Characteristics of Benthic Macroinvertebrate Community Structure in Typical Polders of Dongting Lake and Assessment of Aquatic Ecological Health

As an artificial–natural composite ecosystem, polders face increasingly prominent internal and surrounding water environment problems under sustained strong human activities. To systematically assess the water ecological health status of polders around Dongting Lake, this study took four typical polders around Dongting Lake as research subjects, and two seasonal surveys of benthic macroinvertebrates were carried out in July and December 2024. Through analyses of community structure characteristics and correlations with key environmental factors, the Trophic Level Index (TLI), Water Quality index (WQI), Shannon–Wiener index of benthic macroinvertebrates, and Benthic Macroinvertebrates Index of biotic integrity (B-IBI) were selected to establish an entropy-weighted Bayesian model for comprehensive evaluation of the current ecological health of Dongting Lake polders. In 2024, a total of 47 benthic macroinvertebrate species were found in Dongting Lake polders, including 25 species in the summer and 36 species in the winter. The main groups were Gastropoda, Insecta, and Bivalvia, and the community composition did not show significant seasonal or spatial differences. The benthic macroinvertebrate community structure in the summer was mainly influenced by TN/TP, NO3−-N, WD, Tur, and Chl.a, suggesting eutrophication as a critical concern, while in the winter it was mainly regulated by WSW, WT, Tur, and DO, highlighting hydrological conditions as pivotal. The entropy-weighted Bayesian assessment indicated an overall aquatic ecological health status of “moderate” in 2024, with summer conditions superior to those in the winter and notable spatial heterogeneity observed in winter sampling sites within ditches, among which Polder Chengxi exhibited the best condition, while Polder Junshan and Polder Xiangbin Nanhu showed degraded states. Notably, Polder Junshan (JS2 and JS3) displayed clear signs of ecological degradation during the winter, warranting immediate initiation of targeted restoration measures. This study provides a systematic diagnosis and scientifically grounded evaluation of aquatic ecological health in Dongting Lake polders, offering a robust theoretical framework and empirical data for future water environmental protection strategies and ecological restoration practices.