Integrating habitat configuration and biotic integrity into an ecological flow framework for the heavily regulated Huai River Basin

Study region The Huai River Basin (HRB), China, is a heavily regulated floodplain river system characterized by intensive flow regulation, longitudinal fragmentation, and chronic nutrient loading. Its aquatic ecosystems face compounded stressors from altered hydrological regimes and progressive habitat degradation, making it a representative case for ecological flow (e-flow) optimization in human-altered basins. Study focus This study develops a multidimensional, indicator-based e-flow framework that transitions from conventional hydrological indices to integrated hydro-biological metrics. Synthesizing data from five field campaigns across 27 monitoring sites, we coupled two-dimensional hydrodynamic-habitat simulations for the indicator species Parabramis pekinensis with multi-metric biotic integrity assessments. The framework explicitly quantifies life-stage-specific habitat requirements by prioritizing the spatiotemporal sensitivity of hydro-morphological indicators—including Weighted Usable Area (WUA), spatial connectivity, and fragmentation metrics—while decoupling flow-driven habitat optimization from load-based water quality management. New hydrological insights for the region The HRB’s aquatic communities are hydrologically co-constrained by physical habitat fragmentation and disrupted longitudinal connectivity, with chronic nutrient loading and low-flow regimes acting as primary ecological bottlenecks. The proposed framework significantly outperformed the conventional Tennant method, increasing spawning Habitat Suitability Index (HSI) values to 9.31~13.62 and improving connectivity metrics by 19~26% ( p < 0.05), while reducing spawning habitat fragmentation to 6.51~8.77 hm². We identified a critical spawning discharge threshold of ~1500 m³ /s and a minimum overwintering depth of 2.5~3.0 m. The optimized e-flow, representing approximately 33% of the mean monthly reference flow, provides a mechanistic, biologically grounded benchmark for balancing ecological restoration with regional water security. This phased, indicator-driven approach offers a transferable hydrological template for adaptive e-flow management in regulated floodplain rivers globally.