This study investigates the role of nitrogen (N) and phosphorus (P) in controlling phytoplankton growth in eutrophic Lake Taihu, China. The research shows that nutrient limitation varies seasonally, with phosphorus (P) being the primary limiting nutrient in spring and winter, while nitrogen (N) becomes the main limiting nutrient during summer and fall. The study conducted field sampling and in situ nutrient enrichment bioassays to determine seasonal patterns of nutrient limitation and thresholds for phytoplankton growth. The results indicate that during spring and winter, phytoplankton growth is primarily limited by P, while during summer and fall, N becomes the main limiting factor. When both N and P are added, the growth of the toxin-producing cyanobacteria Microcystis spp. is not nutrient limited. The study highlights that the availability of N during summer is a key factor in the proliferation and maintenance of toxic Microcystis blooms. Although reducing P load is important, reducing N load is essential for controlling the magnitude and duration of algal blooms in Taihu. The study also emphasizes the need for long-term nutrient management strategies that include reductions in both N and P inputs to mitigate eutrophication and harmful algal blooms in the lake.This study investigates the role of nitrogen (N) and phosphorus (P) in controlling phytoplankton growth in eutrophic Lake Taihu, China. The research shows that nutrient limitation varies seasonally, with phosphorus (P) being the primary limiting nutrient in spring and winter, while nitrogen (N) becomes the main limiting nutrient during summer and fall. The study conducted field sampling and in situ nutrient enrichment bioassays to determine seasonal patterns of nutrient limitation and thresholds for phytoplankton growth. The results indicate that during spring and winter, phytoplankton growth is primarily limited by P, while during summer and fall, N becomes the main limiting factor. When both N and P are added, the growth of the toxin-producing cyanobacteria Microcystis spp. is not nutrient limited. The study highlights that the availability of N during summer is a key factor in the proliferation and maintenance of toxic Microcystis blooms. Although reducing P load is important, reducing N load is essential for controlling the magnitude and duration of algal blooms in Taihu. The study also emphasizes the need for long-term nutrient management strategies that include reductions in both N and P inputs to mitigate eutrophication and harmful algal blooms in the lake.