In this study, effects of phosphorus concentration on aquatic microcosms were evaluated in terms of microbial population dynamics and the photosynthetic activity that is the starting point of the material flux. Microcosms that consist of algae Chlorella vulgaris, bacteria Pseudomonas putida and ciliate Cyclidium glaucoma were incubated using media that contain various concentrations of KH2PO4. At 14-16 days after the incubation started, sodium [14C] bicarbonate was added to each microcosm, and then microbial uptake of radioactive substrate was monitored. The population density of C. uulgaris and C. glaucoma increased, as initial phosphorus concentration was higher, while that of P.putida was constant. The photosynthetic activity of C. vulgaris also increased, as initial phosphorus concentration becomes higher. In the microcosm, P. putida uses metabolites produced by C. vulgaris and is preyed by C. glaucoma. Therefore, the growth activity of P.putida increased as the photosynthetic activity of C. vulgaris increased, but predation pressure by C. glaucoma also increased. Consequently, the population of P. putida kept constant. These findings lead to the conclusion that phosphorus concentration strongly affects the microbial population dynamics and material flux in the microbial loop in the aquatic ecosystem. However, the mass balance of each organism should keep steady state in the environment where the population of predator such as protozoa, invertebrates, fish and so on was kept suitable.