Growth Responses of Harmful Algal Species Microcystis (Cyanophyceae) under Various Environmental Conditions

Microcystis is a flagrant cyanobacterial genus forming heavy blooms in aquatic environments and often cause serious problems in the management of water quality. In Japan, Micorcystis aeruginosa and Microcytis wesenbergii are commonly found in natural eutrophic water bodies. Although they frequently occur in the same habitat, the mechanisms determining the dominance of the two Microcystis species are not yet clearly understood. To clarify the difference of growth characteristics between M. aeruginosa and M. wesenbergii, we examined their growth rates, using their isolated strains, under various environmental conditions such as water temperature, light intensity and nutrient concentrations. We also examined the effect of grazing on their growth. Growth rate of M. aeruginosa was significantly higher than that of M. wesenbergii under high temperatures (30 and 35°C). However, growth rates of the two species were similar at lower temperatures (20 and 25°C). No significant difference in growth rates between the two species was found under varying light intensities, nutrient concentrations or the presence of grazers. We suggest that, among the environmental factors examined in the present study, water temperature is the most important factor determining the dominance of the two Microcystis species.