The relative abundance of mixotrophic and heterotrophic ciliates in an oligotrophic lake

Between July and December 1998, mixotrophic ciliates ranged in concentration from 751 to 33,600 cells/L in a small oligotrophic lake in northeast PA. Mixotrophic ciliates, primarily oligotrichs, comprised on average 58% of the total ciliate fauna and 88% of the total ciliate biovolume during summer and fall. Stombidium viride appeared to be the dominant mixotroph. Average ciliate abundance did not change substantially during this time period. However in December, after lake turnover, only 5% of all ciliates were mixotrophic, contributing 50% of the total ciliate biovolume. The cell density of mixotrophic ciliates was not significantly correlated with Chl-a concentration. Grazing experiments conducted in August and October, using fluorescently labeled beads (0.5 μm diameter) indicated that mixotrophic ciliates removed 30.6 and 17.4 bacteria ciliate^-1 h^-1 and heterotrophic ciliates removed bacteria at a rate of 18.2 and 3.0 bacteria ciliate^-1 h^-1. Overall, the ciliates removed less than 1% of the bacterial standing stock per hour.