While aquatic invertebrates in arid streams are adapted to highly dynamic flow regimes (e.g. floods and droughts), many rivers in the arid southwest US and around the globe have had this dynamism removed through damming. These alterations to the natural flow regime can have major consequences on the structure and function of aquatic invertebrate communities. We examined the consequences of a series of prescribed flood events (2006-2008) and flow homogenization (steady baseflow and no high flow events) on aquatic invertebrate communities in the Bill Williams River in Arizona, USA. We hypothesized that: 1) prescribed flow releases would shift the communities towards a similar state found in the upstream communities; and 2) nine years of flow homogenization following the releases would shift the community back to pre-flood conditions. To test our hypotheses, we sampled aquatic invertebrates above (n = 4 sites) and below Alamo Dam (n = 2 sites) before planned flood releases, and then subsequently 1 and 12 days after. Samples were collected again in 2016, representing a nine-year period of flow homogenization. We quantified changes in community structure and functional trait diversity in the presence and absence of natural flows using a combination of univariate and multivariate methods.
Results/Conclusions
Following the prescribed floods there was a significant shift in communities downstream of the dam from pool inhabiting (lentic) species to more early colonizing species found in flowing waters (lotic). Alpha diversity (α) and community evenness were also influenced by floods at each site. During the nine years of stable base flows following the prescribed flow releases, beavers have proliferated and altered the geomorphology of the river channel to a more lentic state than found under natural historical flows. Early results suggest that this flow homogenization shifted the communities to more homogenized state in terms of community structure, and taxonomic and functional trait diversity when compared to pre-flood conditions, as well as shifting communities to those more representative of lentic habitats. We are currently exploring both the functional turnover and convergence/divergence hypotheses to explain these community shifts in relation to functional attributes of the community. These results demonstrate the profound effects that flow homogenization can have on aquatic invertebrate communities downstream of dams through both direct effects on invertebrate communities and also indirect effects by facilitating the dominance of an ecosystem engineer (beaver) that alters the geomorphology of the river into a more lentic state.