Hydrological Regulation Restructures Energy Pathways and Ecosystem Multifunctionality in a Large River‐Reservoir System

Large reservoir operations impose deterministic, seasonal hydro‐physical stress, driving river ecosystems to alternate between turbulent lotic and stable lentic regimes. How ecosystems maintain functional integrity under such rigid anthropogenic forcing, specifically the relative roles of structural complexity versus process efficiency, remains a critical knowledge gap. Using the Three Gorges Reservoir as a case, this study integrated stable isotope analysis, food web topology and structural equation modelling to explore the underlying functional maintenance pathways across a seasonal contrast. Temporally, we identified a fundamental alternation of functional pathways: The ecosystem relies on a complex‐redundant configuration to buffer turbulent hydrodynamic stress during the flood period but appears to shift opportunistically to a high‐efficiency trophic configuration to capitalize on stable conditions during the impoundment phase. Spatially, the abrupt hydrodynamic deceleration corresponds with a severe ecological bottleneck in the transitional zone, rendering it the most ecologically vulnerable link characterized by a double deprivation of both structural redundancy and energetic efficiency. Crucially, in this physically forced system, the capacity for rapid functional pathway reconfiguration, specifically towards shorter, high‐efficiency process transfer, represents a key adaptive mechanism for sustaining overall ecosystem multifunctionality (EMF). Concurrently, biodiversity ensures the stability of these functional relationships, potentially by adaptively switching its mechanistic roles: from providing ecological insurance under lotic stress to facilitating niche complementarity under lentic stability. These findings suggest that the management of large reservoirs should shift towards a holistic paradigm that simultaneously considers biodiversity conservation and the optimization of energy flow pathways.