Planktonic Ciliates: Their Distribution, Abundance, and Relationship to Microbial Resources in a Monomictic Lake

Abstract

The abundance, biomass, vertical distribution, and seasonal succession of planktonic ciliates were studied over a 13-mo period in a small monomictic lake. Measurements made concurrently of bacterial abundance (attached and free), chlorophyll a, and pheopigments were analyzed for correlations with the biomass of the major protozoan species and total protozoan biomass. Ciliate community structure varied primarily between periods of winter mixis and summer stratification. Oligotrich, tintinnid, and peritrich ciliates dominated the community during mixis (November to March). Vertical distributions tended to be uniform except for some species containing algal symbionts which occurred primarily in the photic zone. A more diverse community occurred during stratification. At this time species either tended to be restricted to the epilimnion or had sharply defined metalimnetic or hypolimnetic maxima. With the development of anoxia in the hypolimnion (June), bacterivorous scuticociliates reached high densities in the metalimnion and upper hypolimnion and dominated the community in terms of density and to a lesser extent biomass. Total protozoan (ciliates and the amoeba Difflugia) density varied over the annual cycle between 1 and 50 × 10⁷∙m⁻² (0.1 to 189 × 10³∙L⁻¹) while biomass as measured by volume varied between 6 and 60 mm³∙m⁻² (61–610 mg dry wt∙m⁻² and 1.57–1260 μm³∙L⁻¹). Highest densities occurred from August through October when scuticociliates were abundant, but peak biomass occurred in November with a bloom of the large peritrich Rhabdostyla sp. Total ciliate biomass was correlated only with the concentration of pheopigments, but when the biomass due to Rhabdostyla was subtracted from the total, significant correlations were observed between protozoan biomass and chlorophyll a, pheopigments, attached bacteria, and free bacteria. Correlations between the biomass of specific ciliates and microbial parameters, as well as observations of their vertical and seasonal distributions, suggest that concentrations of appropriate food resources are important in determining the abundance of heterotrophic ciliates in the plankton.Key words: ciliates, plankton, protozoa, amoebae, lakes, monomictic, zooplankton, chlorophyll a, pheopigments, bacteria