Benthic flagellates and ciliates in fine freshwater sediments: Calibration of a live counting procedure and estimation of their abundances

Despite the recognized importance of protozoans (flagellates and ciliates) as predators of bacteria, there are very few estimates of their abundance in fine sediments of freshwater lakes. This is due, in part, to the lack of a standard methodology. Because of the low concentration of protists in relation to particles, epifluorescence counts can not always be used. Instead, dilution followed by live counting was used to solve the masking by sediment particles. One to twenty 1 sample aliquots were diluted with filtered lake water in a Palmer-Maloney counting slide. Four to eight replicates were sufficient to minimize the counting error, while minimizing effort. The method is highly replicable and could potentially be calibrated for different sediment types because sediment masking depends on the mean particle size of the sediment. When this method was applied in a survey of benthic sites in Quebec lakes, flagellate abundances were found to range from 100 to 180,000 cells ml^–1, while ciliate numbers ranged from 26 to 11,000 cells ml^–1. Bacteria are 105 to 10⁷ times more abundant than protists and, thus, the impact of these protists on sediment bacterial dynamics is likely to be minimal.