Mill Dam Effects on Freshwater Mussel Growth in an Alabama Stream

Small dams are common in the southeastern U.S., yet few studies have quantified their effects on the region’s aquatic biota such as imperiled freshwater mussels. I investigated why freshwater mussels are more abundant and larger immediately downstream from a small dam than conspecifics in up- or downstream reaches. I attempted to answer 2 questions. First, is the larger size of mussels immediately below mill dams attributable to faster growth or greater age? Second, do sites up-and downstream from dams differ in mussel food quantity or quality? I thin-sectioned shells to age mussels and compare growth rates between populations. Additionally, I measured total suspended solids (TSS) from filtered water samples seasonally. I analyzed length-at-age data using multiple growth models and found that mill reach mussels grew faster than up- and downstream populations. TSS quantity varied seasonally but was generally highest in the impoundment and mill reach from spring-fall. TSS organic-to-inorganic ratios were highest in the upstream reach from spring-fall but highest in the impoundment and mill reach during winter. Temperature was consistently higher in all seasons in the impoundment and mill reach. My data suggest that some small impoundments enhance mussel food resources and growth conditions in downstream reaches. Increased food quantity and quality combined with elevated temperatures are the likely mechanisms responsible for promoting rapid shell growth downstream from some impoundments. These heretofore undocumented positive effects of small dams suggest that some older, more stable dams may actually benefit or promote the persistence of imperiled mussel populations. Positive effects of small dams and the degree of imperilment of mollusk populations should be factored into cost-benefit analyses when prioritizing sites for dam removal projects.