Abstract
Lake Biwa in Shiga Prefecture is, as is known, of an oligotrophic condition. The results of limnological observations made by the Shiga Pref. Fisheries Exp. Station for forty years since 1931 have shown that the amounts of oxygen dissolved in the deep water before the season of whole circulation have been changing as follows : 6.64cc/l in 1931, 5.26cc/l on the average from 1950-1959, 4.39cc/l on the average for nine years since 1931, and in 1969 the amount became the smallest value of 3.25cc/l. Such results suggest that there is a tendency to a progressed eutrophication of Lake Biwa during the past forty years.
