Relationship between the dominant occurrence of Microcystis aeruginosa and water quality or meteorological factors in the south basin of Lake Biwa

In the south basin of Lake Biwa, Microcystis aeruginosa dominantly occurred and the water blooms were formed at the limit area in late summer of 1983, 1987, 1988, 1990, and 1994. The relationship between the dominant occurrence of M. aeruginosa and water quality and the meteorological condition in the south basin of Lake Biwa was examined, referring to data from Shiga Prefecture and the meteorological Agency. The changes of water quality in the south basin in early September of 1980-1994 showed a remarkable fluctuation of TP (total phosphorus), DIN (dissolved inorganic nitrogen), DIP (dissolved inorganic phosphorus), DIN:DIP, TN (total nitrogen):DIN, and TP:DIP ratios, and M. aeruginosa usually occurred in the water with comparatively low DIN:DIP ratio. The comparison of each average water quality with the dominant species of phytoplanktons in early September of 1980-1994 showed that M. aeruginosa dominantly occurred in the waters with low DIN, DIP, and DIN:DIP ratio and high TN:DIN and TP:DIP ratios. The species occurred in the years with the meteorological conditions of high air temperature, long sunshine time, and comparatively low rainfall (precipitation) during August. These features of M. aeruginosa were contrasted remarkably with those of Rhodomonas sp.