Managed freshwater replenishment is a significant restoration method in the Yellow River Delta. However, their impacts on plankton communities, which are key bioindicators of aquatic ecosystem health and sensitive to the changes in the environment, remain poorly quantified. In this study, we conducted plankton surveys across wetlands subjected to freshwater restoration durations ranging from 5 to 22 years. We assessed shifts in phytoplankton and zooplankton community structure, biomass, diversity, and their relationships with environmental drivers. Results revealed distinct temporal dynamics: phytoplankton biomass and diversity followed a “U-shaped” trajectory (initial decline followed by recovery), while zooplankton biomass decreased but diversity increased with restoration duration. Canonical Correspondence Analysis (CCA) and Partial Least Squares Path Modeling (PLS-PM) identified salinity (Cl⁻, SO₄²⁻) and dissolved nitrate (NO₃⁻) as primary environmental controls for both groups. Cyanobacteria dominated phytoplankton biomass initially but declined with restoration age, while rotifers replaced copepods as the dominant zooplankton taxon over time. These findings demonstrate that freshwater restoration restructures plankton communities through salinity-mediated physiological constraints and altered nutrient availability, with implications for ecosystem function and adaptive management in anthropogenically influenced deltas.

Reservoirs, driven jointly by artificial regulation and natural processes, are semi-artificial complex ecosystems whose equilibrium states affect the water ecological security and management effectiveness of a river basin. Understanding how operational dispatch alters hydrodynamic processes, habitat structures, and biological succession pathways is necessary, as traditional steady-state theories based on natural lakes struggle to fully explain the responses of reservoir ecosystems. Therefore, this review constructs a theoretical framework for reservoir ecosystem regimes, defining the core connotation of their “conditional metastable state” as the long-term maintenance of physicochemical, biological, and hydro morphological parameters within limits that allow ecosystem functioning under anthropogenic regulation and natural disturbances. It identifies typical characteristics such as habitat reconstruction-driven dynamics, altered ecological resilience, dual threshold effects, and regulation-modified hydrological processes. We propose that reservoir regime shifts can be analyzed from three levels: the evolutionary sequence of the full life cycle, stage transitions during the operational period, and disturbance responses at multiple time scales.

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