• • Recorded 113 phytoplankton species across 7 phyla and 57 genera; density peaked in summer, with cyanobacterial dominance in summer-autumn indicating elevated bloom risk.
• • Water temperature exerted a total positive effect on biomass (β=0.96), decomposed into direct (β=0.94, P<0.05) and indirect via abundance (β=0.02), highlighting temperature as a primary driver.
• • Total nitrogen had a negative total effect on biomass (β=-0.98), with direct (β=-0.97, P<0.05) and indirect via abundance (β=-0.01) components, plus a direct negative effect on abundance (β=-0.15, P<0.05), underscoring nitrogen's suppressive role.
• • Dissolved oxygen and TN both negatively affected biomass, while total phosphorus and pH showed complex pathways with positive total effects but negative indirect effects via reduced abundance, informing targeted eutrophication management.