Planktonic ciliate species diversity as an integral component of ecosystem function in a freshwater pond

A diverse and dynamic community of ciliated protozoa lives in the stratified water column of the productive freshwater pond known as 'Priest pot'. As part of a long-term continuous monitoring programme, this community was examined with 10 cm-scale vertical sampling in August 1995 and June 1997, and found to be dominated by species with endosymbiotic algae (1995), or by a quite different set of species, feeding on the dinoflagellate Peridinium (1997). On both occasions, the community structure was comprehensible in terms of the preceding sequence of reciprocal interactions involving microbiological, physical and chemical factors (e.g. oxygen depletion, thermal gradient, essential nutrient concentrations). In this one pond, very different ciliate communities appear at different times, yet each community may be nothing more than a transient bi-product of dynamic ecosystem functions. The facility with which the ciliate community (or any other microbial community) transforms in a continuously changing environment probably depends on a large local diversity of rare and encysted species and the rapidity with which these species fill vacant niches.