Dam removal is a river management tool that is becoming more common. Removing dams reestablishes upstream-downstream connectivity in rivers, with ensuing effects on fish habitat and community structure. In cases where large dams are removed, fish movements to upstream reaches can be restored; however, in cases of smaller dams that partially impede fish movement, the effects of dam removal on fish community structure and associated functions remain unclear. To address this question, we tracked fish community structure before, and for 5 years after the removal of two small, lowhead (<7.5 m) dams located within the Olentangy and Scioto Rivers (Ohio, USA) using a modified before-after-control-impact (BACI) design. We quantified fish community composition, individual body mass, and feeding guilds in two reaches still influenced by intact dams, two reaches downstream of former dams, and three reaches in former upstream reservoirs. We then used these data to approximate community-level fish excretion with a recently published allometric model based on metabolic and stoichiometric theory. We compared community-level nutrient excretion metrics before and after dam removal, and among reference reaches that were still impacted by dams and reaches that were affected by the dam removal disturbances.
Results/Conclusions
Ordination analyses revealed that dam removal shifted reach-level fish community structure, with strongest effects 1-2 years post-removal. Dam removal effects on fish communities were more pronounced in the Olentangy vs. the Scioto River and in former reservoirs vs. downstream of removed dams. Comparing reference reaches and former reservoirs 1-3 years post-dam, fish species richness and individual body size decreased (up to 2× and 86×, respectively) in favor of smaller-bodied, insectivorous fishes. Nutrient excretion rates depend on body size, such that in the same locations predicted aggregate N and P excretion (mg h-1) decreased by up to 158× and 84×, respectively. The N:P ratio of fish excretion decreased from a maximum of 80:1 to 60:1 post-dam removal. Fish community biomass, and nutrient excretion rates, increased in impacted reaches by the final year of the study, approaching both reference, and pre-dam removal levels. Our findings imply that changes to river habitat structure and availability following dam removal affect fish abundance, species composition, and biomass within former dammed reaches, potentially leading to shifts in biogeochemical hotspots. Overall, these results illustrate how monitoring relatively simple characteristics of riverine fish communities can be used to predict community- and ecosystem-level functioning after significant management interventions.