This study investigates the control of phytoplankton growth in Lake Taihu, China, by nitrogen (N) and phosphorus (P) inputs. Field sampling and in situ nutrient enrichment bioassays were conducted to determine seasonal patterns of nutrient limitation and nutrient thresholds for phytoplankton growth. The results show that the TN:TP and TDN:TDP mass ratios in the ambient water exhibit high seasonal variation, with ratios ranging from 33–80:1 and 52–212:1 in winter and spring, respectively, and declining to below 20:1 in summer. In spring and winter, total phytoplankton biomass and growth rates increased significantly with P additions, suggesting P limitation. During the summer and fall bloom periods, N additions alone revealed a significant positive effect on phytoplankton growth, while P additions only stimulated growth once N had been added, indicating that N is the primary limiting nutrient, with P being a secondary limiting nutrient. When P enrichment was ≥ 0.20 mg P L−1 and N enrichment ≥ 0.80 mg N L−1, growth of the toxin-producing cyanobacteria Microcystis spp. was not nutrient limited. This study suggests that N availability during the summer is a key factor in the proliferation and maintenance of toxic Microcystis spp. blooms. Therefore, while P load reduction is important, N load reduction is essential for controlling the magnitude and duration of algal blooms in Taihu.This study investigates the control of phytoplankton growth in Lake Taihu, China, by nitrogen (N) and phosphorus (P) inputs. Field sampling and in situ nutrient enrichment bioassays were conducted to determine seasonal patterns of nutrient limitation and nutrient thresholds for phytoplankton growth. The results show that the TN:TP and TDN:TDP mass ratios in the ambient water exhibit high seasonal variation, with ratios ranging from 33–80:1 and 52–212:1 in winter and spring, respectively, and declining to below 20:1 in summer. In spring and winter, total phytoplankton biomass and growth rates increased significantly with P additions, suggesting P limitation. During the summer and fall bloom periods, N additions alone revealed a significant positive effect on phytoplankton growth, while P additions only stimulated growth once N had been added, indicating that N is the primary limiting nutrient, with P being a secondary limiting nutrient. When P enrichment was ≥ 0.20 mg P L−1 and N enrichment ≥ 0.80 mg N L−1, growth of the toxin-producing cyanobacteria Microcystis spp. was not nutrient limited. This study suggests that N availability during the summer is a key factor in the proliferation and maintenance of toxic Microcystis spp. blooms. Therefore, while P load reduction is important, N load reduction is essential for controlling the magnitude and duration of algal blooms in Taihu.