Effects of impoundments on stream crayfish assemblages

Life-history timing, trophic interactions, and colonization potential of stream benthos are linked to a stream’s flow regime. Modifications to flow regimes often alter the composition of benthic assemblages. We assessed the impacts of relatively large storage dams (>15 m in height, >400-ha impoundments) on crayfish assemblages in southern Appalachian Mountain streams, Alabama, USA. We sampled crayfishes at multiple sites in flowing sections upstream and downstream of dams in 3 impounded streams and along similar lengths of 2 unimpounded streams in the Bear Creek and Cahaba River drainages. We analyzed at 2 scales: within and between drainages. This multi-scale approach allowed us to assess the effects of impoundments on crayfish assemblages in 1 drainage and then assess whether impoundment effects could be generalized across drainages. We compared crayfish assemblages between impounded and unimpounded streams and examined changes in crayfish assemblages with distance from impoundments, considering land-use history as a potential confounding factor. Adult densities averaged 45% higher, and sizes 10% smaller, in downstream sections of unimpounded streams compared with impounded streams. Densities and sizes of adults in upstream sections did not differ between unimpounded and impounded streams. In both drainages, unimpounded stream crayfish assemblage structure gradually shifted along the stream length, with species present upstream differing from those present downstream. Conversely, in all impounded streams, species assemblages did not differ between upstream and downstream sections, illustrating that dams diminish crayfish diversity. Nonetheless, finer-scale assessments showed that crayfish density and richness increased with distance downstream of impoundments. This suggests that assemblage recovery is possible with increasing distance downstream of impoundments. Additionally, we detected fewer crayfish assemblage differences between impounded and unimpounded streams in the Cahaba River drainage than the Bear Creek drainage. In the Cahaba River drainage, urbanization and the presence of an invasive crayfish species in both streams may have hindered our ability to distinguish impoundment effects on crayfishes. Our results show that large dams negatively affect crayfishes and may affect hundreds of crayfish species in thousands of stream km, possibly altering stream systems by interfering with the numerous ecosystem functions influenced by crayfishes.