Factors controlling pelagic populations of ciliates and heliozoans— late summer investigations in an acidic lake before and after liming

Ciliates and heliozoans were studied in a small oligotrophic Swedish lake with the objective of investigating what controlled the community structure and population dynamics. Enclosure experiments were performed with five levels of inorganic nutrient concentrations and the presence or absence of crustacean zooplankton. The experiments were performed during the summers immediately preceding and following liming of the lake. The responses of protozooplankton to these manipulations were investigated and compared with the responses of bacterioplankton, phytoplankton and metazooplankton. In contrast to most studies on protozoan zooplankton, ciliates were determined to the lowest taxon possible, and biomasses and abundances were calculated for the different taxa. The protozooplankton biomass of the lake, expressed as summer average, did not change between the years. However, the community composition changed from smaller to larger species as the small prostomatid ciliates Urotricha and Balanion were largely replaced by oligotrichs of the genera Strombidium and Strobilidium. The experiments suggested that the protozooplankton community in the lake was controlled by metazooplankton in the acidic environment, while food limitation was the most important controlling factor after liming. Before liming, Urotricha and Balanion were unaffected by nutrient additions as well as by metazooplankton, while other ciliates and Heliozoa showed a significantly negative response to metazooplankton. After liming, Urotricha and Balanion showed a significant positive response to nutrient addition as well as to bacterioplankton biomass. A possible explanation for this response in these algivore genera is that the response was indirect and mediated through small chrysoflagellates.