PELAGIC CILIATED PROTOZOA IN 2 MONOMICTIC, SOUTHERN TEMPERATE LAKES OF CONTRASTING TROPHIC STATE - SEASONAL DISTRIBUTION AND ABUNDANCE

Two lakes of contrasting trophic state in the central North Island of New Zealand were sampled monthly for protozoan ciliates and potential food resources. Oligotrichs dominated numbers in both lakes. Subdomina nts in oligotrophic Lake Taupo included Askenasia, Pseudobalanion and Urotricha, and in eutrophic Lake Okaro Prorodon, Coleps, Urocentrum, S tenter and Spirostomum were important. Biomass was dominated by large predatory ciliates and Stenter in Lake Taupo, and Prorodon and Stentor in Lake Okaro. The importance of Prorodon and Stenter to ciliate biom ass is unusual and has not been reported for northern hemisphere lakes . Small ciliates (<20 mu m) capable of consuming particles <2 mu m wer e a major component of the ciliate community in Lake Taupo. Peaks in c iliate abundance occurred at the same time in both lakes: in autumn, a t the beginning of mixis and in spring. Ciliates were vertically strat ified during mixis and stratification in both lakes. The effect was mo re pronounced during deoxygenation of the hypolimnion in Lake Okaro wh ich excluded oligotrichs and introduced benthic ciliates. Ciliates wer e less abundant (mean 4000 1(-1) in Lake Okaro and 900 1(-1) in Lake T aupo) than in comparable northern temperate lakes. There was no correl ation between the seasonal distribution of ciliates and chlorophyll a, primarily caused by a winter peak in chlorophyll a dominated by large species of phytoplankton in Lake Taupo, at a time when ciliate number s were low. The only consistent, significant correlations were total c iliate numbers and individual species of ciliates with bacterial conce ntrations in both lakes and with picophytoplankton in Lake Taupo.