Abstract:In order to understand the response of phytoplankton community in P-restricted water from Liuxihe Reservoir in Guangdong Province to different N/P ratios, the structure of phytoplankton community was studied by adding N and P to create a gradient of N/P. The results showed that the growth of phytoplankton had been promoted significantly with the addition of N and P, and the effects of P was greater than that of N. The species specificity of phytoplankton led to the inconsistent response of phytoplankton community to nitrogen and phosphorus nutrients, phytoplankton abundance was not correlated with N/P ratio. Cyanobacteria, such as Aphanocapsa sp., Cylindrospermopsis raciborskii and Pseudanabaena sp., was more favored in conditions with high level of N and P. Dominant species of green algae, Scenedesmus bijuga preferred medium N and high P. Bacillariophyceae, such as Achnanthes sp. and Cyclotella sp. had advantages in environment with low N and low P. The threshold of P concentration for alkaline phosphatase activity (APA) ranged from 0.8 to 2.0 μmol/L. When P concentration was above 2.0 μmol/L, APA was inhibited, and the optimal P concentration for phytoplankton to maintain growth may be around 2.0 μmol/L, and the ratio of cellular N/P reached equilibrium. The cellular N/P and C/P were significantly positively correlated with supplied P concentration and supplied N/P ratio, while cellular C/N was more correlated with supplied N (P<0.05). So, these would provide a theoretical reference for water quality management in tropical and subtropical reservoirs.