Impacts of Dams on Fish Species Assemblage and Functional Trait Composition Within Two Alabama Drainages

ABSTRACT Modifications to flow regimes may substantially alter the structure and function of aquatic systems and biotic assemblages. We assessed the impacts of relatively large dams in Alabama, USA, on fish assemblages and functional trait composition at two scales—within one drainage (Bear Creek, BCD) and between two drainages (BCD and Cahaba River, CRD). We collected fishes in shallow waters (< 1 m) of lotic sections of three impounded and two unimpounded streams. Impoundment effects were similar at both scales. Assemblage structure differed up‐ and downstream of impoundments, but few differences were detected between unimpounded stream sections. Additionally, there were few functional trait composition differences between up‐ and downstream sections of impounded or unimpounded streams. However, we detected numerous functional trait composition differences between impounded and unimpounded streams, indicating that ecosystem functions may differ in each stream type due to differences in fish functional traits. Fish assemblage and functional trait variations were best explained by the percent cover of aquatic vegetation, temperature, and discharge variation. Fish species that preferred slow‐moving waters were indicators of impounded streams, and species that preferred high percentages of vegetation were indicators of unimpounded streams. Species sensitive to habitat degradation, such as leuciscids and percids, were less common, and more tolerant species, such as centrarchids, were more common in the current study than in historical surveys from the 1960s and 2000s. Although we detected assemblage differences by taxa between impounded and unimpounded streams, functional trait composition may give an operational understanding of assemblage changes, allowing managers to target functional groups for conservation efforts, as well as providing a metric that can be applied to other systems.