We studied the seasonal variation in the abundance of metabolically active bacteria (cells with an active electron transport system based on the INT-reduction method) and the proportion of active cells in the euphotic zone of a hypertrophic lake with the oblective of determining the main factors related to their dynamics.The annual average proportion of active cells was -47 % and changed from -17 % in winter to 100% in autumn.Both total direct counts (range: 1.5 to 20.4 X 10' m]-') and abundance of active cells (range: 0 4 to 6.0 X 10' ml-l) varied annually by 14-fold and were highly correlated (r = 0.74, p < 0.001).Bacterial activity, based on ['"C]-leucine uptake, was more strongly correlated with active bacterial abundance (r = 0.83.p < 0.001) than with total bacterial counts (r = 0.70, p < 0.001).Water temperature explained -81 "o of the temporal variability of active bacterial abundance but only 55% of total bacterial numbers.Chlorophyll a concentration (range.99 to 335 pg 1-l) was only weakly correlated to the abundance of active bacteria ( I = 0.34, p < 0.05).Compared with data from other freshwater systems, our results show that although in this lake the average proportion of active bacteria was higher, their absolute abundance was lower than in less enriched systems.
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Sommaruga et al. (1997) studied this question.