Abstract: Suzhou urban water network is a typical plain river-lake composite system regulated by sluices. To clarify the spatiotemporal variations and driving mechanisms of phytoplankton functional groups (PFGs) under intensive anthropogenic interference, we conducted a systematic investigation across 102 sampling sites, including backbone rivers, urban rivers, and lakes, during flood and non-flood seasons from 2022 to 2023. Results showed that a total of 198 species belonging to 8 phyla were identified, with Chlorophyta (42.4%), Bacillariophyta (25.2%), and Cyanophyta (13.1%) as the dominant phyla. Fourteen dominant functional groups were identified, including eutrophic mesothermal (M), mixed eutrophic (P), clear-water diatom (J), stagnant-water planktonic (T), stagnant-water mixotrophic (Y), turbidity-tolerant (D), and mesotrophic (B). The average density and biomass were 6.87×10⁵ cells·L^-1 and 0.09 mg·L^-1, respectively. Phytoplankton biomass in the Suzhou water network exhibited significant spatiotemporal heterogeneity, being generally higher in the flood season than in the non-flood season. Y, D, T, P, and B were the core dominant groups across the entire region. The seasonal succession was driven by the dual effects of natural rhythms and artificial regulation. The flood season was dominated by the high-nutrient-tolerant groups M and Lo, while the non-flood season shifted toward T and B, which were adapted to mesothermal and stable habitats. The extreme heat event in 2022 significantly enhanced the year-round dominance of thermophilic groups Y and T. Results of structural equation modeling (SEM) indicated that phytoplankton biomass in lakes was primarily driven by water temperature and total phosphorus, whereas electrical conductivity exerted a more significant influence on rivers. This study elucidated the response mechanisms of phytoplankton functional groups to multiple environmental stressors in subtropical urban river networks, providing a theoretical basis for the ecological management of sluice-controlled river-lake systems.

Key words: phytoplankton, functional group, environmental factor, structural equation model