Seasonal phytoplankton dynamics in a river–lake continuum

Hydrological exchange and seasonal transitions can reorganize resource supply, light conditions, and water transport in river–lake systems, yet their joint effects on phytoplankton remain poorly resolved. We investigated the inflowing river reach (HP-IR)–main-lake area (SJH)–outflow river reach (HP-OR) continuum of Shengjin Lake at 32 fixed sites in summer, autumn, and winter 2025 and spring 2026, yielding 110 samples, 11 environmental variables, and 216 phytoplankton taxa. Excluding chlorophyll a, 10 exogenous environmental variables characterized environmental gradients and heterogeneity. Multivariate analyses and random forests resolved compositional and quantitative responses. The first three PCA axes explained 73.9% of environmental variation, and environmental heterogeneity showed strong hydrological-unit × season dependence. Cyanobacteria and Bacillariophyta accounted for 62.8% and 23.8% of total cell density; Bacillariophyta dominated in spring (71.96%), whereas Cyanobacteria reached 80.39% in summer, when cell density peaked at 5.32 × 10⁷ cells L⁻¹. Hydrological unit, season, and their interaction explained 6.80%, 8.96%, and 9.63% of community variation, respectively (P < 0.001). After conditioning on season, the 10 environmental variables explained 14.58% of community variation (adjusted R² = 9.27%), with SD, WD, TN, WT, TAN, and TP retained by forward selection. Random forests identified WT as the strongest predictor of total cell density, followed by TAN, SD, and pH, with clear nonlinear responses. Overall, seasonal thermal conditions set the temporal context for succession, while water depth, light availability, and nutrient status jointly modulated spatial differentiation along the hydrological continuum.