Responses of three freshwater planktonic ciliates with different feeding modes to cryptophyte and diatom prey

The filter feeding oligotrich ciliate Strobilidium lacustris, the raptorial prostome ciliate Balanion planctonicu~n and the diffusion feeding scuticociliate Histiobalantium bodamicurn could be cultivated for months/years on a sole &et of Cryptomonas sp., whereas the diatom Stephanodiscus hantzschii did not support their growth. With Cryptornonas sp. as food, numerical responses of all ciliates followed a modified Michaelis-Menten model, which at lSC yielded maximum growth rates of 0.96. 1.87 and 0.33 d-' and threshold concentrations of 61, 7 8 and 290 ng C ml-' for S. lacustris, B. planctonicum and H. bodamicum, respectively. Functional response patterns differed between species. In all investigated ciliates, growth rates reached a maximum earlier than ingestion rates, and there were no threshold concentrations for zero ingestion. Food selectivity depended on feeding mode. H. bodamicum was not able to ingest the non-motile diatoms. Both S. lacustris and B. planctonicum selectively preferred cryptophytes when offered a mixed diet. This effect was more pronounced in the raptorial feeder compared to the filter feeder. Our results indicate that during the phytoplankton spring bloom in Lake Constance prostome and oligotrich ciliates mainly exploit cryptophytes, and that the scuticociliate H. bodamicum, due to its slow growth, is an inferior competitor during this season. The observed threshold concentrations suggest that during the rest of the year prostomes and oligotrichs must rely on small-scale patches of this food, whereas H. bodamicum, with maximum development in late summer and autumn, presumably consumes a much larger variety of prey.