Abstract: Species sensitivity distributions (SSDs) methods in both forward and reverse modes were used to evaluate the ecological risk and determine the contaminant concentration threshold for the protection of aquatic species and ecological quality. In this study, the existing toxicity data of freshwater organisms were fitted to SSD functions to estimate the hazardous concentrations for 5% of the species (HC₅) for microcystins, ammonia and nitrite, and the ecological risk of their mixtures. The potentially affected fractions (PAFs) of various concentrations of microcystins, ammonia and nitrite were also calculated. Results showed that microcystins exhibited a higher ecological risk than ammonia and nitrite. The HC₅ value for microcystins exposure was 19.22 μg·L^-1 whereas the HC₅ values for ammonia and nitrite exposure were 6583.94 μg·L^-1 and 334.33 μg·L^-1, respectively. The sensitivity of freshwater organisms varied with exposed concentrations of microcystins, ammonia and nitrite. Crustaceans were more sensitive than fishes to microcystins, and less sensitive than fishes to nitrite when the concentrations of microcystins and nitrite were below 125.04 μg·L^-1 and 2989.40 μg·L^-1, respectively, and vice versa when exposed to higher concentrations of microcystins and nitrite. No significant difference was observed for the sensitivities of fishes and crustaceans exposed to ammonia. In studies with selected lakes in China, our results showed that the ecological risk in both Tai and Hongfeng lakes exceeded the permissible HC₅ threshold, and the multiple substance potentially affected fractions (msPAFs) of microcystins, ammonia and nitrite were 2.6%-5.6%, indicating that the ecological risk of their mixtures was more threatening than each individual contaminant being investigated.