Relationships between the Dominant Occurrence of Uroglena americana and Water Quality and Meteorological Factors in the North Basin of Lake Biwa.

In the north basin of Lake Biwa, harmful dinoflagellate Uroglena americana occurred dominantly, causing bad-smelling tap water in late spring for several years. The relationships between the abundance of U. americana and water quality in the north basin and the meteorological conditions of Lake Biwa were examined. The changes of water quality in the north basin (Station A) of Lake Biwa in early May of 1980-1993 showed that the fluctuation of WT (water temperature), DIP (dissolved inorganic phosphorus), TN: TP (total nitrogen: total phosphorus), DIN:DIP (dissolved inorganic nitrogen: dissolved inorganic phosphorus), and TP:DIP ratios were remarkable. The comparison of each average water quality with regard to the dominant phytoplankton in early May of 1980-1993 showed that U. americana occurred in the waters with water qualities near the mean values, excepting comparatively high WT. The species also occurred in the years with meteorological conditions from mid April to early May close to the mean. These features of water quality and meteorological conditions suggested that the alga was a common species in early May and the control of the dominant occurrences resulted from abnormal water quality and meteorological conditions.