Factors controlling pelagic communities of freshwater ciliates and heliozoans

Sammanfattning: Ciliate diversity and seasonal succession were studied during two years in oligotrophic,acidic Lake Njupfatet. Maximum ciliate abundance occurred shortly after ice-out. After thefirst year, the lake was neutralised by liming. Liming caused changes in the bacterioplanktonand phytoplankton communities, but no change in ciliate diversity. After liming, the ciliatecommunity showed two summer abundance maxima. Both years, the ciliate community wasnumerically dominated by two small, raptorial prostomatids, Urotricha and Balanion. The biomass was dominated by a few species of the filter feeding oligotrichs Strombidium and Strobilidium. The same taxa were present after liming, but the dominance of prostomatids decreased and the oligotrichs increased in relative importance.Enclosure experiments showed that inorganic nutrients and predation by metazooplankton simultaneously regulated the total biomass of ciliates and heliozoans. The stimulatory effect of nutrients was an indirect effect of increased production of prey organisms, which included bacterioplankton, small phytoplankton and flagellates. Taxa responded differently to the enclosure manipulations. The most common ciliates, Urotricha/Balanion, were regulated mainly by their food supply. Most susceptible to calanoid copepod predation were two species of Strombidium.Heliozoans were abundant in the lake in June prior to liming, and in August after liming;but were absent during other periods. They were negatively correlated with metazooplankton. A sudden bloom and collapse of the heliozoan community in the experimental enclosures was most likely due to a rapid utilisation, followed by a depletion, of food resources.Data from the enclosure experiments and from the literature suggest that higher nutrientconcentrations can support a more diverse ciliate community, and often a higher biomass.Very slight changes in nutrient concentrations in oligotrophic environments are likely toinfluence the supply of food items, and thereby induce changes in the species compositionof ciliates.