1Ibaraki Kasumigaura Environmental Science Center, Tsuchiura, Ibaraki 300-0023, 2Department of Environmental Sciences, Faculty of Science, Shinshu University, Matsumoto, Nagano 390-8621, Japan
Influences of nitrogen and phosphorus concentrations on Microcystis morphospecies were confirmed from the results of an investigation of Lakes Nishiura and Kitaura between July and September 2005. During the exponential growth phase, the growth of the Microcystis population was limited by phosphorus (DIN: DIP>7) and M. aeruginosa was dominated in Nishiura. In Kitaura, the limiting factor for Microcystis growth was nitrogen (DIN: DIP<7) and four morphospecies of Microcystis, M. aeruginosa, M. ichthyoblabe, M. viridis and M. wesenbergii were observed. These results suggested that Microcystis morphospecies compositions were affected by nutrient concentrations in lake water.