by increasing environmental variability. Thus, flow management plans for regulated rivers based on minimum flows may be most effective when implemented in conjunction with plans that reduce the spatial and temporal variability of habitat availability, particularly in shallow, slow‐flowing habitats.