Optimizing reservoir operation rules for ecological sustainability by identifying dual effects of flow regime and water quality on metacommunity dynamics

Reservoir operation causes spatiotemporal variations in outflow, which influence the dynamics of downstream aquatic communities. However, empirical evidence of community responses to flow regime (FR) and water quality (WQ) remains limited for dam-regulated rivers. This study focused on identifying the influences of both FR and WQ on metacommunity dynamics downstream of the reservoir. First, the metacommunity dynamics model (MDM) was used to simulate aquatic community dynamics under changing FR and WQ. Then, the flow-ecology relationship was established to identify community response to reservoir outflow. Third, the novel ecological indicators were proposed to evaluate the resilience and resistance of multi-population systems. Finally, the reservoir operating rule curves were optimized by considering tradeoffs between socioeconomic and ecological objectives. The coevolution processes of multi-population systems (fish, phytoplankton, zooplankton, zoobenthos, and macrophytes) were simulated by MDM for each local community. The population densities of stable states showed continuous downward trends with increasing alteration degree of FR and WQ for multi-population systems, and aquatic community systems could be destroyed when alteration reached its acceptable maximum. The greater the alteration degree of FR and WQ, the longer the recovery time from an unstable to a stable state, and the weaker resistance for each population system. The resilience and resistance of downstream multi-population systems can be enhanced by optimizing reservoir outflow. The optimization results illustrated that all the performances of the multiple objectives of water supply, hydropower generation, and ecological benefits were improved by no less than 2.5% compared with the conventional operation. This study provided an approach to identify dual effects of FR and WQ on aquatic community systems, which is helpful in guiding ecological restoration for river ecosystems.