PLANKTONIC CILIATE SPECIES-DIVERSITY AS AN INTEGRAL COMPONENT OF ECOSYSTEM FUNCTION IN A FRESH-WATER POND

A diverse and dynamic community of ciliated protozoa lives in the stra tified water column of the productive freshwater pond known as 'Priest pot'. As part of a long-term continuous monitoring programme, this co mmunity was examined with 10 cm-scale vertical sampling in August 1995 and June 1997, and found to be dominated by species with endosymbioti c algae (1995), or by a quite different set of species, feeding on the dinoflagellate Peridinium (1997). On both occasions, the community st ructure was comprehensible in terms of the preceding sequence of recip rocal interactions involving microbiological, physical and chemical fa ctors (e.g. oxygen depletion, thermal gradient, essential nutrient con centrations). In this one pond, very different ciliate communities app ear 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) transf orms in a continuously changing environment probably depends on a larg e local diversity of rare and encysted species and the rapidity with w hich these species fill vacant niches.