Non-equilibrium Recovery of Benthic Macroinvertebrate Communities Following Ecological Restoration in a Large Shallow Lake: Evidence from Five-Year Monitoring.

Baiyangdian Lake, the largest freshwater lake in the North China Plain, plays a crucial role in regional ecology and hydrology. Benthic macroinvertebrates are widely recognized as important bioindicators for assessing the ecological health of lake ecosystems. Following the establishment of the Xiong'an New Area, comprehensive ecological restoration was implemented from 2019 to 2023, yet the recovery status of benthic communities remains inadequately understood. In this study, we conducted a comprehensive analysis of the temporal and spatial dynamics of benthic macroinvertebrates using Generalized Additive Models (GAMs). A total of 449 individuals of benthic macroinvertebrates were identified, belonging to 3 phyla (Mollusca, Annelida, and Arthropoda), 6 classes, 7 orders, 7 families, and 20 genera. Five dominant species were identified: Propsilocerus akamusi, Bellamya aeruginosa, Cipangopaludina cathayensis, Chironomus flaviplumus, and Bellamya purificata. The Shannon-Wiener diversity (H') and Margalef richness (D) indices showed non-significant upward trends (p > 0.05), increasing from 0.708 to 1.854 and from 0.638 to 2.875, respectively. Biomass showed a significant increasing trend, peaking at 427.02 g/m2 in 2023 (357.9% increase relative to 2019), whereas density fluctuated without directional pattern. Significant phylum-level restructuring occurred: Arthropoda abundance declined, with C. flaviplumus showing a highly significant decreasing trend (edf = 1.49, p < 0.001) and P. akamusi collapsing from 98 individuals (2019) to 2 (2023); conversely, Annelida richness exhibited a significant non-linear increasing trend (edf = 1.91, p = 0.045) and Mollusca abundance showed a marginally significant quadratic trend (edf = 1.92, p = 0.060). Negative correlations between Arthropoda versus Mollusca (r = -0.60) and Annelida (r = -0.45) abundances indicate competitive replacement dynamics. Pronounced spatial heterogeneity characterized recovery trajectories: former aquaculture sites (Quantou) exhibited pioneer species collapse without immediate replacement, while macrophyte-revegetated areas (Nanliuzhuang) demonstrated successful gastropod colonization. These findings indicate that while water quality improvement has initiated community reorganization from r-selected, pollution-tolerant assemblages toward K-selected, competitive dominants, the benthic macroinvertebrate community remains in a non-equilibrium, early-stage recovery state. The observed temporal asynchrony and spatial variability underscore that ecosystem stabilization has not yet been achieved. This study provides a scientific basis for adaptive management of restored lake ecosystems and underscores the necessity of long-term monitoring to track the trajectory toward full ecosystem stabilization.