To test the growth characteristics of Microcystis aeruginosa Kutz (M.) and Scenedesmus obliquus (Turp.) Kutz (S.) under non-steady-state conditions, a culturing experiment was performed using the "pulse" adding method. The culture solutions were matched according to different mass ratios of nitrogen and phosphorus (35 ∶ 1, 70 ∶ 1 and 15 ∶ 1). The two kinds of cells were also separately cultured with a ratio of nitrogen to phosphorus of 35 ∶ 1, demonstrating the similar ratio of growth in the same culture solution with sufficient nutrients. However, different ratios of growth were observed under conditions of limited nutrients. From these findings, a ratio of 35 ∶ 1 was considered to be optimal. The experimental results showed that M. was almost always the predominant algae when the frequency of addition was once per day during the whole culturing process, regardless of the availability of nutrients. M. also gained a competitive advantage as the limited time\\average concentration of nitrogen varied from 0.3 to 2.4 mg/L. S. was only dominant at the initial growth stage as the limited phosphorus concentration varied from 0.018 to 0.035 mg/L, with M. finally becoming dominant. When nitrogen and phosphorus were both limited at a ratio of 35:1, the symbiosis of these alga was observed under two different frequencies of nutrient addition, i.e., once per day and once every eight days. However, when cultured at the former frequency, symbiosis occurred and both algae showed similar growth characteristics. When the nitrogen:phosphorus ratio departed from the optimum value, symbiosis did not occur at any of the frequencies of addition. M. was the predominant algae when the time-average concentration of nitrogen ranged from 0.6 to 4.8 mg/L, and S. became predominant when the concentration varied from 0.15 to 0.3 mg/L. Images of the cells were recorded and used to analyze variations in size. The results showed that the cell size of the two algae changed with the frequency of addition. The cell size of S. increased as the nutrient concentration decreased when simple nutrients were limited, and larger changes in cell size were observed when phosphorus was limited. Similarly, S. cells grew larger when two nutrients were limited than when simple nutrients were limited, with the largest change in size being from 26.33 μm² to 57.60 μm². However, the size of M. cells was not changed as significantly as S. cells, with differences of no more than 3 μm² being observed whatever the nutrient availability. Analysis of the experimental results indicated that competition between the two algae was related to the frequency of addition of nutrients. According to the theory of gleaner-opportunist trade-off, M. showed the characteristics of an opportunist and S. was more likely to be a gleaner. The symbiosis of the two algae coincided with the intermediate disturbance hypothesis. The two algae were able to change their cell size to strive for more nutrients. The gleaner characteristic of S. was also confirmed by cell size changes at low concentration.