Multi-Dimensional Characterization of Seasonal Phytoplankton Community Dynamics in Urban Water Bodies of Beijing

Phytoplankton play a central role in aquatic ecosystems as primary producers and serve as key bioindicators of water quality. This study systematically examined the seasonal dynamics (spring, summer, autumn) of phytoplankton communities in Beijing’s urban water bodies by integrating α-diversity, co-occurrence networks, β-diversity decomposition, and environmental driver analysis. Results indicated that spring exhibited the highest α-diversity (Margalef index: 2.95, Shannon index: 2.99) and optimal ecological conditions, with community assembly primarily influenced by spatial processes. Summer was characterized by cyanobacterial dominance, a peak in algal density (957.35 ± 4818.65 ind./L), and tightly connected, cooperative networks with high clustering and positive interactions. In autumn, β-diversity increased significantly (0.9030), driven predominantly by taxa turnover, while co-occurrence networks became more modular and less connected, indicating enhanced environmental filtering. Key environmental drivers, including temperature, total phosphorus, total nitrogen, and organic pollution indices, shaped community structure, with their relative influence shifting seasonally. A random forest model, trained on multiple biodiversity indices and algal density, effectively captured nonlinear ecological patterns, confirming the highest ecological quality in spring and a marginal decline in autumn. These findings highlight the seasonal transition in assembly mechanisms—from spatial to environmental processes—and support tailored management strategies.