Bacterioplankton in Lake Biwa: Annual fluctuations of bacterial numbers and their possible relationship with environmental variables

Annual fluctuations of bacterial numbers were surveyed at four stations in Lake Biwa between 1981-1982. The total count obtained by acridine orange direct microscopic count method changed in the order of 105-106cells per ml at St. Ie (sited in the pelagic area of the north basin), and 106-107cells per ml at three stations in the south basin. The percentage of the viable count to the total count (V/T ratio) ranged from 0.009 to 1.81%, and the ratio tended to be higher in more eutrophic water. Generally, at St. Ie, the vertical profiles of both total and viable counts clearly correlated with those of chlorophyll-a, pH and dissolved O2 (% saturation); however, hypolimnetic O2 depletion had no effect on bacterial numbers. Rank correlation analysis for the samples of epilimnion also showed significant correlations (p