PROTISTAN BACTERIVORY IN AN OLIGOMESOTROPHIC LAKE - IMPORTANCE OF ATTACHED CILIATES AND FLAGELLATES

Seasonal and depth variations of the abundance, biomass, and bacterivo ry of protozoa (heterotrophic and mixotrophic flagellates and ciliates ) were determined during thermal stratification in an oligomesotrophic lake (Lake Pavin, France). Maximal densities of heterotrophic flagell ates (1.9 x 10(3) cells ml(-1)) and ciliates (6.1 cells ml(-1)) were f ound in the metalimnion. Pigmented flagellates dominated the flagellat e biomass in the euphotic zone. Community composition of ciliated prot ists varied greatly with depth, and both the abundance and biomass of ciliates was dominated by oligotrichs. Heterotrophic flagellates domin ated grazing, accounting for 84% of total protistan bacterivory. Maxim al grazing impact of heterotrophic flagellates was 18.9 x 10(6) bacter ia 1(-1) h(-1). On average, 62% of nonpigmented flagellates were found to ingest particles. Ciliates and mixotrophic flagellates averaged 13 % and 3% of protistan bacterivory, respectively. Attached protozoa (ci liates and flagellates) were found to colonize the diatom Asterionella formosa. Attached bacterivores had higher ingestion rates than free b acterivorous protozoa and may account for 65% of total protozoa bacter ivory. Our results indicated that even in low numbers, epibiotic proto zoa may have a major grazing impact on free bacteria.