Mill dams enhance mussel growth rates in Alabama streams

Although the effects of large dams on stream ecosystems are well-known, the effects of more ubiquitous smaller dams on stream biota remain understudied.  We investigated differences in Elliptio spp. density and body size near mill dams in 3 East-central Alabama Piedmont streams.  Elliptio arca populations in Sandy Creek (Tallapoosa River Drainage) immediately downstream of the mill dam (mill reach) occurred at significantly higher densities (ANOVA F_2,262=12.45, P<0.0001) and were significantly larger than individuals collected up-or downstream (ANOVA, F_{2, 391}=26.02, P< 0.0001). We observed similar length difference patterns in E. arctata in Loblockee Creek (Tallapoosa River Drainage) (ANOVA F_2,335=18.74, P<0.0001).  Elliptio pullata in Halawakee Creek (Chattahoochee Drainage) were significantly larger in the mill reach than individuals upstream (ANOVA F_2,125=6.56, P<0.02). We thin sectioned E. arca shells from Sandy Creek, obtained internal age estimates, and observed significant growth rate differences between mill, up- and downstream reaches (ANOVA F_2,24=7.14, P<0.004).  Moreover, LSD Post hoc analysis showed that mussel growth rates in the mill reach are significantly greater than both up-and downstream reaches (P<0.0001 and P=0.05, respectively).  We hypothesize that shell growth rates may be greater below mill dams because impoundments increase downstream temperatures and possibly mussel food quality and/or abundance.