Introduction
Dams cause physical and ecological impacts to aquatic ecosystems disrupting over half of the world’s major river systems (Nilsson et al., 2005). Although dams vary in their characteristics (e.g., size, function), they have some common effects on habitat, discharge, and energy inputs. All dams interrupt the movement of water and aquatic organisms and can disrupt energy and nutrient flows. Organisms in river systems rely on energy inputs from both upstream and downstream habitats, and structures interrupting river connectivity generally have watershed-scale effects (Vannote et al., 1980; Ward, 1989; Stanford & Ward, 2001; Schindler et al., 2013). Upstream, inundation eliminates terrestrial habitats, increases water depth, and removes turbulent flow, significantly affecting lotic specialists (Nilsson et al., 2005). Downstream, releases from the dams alter habitats, create unnatural temperature variation, alter natural flow variation, limit channel development/maintenance, and reduce system productivity (Nilsson et al., 2005). The serial discontinuity concept (SDC) details theoretical ecosystem responses to altered conditions downstream of such disturbances, identifying how certain variables (ecological connectivity, biodiversity, particulate matter abundance) are expected to decrease in tailrace areas before gradually increasing along a longitudinal gradient (Ward & Stanford, 1989; Stanford & Ward, 2001).
Hydropeaking dams in particular can have effects on downstream waters, including artificial flow variation and alteration to the natural thermal regime (Irwin & Freeman, 2002; Twardek et al., 2022). R.L. Harris Dam on the Tallapoosa River in east-central Alabama is such a facility that operates on a hydropeaking schedule. Constructed in 1983, the dam initially operated with hypolimnetic release and no partial flow between generation periods which resulted in both thermal and flow modification of the downstream area. In 2005, stakeholders adopted an adaptive management plan, (called the “Green Plan”) that established regulations on dam operations, including base flow requirements between generation periods (Irwin & Freeman, 2002; Kleinschmidt Associates, 2018). Although the Green Plan mitigated some of the effects of dam operation, water was still released from the hypolimnion, leading to pulses of higher flow with colder water temperatures during summer. Downstream effects of dam operations under such a modified hydropeaking schedule on fishes have not been thoroughly described prior to this work. The subsequent change in downstream water temperature between generation versus non-generation periods could reach 10°C and the change remained measurable many kilometers downstream (Irwin & Freeman, 2002), although the majority of hourly temperature changes were less than 2°C (DeVries et al. 2022).
Alabama is a hotspot of aquatic biodiversity, and each river system in the state has unique aspects of its native fish assemblages (Lydeard & Mayden, 1995; Mettee et al., 1996; Freeman et al., 2005). Historically as many as 126 fish species have been identified in the Tallapoosa River, and the broader Mobile drainage is one of the most diverse freshwater fish assemblages in North America (Lydeard & Mayden, 1995; Travnichek & Maceina, 1994; Freeman et al., 2005). Protecting biodiversity in Alabama rivers is increasingly important as anthropogenic impacts increase. To protect aquatic resources, it is necessary to identify how various threats (such as a dam) affect the ecology and life history of resident organisms. The Tallapoosa River from the tailrace of Harris Dam downstream to the headwaters of Lake Martin is diverse in both habitat and resident fishes and is considered one of the highest quality river segments in the piedmont region of Alabama (Irwin & Freeman, 2002). Although little information about the Tallapoosa River fish assemblage is available from before Harris Dam was constructed, several studies of the fish assemblage have been conducted since 1983 (Kinsolving & Bain, 1993; Travnichek & Maceina, 1994; Irwin & Freeman, 2002; Freeman et al., 2005; Irwin et al., 2019). Beyond the effects of hydropeaking dam operation discussed earlier, Travnichek and Maceina (1994) showed that the diversity and richness of fish inhabiting shallow downstream waters was decreased in regulated portions of the Tallapoosa River versus upstream/unregulated areas. However, little work has been conducted after the implementation of the modified hydropeaking operations on the overall fish community; as such, we quantified the fish assemblage across a spatial gradient downstream from the dam under the current “Green Plan” to determine the magnitude of continued effects of Harris Dam.