Abstract:The effects of PO3-4-P coexisting with NO-3-N and NH+4-N on the proliferation and toxin production of Microcystis aeruginosa were studied by orthogonal experiments. The results showed that under the coexistence of PO3-4-P and NO-3-N, when ρ(PO3-4-P)≤ 0. 10 mg/L, the growth of algae was restricted, and the growth of algae was not promoted significantly by the increase of ρ(NO-3-N ). When the phosphorus nutrition was suitable, the excessive growth of algae could be effectively controlled when ρ(NO-3-N )≤ 5. 0 mg/L. Under the coexistence of PO3-4-P and NH+4-N, only when ρ(PO3-4-P)≤0. 05 mg/L could the promotion of NH+4-N on algal cell growth be limited. When the phosphorus nutrition was suitable, the excessive growth of algae could be effectively controlled only when ρ(NH+4-N)≤1. 0 mg/L. MC-LR is the main algal toxin produced by Microcystis aeruginosa. Under the cultivating condition of NO-3-N, when ρ(NO-3-N )> 10. 0 mg/L, the effect of ρ(PO3-4-P)on the toxicity was significantly promoted, the greater the toxicity was. When ρ(NO-3-N )<10. 0 mg/L, the effect of PO3-4-P was not obvious. Under the cultivating condition of NH+4-N, the toxicity of all concentrations of PO3-4-P reached the maximum at ρ(NH+4-N)= 10. 0 mg/L. NH+4-N is a priority nitrogen form in the process of eutrophication control. Excessive ρ(NO-3-N )(>10. 0 mg/L)and ρ(NH+4-N)(>5. 0 mg/L)will greatly stimulate the synthesis of algae toxins.