Effects of N:P Ratio and Temperature on Growth and Competition of Microcystis aeruginosa and Cyclotella sp. in Eutrophic Condition

The pure and mixed culture experiments of blue-green alga Microcystis aeruginosa and diatom Cyclotella sp. were carried out under N:P ratios of 1, 10, 50 and 100 and temperatures of 15, 20 and 25 ℃ to determine their growth characteristics and competition abilities. In the pure culture experiment, the N:P ratio did not affect the growth rate of both species at the same temperature. In the mixed culture experiment, the higher temperature of 25 ℃ would favor the dominance of M. aeruginosa , whereas Cyclotella sp. would become a dominant species at lower temperature of 15 ℃ without any influences of N:P ratio in the competitive circumstance. The dominance of these species at the intermediate temperature around 20 ℃ was strongly affected by N:P ratio. Although generally accepted N:P ratio rule suggested that M. aeruginosa favored low N:P ratio, our results clearly indicated that M. aeruginosa could grow and outcompete Cyclotella sp. at higher N:P ratios ranged from 10 to 100 rather than lower N:P ratio of 1. Moreover, M. aeruginosa could not become dominant species under the N:P ratio of 1 at 20 ℃ , suggesting that the dominance of M. aeruginosa could be inhibited by N-limited condition rather than N:P ratio.