Pelagic ciliated protozoa in two 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. Subdominants in oligotrophic Lake Taupo included Askenasia, Pscudobalanion and Urotri-cha, and in eutrophic Lake Okaro Prorodon, Coleps, Urocentrum, Stentor and Spirostomum were important. Biomass was dominated by large predatory ciliates and Stentor in Lake Taupo, and Prorodon and Stentor in Lake Okaro. The importance of Prorodon and Stentor to ciliate biomass is unusual and has not been reported for northern hemisphere lakes. Small ciliates (<20 μm) capable of consuming particles <2 μm were a major component of the ciliate community in Lake Taupo. Peaks in ciliate abundance occurred at the same time in both lakes: in autumn, at the beginning of mixis and in spring. Ciliates were vertically stratified during mixis and stratification in both lakes. The effect was more pronounced during deoxygenation of the hypolimnion in Lake Okaro which excluded oligotrichs and introduced benthic ciliates. Ciliates were less abundant (mean 40001−1 in Lake Okaro and 9001−1 in Lake Taupo) than in comparable northern temperate lakes. There was no correlation 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 numbers were low. The only consistent, significant correlations were total ciliate numbers and individual species of ciliates with bacterial concentrations in both lakes and with picophytoplankton in Lake Taupo.