Food web structure and energy flux synergistically enhance multifunctionality: Evidence from a river-lake ecosystem

1. Biodiversity–Ecosystem Multifunctionality (BEMF) frameworks underpin function-oriented management of freshwater ecosystems. While food web topology and energy flux critically regulate multifunctionality (MF), their synergistic roles remain unresolved.2. We investigated the effects of food web structure and energy flux on ecosystem multifunctionality (MF) across four river-lake habitats (river, ecotone marsh, lake center, outflow aquaculture zone) in Dongping Lake, China. Eleven functional variables (nutrient cycling, ecosystem metabolism, primary/secondary production, microbial abundance) were integrated to quantify MF using averaging and threshold approaches (30%, 50%, 80% effectiveness threshold). Bayesian linear models and Partial Least Squares Path Modeling (PLS-PM) identified mechanistic drivers.3. MF displayed pronounced spatiotemporal heterogeneity, peaking in the ecotone and minimizing in the river. Food web structural complexity maximized in the ecotone (elevated nodes/links/robustness/principal component scores) and minimized in the river. Total energy flux was lowest in the river, peaking at the aquaculture zone (spring) and ecotone (summer). The ecotone maintained the highest herbivory-derived flux across seasons. Both food web structure and energy flux enhanced MF, but structural properties only promoted low-threshold MF (30%), while energy flux elevated MF across all thresholds.4. Our findings highlight the synergistic effects of food web structure and energy flux on multifunctionality, and identify the critical role of ecotones in sustaining such multifunctionality, which will advance the current BEMF frameworks and provide a robust foundation for conservation efforts in river-lake ecosystems.