Differential impacts of small hydropower plants on macroinvertebrate communities upstream and downstream under ecological flow

Hydropower dams influence freshwater biodiversity by altering river flow patterns and habitat conditions. With the global surge in small hydropower plants (SHPs), their impacts on aquatic ecosystems have become increasingly recognized. However, most previous studies did not consider the recently implemented ecological flows. Consequently, the effects of SHPs under ecological flow conditions on aquatic organisms, such as macroinvertebrate communities, remain unclear. We surveyed 15 SHPs in the Oujiang region, establishing sampling sites upstream of the intake dams (S1), in dam-induced reservoirs (S2), in dewatered sections downstream of the dams with ecological flows (S3), and in sections with restored natural flow (S4). By comparing macroinvertebrate community composition, diversity, functional feeding groups, and network structures in these areas, we assessed the ecological response of macroinvertebrates to SHPs under ecological flows. Our research found that SHPs significantly impact macroinvertebrate communities. Specifically, at site S2, stagnant water species replaced those typically found in flowing conditions, resulting in a marked difference in species composition between S2 and other sites. Compared to S1 and S4, diversity indices at S2 and S3 were lower, with filterers and collectors dominating the functional feeding groups at S2 and S3. Co-occurrence network analysis revealed that network complexity at S2 and S3 was lower than at S1 and S4. Additionally, S3 was less affected by SHPs than S2, underscoring the importance of ecological flow replenishment. Overall, our research confirmed the remarkable influence of SHPs on S2 macroinvertebrate community, and emphasized the importance of maintaining sufficient ecological flow to the downstream aquatic organism of S3 reach. We suggest a comprehensive assessment of the potential environmental impacts of SHPs, particularly the negative effects caused by insufficient ecological flow, to ensure the sustainable development of ecosystems.