Responses and drivers of phytoplankton to the anthropogenic water-sediment regulation in the Yellow River.

The operation of the Xiaolangdi Reservoir's intensive water-sediment regulation imposes a significant, pulsed anthropogenic disturbance on the downstream Yellow River, substantially altering its hydrodynamic and sedimentary habitats. These alterations profoundly impact phytoplankton communities, key bioindicators of riverine ecosystem health, yet the mechanisms and ecological thresholds governing their response remain unclear. Through synchronous monitoring across three distinct regulation phases in 2024, we identified 226 phytoplankton species from 7 phyla in the mainstream and tributaries. By integrating Random Forest modeling with SHAP analysis, we analyzed the response patterns of phytoplankton community structure and stability dynamics. The results demonstrated that: (1) Phytoplankton community composition differed significantly among phases (P = 0.001), with clear-water pulse phase favoring Chlorophyta (dominating with 53.56% and 73.09% of total biomass in the mainstream and tributaries, respectively), and high-sediment flow phase promoting the recovery of Bacillariophyta and Cyanophyta, indicating a structural change under light-limited conditions. (2) Distinct response patterns emerged between the mainstream and tributary ecosystems. The mainstream exhibited lower diversity, simplified structure, and reduced stability and resilience, whereas the tributaries maintained richer species diversity and higher biodiversity, demonstrating stronger recovery capacity. (3) Machine learning models identified key environmental drivers and quantified their ecological thresholds, with NO3--N exhibiting inhibitory effects above 1.06 ± 0.05 mg/L, and TSS suppressed phytoplankton beyond 131 ± 9 mg/L. These findings identify key environmental drivers and ecological thresholds associated with phytoplankton responses to anthropogenic hydraulic disturbance, and provide actionable guidance for designing ecologically sustainable water-sediment regulation strategies to mitigate adverse effects on riverine biodiversity.