Bacterial numbers and substrate utilization as related to phytoplankton activity in West Blue Lake, Manitoba

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Brandt, Lester John
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Bacterial numbers in West Blue Lake, Manitoba, as determined by plate counting, showed fluctuations of a 14 day or longer duration which were apparently related to changes in chlorophyll a. Seasonal cycles were measured for both bacterial populations and chlorophyll a concentrations. These were found to be similar for some of the depths sampled, particularly the epilimnial depths. Chlorophyll a values in the epilimnion showed spring and mid-summer maxima, with some indication of an impending early fall peak. Bacterial plate counts from Cytophaga medium were out of phase relative to chlorophyll a, that is, their peaks, particularly the epilimnial midsummer peaks, succeeded chlorophyll a peaks. The most significant correlation as determined by multiple regression was the negative correlation of temperature with chlorophyll a. Times of maximum temperature change were usually coincidental with maximum carotenoid:chlorophyll a ratio changes. Although heterotrophic potential was being meeasured by the direct inoculation of both Cytophaga agar and succinate (basal salts solution) agar, comparative measurements were made on a variety of organic acids, glutamate and glucose by means of replica plating from Cytophaga medium. The proportion of Cytophaga agar counts able to use the various media varied with respect to time and substrate. But the mean proportion of each substrate was specific resulting in a relative sequential order as follows; glutamate (highest proportion), glucose, succinate, malate, fumarate, pyruvate, acetate, citrate, tartarate, glycolate and lactate. As substrates in West Blue Lake the latter two are probably insignificant. The proportions varied from 0% to 50% and higher. The calculated absolute numbers for the replica plating media correlated very highly with Cytophaga agar plate counts.