Effects of a small dam on freshwater mussel growth in an Alabama (U.S.A.) stream

1. Dams, ubiquitous features in many lotic ecosystems, are believed to have many broad-ranging and predominantly negative effects on stream biota. Whereas the impacts of larger dams are well studied, few studies have quantified effects of small dams on streams. 2. Recent surveys found numerous locations where mussels were abundant and larger in reaches immediately downstream from small dams. We examined mussel shell growth and resource conditions in Sandy Creek, a small (third-order) tributary of the Tallapoosa River in east-central Alabama (U. S. A.), to determine whether larger populations and individuals result from more rapid growth or longer lifespans of mussels downstream from the dam. 3. Growth rates for populations occurring immediately downstream from the dam (mill reach), c. 5 km downstream from the dam (downstream reach) and upstream from the impoundment (upstream reach) were compared with environmental conditions (seasonal measures of nutrient concentrations and water chemistry) and food availability [ total suspended solids (TSS)]. Water temperature was continuously monitored using data loggers. 4. Analysis of length-at-age data using multiple growth models found that mill reach mussels grew faster than both up-and downstream populations. This dam appears to substantially increase water temperatures and may extend the shell growth period in the mill reach. TSS quantity varied seasonally between sites but was generally highest in the impoundment and mill reach during spring and autumn. TSS quality was highest in the upstream reach from spring through autumn but was highest in the impoundment and mill reach during winter. 5. Our data suggest that some small impoundments enhance conditions for freshwater mussel growth in downstream reaches. However, we do not know how far downstream this subsidy extends or how different species respond to mill dam augmentation. Regardless, mounting evidence suggests that this phenomenon is geographically and taxonomically widespread in eastern North America. Heretofore, undocumented positive effects of small dams suggest that some older dams may warrant protection or restoration if downstream reaches support imperilled mussel populations. Further, some small dams may prove useful conservation tools for natural resource managers attempting to identify sites for mussel culture facilities or translocation refugia.