Short-term consequences of lowhead dam removal for fish assemblages in an urban river system

Abstract

Lowhead or run-of-river dams, which can have signifi cant impacts on river ecosystems, are common on rivers around the world. Although lowhead dam removal is becoming an increasingly viable component of river restoration projects, the quantitative effects of lowhead dam removal on river ecosystems are not well described. In this study, we investigated the short-term (< 2 years) effects of two lowhead dam removals on fish assemblage diversity and structure in the Scioto and Olentangy Rivers of urban Columbus, Ohio (USA). Non-metric multidimensional scaling (NMS) and analysis of similarities (ANOSIM) revealed that upstream assemblage composition shifted signifi cantly from before to after dam removal (ANOSIM; R = 0.714, p = 0.001). Likewise, assemblage shifts were signifi cant between years 1 and 2 at Olentangy River reaches both upstream and downstream of dam removal (ANOSIM; R = 0.136, p = 0.019). Shifts in fish diversity metrics were accompanied by changes in relative abundances of taxa within feeding guilds. For example, reductions in species richness and diversity at upstream reaches were accompanied by the loss of large-bodied omnivorous species. In the second year following dam removal, a signifi cant increase in assemblage diversity at an upstream restored reach (including colonization by sensitive Etheostoma species) was accompanied by an increase in insectivores and a reduction of larger-bodied omnivores and carnivores. Overall, our results suggest that dam removal may act as a pulse disturbance with quantitative short-term impacts on fish assemblages. Fish responses to dam removal likely operate along a temporal trajectory wherein short-term responses will be critical in shaping longer-term responses.