Macroinvertebrate response to run‐of‐river hydropower tailrace inflow: Spatial assessment of community and traits

Run‐of‐river (R‐O‐R) hydropower dams introduce two discontinuities in the river: the dam and the tailrace. Discontinuities can lead to changes in the water quality that may affect the survival of aquatic macroinvertebrates, depending on their traits. We investigated the invertebrate assemblage and community‐level traits in an urban river influenced by a small hydropower tailrace in Hokkaido, Japan. As a result of the tailrace inflow to the main channel, the bypassed reach was warmer, and the areas below the confluence were colder. Temperature was a consistent factor affecting the distribution of indicator taxa and of the five major taxa that contributed the most to dissimilarity, but other environmental factors also showed significant effects. At the community level, the weighted mean of taxa with warm preference showed a significant positive relationship with temperature, but taxa with cold preference did not appear to be filtered by the cold water delivered by the tailrace. However, sensitive taxa such as Ephemerella have already started showing negative responses to temperature. The observed distribution of cold‐preferring scrapers could negatively affect the energy transfer from primary producers to higher consumers in the bypassed reach. Appropriate mitigation of climate change effects in hydropower systems can be achieved by increasing the flow in the bypassed reach, especially during the summer months, to maintain a temperature regime that is adequate for the survival of macroinvertebrate populations and maintenance of ecosystem functions.