The horizontal distribution of chlorophyll a concentration was surveyed in the south basin of Lake Biwa during April 1978 to March 1979. Three types of distribution patterns were observed; (1) nearly uniform distribution, and (2) uneven distribution with higher chlorophyll a concentration along the east side and (3) higher concentrations along both the east and west sides. Pattern (2) with the east (high) -west (low) gradient was observed frequently and seemed to be typical in the south basin. Change in the patterns was mainly due to a great variation of chlorophyll a on the east side during the investigation period. As a whole, the horizontal distribution patterns depended mainly on nannoplankton contribution to total phytoplankton except in August -October, when netplankton were relatively abundant.
Seasonal changes in chlorophyll a and dissolved inorganic nitrogen (DIN) concentrations were compared among stations along the west-offshore-east sides or among offshore stations from the north to the south. Chlorophyll aconcentration tended to change seasonally with three peaks, in which dominant algal species differed among stations. Chlorophyll a and DIN concentrations were extraordinarily higher and fluctuated much more at the east stations than at the west and offshore stations.