Impacts of flood retention dams on benthic invertebrates by affecting bed material size and disturbance in reservoir and downstream sections

The impacts of Flood Retention Dams (FRDs), designed solely for flood control and featuring bottom outlets at the riverbed level, on benthic invertebrate communities have not been sufficiently elucidated. This study investigated the impact of FRDs on benthic invertebrate communities downstream and reservoirs, focusing on differences in riverbed conditions and bed disturbances caused by reservoir sedimentation. We compared benthic invertebrate communities and riverbed conditions at upstream, reservoir, and downstream sites from seven FRDs in Japan. The average community similarity between the upstream and downstream sites across the seven dams was 0.73, comparable to unregulated streams. They were not related to the duration of dam operation, indicating minimal long-term impacts on downstream communities. However, the community similarity between the upstream and reservoir sites was notably lower, at 0.66. Reservoir sites had smaller grain sizes and softer substrates with narrower interstitial spaces than upstream sites. Consequently, taxes that inhabit or move across stone surfaces were more common in reservoirs. Conversely, taxa that prefer stable beds and larger body sizes were less frequent. Additionally, we found that reservoir sedimentation fluctuations, which indicates riverbed disturbance pattern, influence the similarity of invertebrate communities between upstream and downstream sites. To conclude, FRDs have negligible impacts on benthic invertebrate communities at DS sites; smaller grain sizes modulate these communities at RS sites. Predicting reservoir sedimentation fluctuations supports the design of FRDs with minimal impacts on benthic invertebrates.