Lake Taihu (Taihu) is the third largest freshwater lake in China and an important drinking water, fishing, and tourism resource for Jiangsu Province. Recent toxic cyanobacterial blooms caused by excessive human nutrient loading have focused attention on arresting blooms and restoring the lake to acceptable water quality conditions by reducing nutrient 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 TN: TP and TDN:TDP mass ratios in the ambient water showed high seasonal variation and changed from 33‐80 : 1 and 52‐212 : 1, respectively, in winter and spring, and both declined to below 20 : 1 in summer. In spring and winter, total phytoplankton biomass and growth rates increased significantly with additions of P, with no primary effects from N, suggesting P limitation of phytoplankton growth. During the summer and fall bloom periods, however, N additions alone revealed a significant positive effect on phytoplankton growth, and P additions only stimulated phytoplankton growth once N had been added, suggesting that N was the primary limiting nutrient, with P being a secondarily 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, dominant bloom‐forming cyanobacteria Microcystis spp. was not nutrient limited. This study suggests that availability of N during the summer is a key growth‐limiting factor for the proliferation and maintenance of toxic Microcystis spp. blooms. Therefore, although P load reduction is important, N load reduction is essential for controlling the magnitude and duration of algal booms in Taihu.

China is a country with many lakes, about one-third of which are freshwater mainly distributed in the middle and lower reaches of the Yangtze River. Currently most of the lakes are mesotrophic or eutrophic. Lake eutrophication has become one of the major ecological and environmental problems faced by lakes in China and can lead to a series of abnormal ecosystem responses, including extinction of submerged plants, frequent occurrence of cyanobacterial blooms, increased microbial biomass and productivity, decreased biodiversity, accelerated cycles, and a change in the efficient use of nutrients. With development of eutrophication, the whole lake ecosystem suffers decreased biodiversity, simplification of biotic community structure, instability of the ecosystem, and ultimately the clear-water, macrophyte-dominated ecosystem gradually shifts to a turbid-water, algae-dominated ecosystem. This ecosystem succession mechanism is speculated to be caused by different nutrient utilization efficiencies of macrophytes and phytoplankton. The ultimate ecosystem succession trend of seriously eutrophic lakes is that a phytoplankton-dominated autotrophic lake shifts to a heterotrophic lake dominated by micro-organisms, protozoans.