Abstract
The dependence of Microcystis aeruginosa growth on nitrogen and phosphorus concentration and nitrogen/phosphorus mass ratio （N/P ratio） in the medium, which could be regarded as the initial total nitrogen/phosphorus mass ratio （TN/TP ratio） under the existence of Cyclotella sp. was examined through monoculture and monoxenic culture experiment. Furthermore, the relationship between M. aeruginosa dominance and low TN/TP ratio in eutrophic lake was discussed based on the results obtained in this study and reference data. The monoculture as well as monoxenic culture experiment under high nitrogen and phosphorus concentration showed that the growth of M. aeruginosa was affected by nitrogen－deficient condition, and depended on N/P ratio rather than nitrogen and phosphorus concentration. On the other hand, in the monoxenic condition under low nitrogen and phosphorus concentration, the growth depended on both N/P ratio and phosphorus concentration. As the possibility of generally accepted fact that the low N/P ratio （TN/TP < 29） favors the appearance of water bloom, we suggested that low N/P ratio might be a result of water bloom, which would reflect the cellular N/P ratio of water bloom followed by assimilation of nitrogen and phosphorus in lake water.
