SEASONAL VARIABILITY OF METABOLICALLY ACTIVE BACTERIOPLANKTON IN THE EUPHOTIC ZONE OF A HYPERTROPHIC LAKE

We studied the seasonal variation in the abundance of metabolically ac tive bacteria (cells with an active electron transport system based on the INT-reduction method) and the proportion of active cells in the e uphotic zone of a hypertrophic lake with the objective of determining the main factors related to their dynamics. The annual average proport ion of active cells was similar to 47% and changed from similar to 17% in winter to 100% in autumn. Both total direct counts (range: 1.5 to 20.4 x 10(6) ml(-1)) and abundance of active cells (range: 0.4 to 6.0 x 10(6) ml(-1)) varied annually by 14-fold and were highly correlated (r = 0.74, p < 0.001). Bacterial activity, based on [C-14]leucine upta ke, 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 similar to 81% of the temporal variabil ity of active bacterial abundance but only 55% of total bacterial numb ers. Chlorophyll a concentration (range: 99 to 335 mu g l(-1)) was onl y weakly correlated to the abundance of active bacteria (r = 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 bacte ria was higher, their absolute abundance was lower than in less enrich ed systems.