Effects of historic flooding on fishes and aquatic habitats in a Missouri River delta

Understanding the effects of disturbances on aquatic biota is important for management of the world's rivers. Riverine deltas are among the most biologically productive ecosystems, yet the ecological effects of floods in deltas are poorly understood. Therefore, the goal of this study was to examine impacts of an historic flood in the Missouri River basin in 2011 on fishes and aquatic habitats in the Lewis and Clark Delta, located in South Dakota and Nebraska, USA. Fish community structural indices declined in the six years preceding the flood, with species richness and diversity (Fisher's α) decreasing from 25 to 22 and 4.56 to 3.48, respectively. However, the fish community exhibited short-term resistance to the flood as both metrics were similar to pre-flood levels after the disturbance in 2012. Evenness (J ′) declined from 0.88 to 0.73 before the flood but increased to 0.84 after the disturbance. The majority of species exhibited greater relative abundance after the flood regardless of age class (i.e., juvenile, adult), morphology (i.e., small-bodied, large-bodied), introduction history (i.e., introduced, native), or recreational importance (i.e., sport fish, non-recreational). However, the flood reduced relative abundance of juvenile freshwater drum (Aplodinotus grunniens) and white crappie (Pomoxis annularis) and did not affect the three small-bodied species that were studied, including emerald shiner (Notropis atherinoides), red shiner (Cyprinella lutrensis), and spotfin shiner (Cyprinella spiloptera). The flood decreased side channel and backwater frequency and width/area but increased sandbar abundance. Physical alterations evidently had minimal effects on structural indices of the fish community. However, an overall increase in relative abundance across species suggests that interspersed fluvial and slackwater habitats in the delta provided refuge from floodwaters during the disturbance. Maintaining habitat connectivity in deltas during and after floods is particularly important for fisheries conservation. Illustrating the ecological effects and implications of a major flood, this study contributes to the nascent field of delta ecology.