Risk Assessment of Algal Blooms in the Qiantang River Basin, Zhejiang Province, China: Integrating Biomass, Community Dynamics, and Bloom-Associated Functional Traits

Understanding mechanisms governing phytoplankton succession under nutrient fluctuations is essential for algal bloom prevention. Using the Qiantang River Basin as a model system encompassing mesotrophic to mid-eutrophic conditions, we investigated phytoplankton assemblages shifts, bloom-forming traits, and main regulatory mechanisms behind nutrient-driven phytoplankton dynamics. A total of 268 phytoplankton were identified, with abundance and biomass ranging from 2.71 × 10⁴ to 9.42 × 10⁶ ind.·L⁻¹ and 0.025 to 4.49 mg·L⁻¹, respectively. Phytoplankton diversity and biomass increased with nutrient levels, and eutrophication caused taxonomic shifts: Cyanophyta abundance increased significantly, while Bacillariophyta declined. Network analysis revealed that nutrient-rich waters supported stronger phytoplankton–zooplankton interactions, highlighting a zooplankton-mediated top-down regulatory mechanism that could constrain phytoplankton biomass accumulation through intensified grazing pressure. Specific nutrient forms (e.g., ammonium (NH₄–N), nitrite (NO₂–N), phosphate (PO₄–P), and dissolved total phosphorus (DTP)) were primary environmental drivers shaping phytoplankton composition, accounting for 56% of community variation, while zooplankton contributed 25.6%. Mid-eutrophic sites supported more bloom-forming taxa with motility (37.1%–73.6%), toxicity (11.4%–43.0%), and nitrogen fixation (7.8%–24.6%) traits. Mechanistically, bottom-up nutrient supply and top-down zooplankton grazing were both significant regulators of phytoplankton dynamics. These results emphasize the importance of nutrient species, ecological feedbacks, and functional trait dynamics in modulating algal bloom risks under eutrophication.