In the south basin of Lake Biwa, Phormidium tenue (Cyanophyceae) occurred in great portion sand the tap water had been damaged causing concern about the odor in early June of 1982, 1983, 1987, 1989, and 1990. The relationship between the abundance of P. tenue and water quality in the south basin and the weather condition of Lake Biwa were examined.
The changes of water quality in the south basin of Lake Biwa in early June of 1980-1993 showed that DIN (dissolved inorganic nitrogen) and DIN:DIP (dissolved inorganic phosphorus) ratio were comparatively low and TN (total nitrogen):DIN and TP (total phosphorus):DIP ratios were high in the years when P. tenue occurred abnormaly abundant.
The comparison of each average water quality among the dominant phytoplankton in early June of 1980-1993 showed that P. tenue occurred in the waters with low DIN and DIN:DIP ratio, and high WT (water temperature), TN:DIN and TP:DIP ratios. The species occurred in the years with the weather conditions of high air temperature and solar radiation, and low rain fall (precipitation) from 21 May to 10 June, before it become dominant. These features of P. tenue were remarkably contrac-tive as compared with those of Uroglena americana, with the exception of TP:DIP ratio.
