In the present study,Phyto-PAM was applied to measure maximal photochemical quantum yield of PSII (Fv/Fm), actual photochemical efficiency of PSII (Φ_PSII),and rapid light response curve (RLC) of phytoplankton samples collected from Lake Gehu and Lake Luoma in spring.Meanwhile,the cell numbers of phytoplankton were counted by microscopy.The results showed that values of Fv/Fm and Φ_PSII of cyanobacteria in Lake Gehu varied from 0.36 to 0.52 and from 0.27 to 0.53,and Bacillariophyta, from 0.25 to 0.37 and from 0.25 to 0.36,respectively,but there was no photochemical vitality of Chlorophyta.RLC has indicated that cyanobacteria has higher photochemical vitality and are prone to form advantage populations in Lake Gehu.On the other hand,the values of Fv/Fm and Φ_PSII of Chlorophyta and Bacillariophyta in Lake Luoma ranged from 0.45 to 0.65, from 0.41 to 0.49,from 0.25 to 0.32,and from 0.19 to 0.25,respectively,and no photochemical vitality were found for cyanobacteria.The density of phytoplankton in Lake Gehu ranged from 530×10⁴ to 4200×10⁴ cells/L,with an average of 2918×10⁴ cells/L.Cyanophyta,Bacillariophyta,Cryptophyta and Euglenophyta were dominant in the phytoplankton community.The density of phytoplankton in Lake Luoma varied from 260×10⁴ to 510×10⁴ cells/L,with an average of 379×10⁴ cells/L,Bacillaryophyta and Chlorophyta were dominant in the phytoplankton community.According to the comparison between nutritional level,the abundance and photochemical vitality of phytoplankton assemblages in the two lakes,the lake eutrophication can increase the abundance of phytoplankton and photosynthetic activity of cyanobacteria,and become in favor of gaining advantages in phytoplankton assemblages.