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Excessive nitrogen (N) and phosphorus (P) loading of aquatic ecosystems is a leading cause of eutrophication and harmful algal blooms worldwide,and reducing nutrient levels in water has been a primary management objective.To provide a rational protection strategy and predict future trends of eutrophication in eutrophic lakes,we need to understand the relationships between nutrient ratios and nutrient limitations.We conducted a set of outdoor bioassays at the shore of Lake Taihu.It showed that N only additions induced phytoplankton growth but adding only P did not.Combined N plus P additions promoted higher phytoplankton biomass than N only additions,which suggested that both N and P were deficient for maximum phytoplankton growth in this lake (TN:TP =18.9).When nutrients are present at less than 7.75-13.95 mg/L TN and 0.41-0.74 mg/L TP,the deficiency of either N or P or both limits the growth of phytoplankton.N limitation then takes place when the TN:TP ratio is less than 21.5-24.7 (TDN:TDP was 34.2-44.3),and P limitation occurs above this.Therefore,according to this ratio,controlling N when N limitation exists and controlling P when P deficiency is present will prevent algal blooms effectively in the short term.But for the long term,a persistent dual nutrient (N and P) management strategy is necessary.