A field study was carried out from summer to fall in 1988 to clarify the change in the dominant species of Microcystis in Lake Kasumigaura. Microcystis aeruginosa f. aeruginosa and M. aeruginosa f. flos-aquae were dominant in the summer, and M. viridis became dominant in the fall. A high correlation was found between the standing crop of M. aeruginosa and the water temperature, but the standing crop of M. viridis and M. wesenbergii showed no correlation with the water temperature.
Effects of temperature and illumination on the specific growth rates (μ) and the algal yield of M. viridis N-1 and M. viridis NIES 102 were studied with a batch culture. Maximum specific growth rates (μ_max) of M. viridis N-1 and M. viridis NIES 102 were observed at 25 and 30°C, respectively. M. viridis could grow at lower temperature than the other species of Microcystis. M. viridis N-1 could not grow under continuous illumination of 2, 000 lx but did so under periodic illumination. The specific growth rate of M. viridis was lower than for other Microcystis species.
These results suggested that M. viridis was more difficult to dominate than other Microcystis in summer.