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 planctonicum and the diffusion feeding scuticoci liate Histiobalantium bodamicum could be cultivated for months/years on a s ole diet of Cryptomonas sp., whereas the diatom Stephanodiscus hantzschii d id not support their growth. With Cryptomonas sp: as food, numerical respon ses of all ciliates followed a modified Michaelis-Menten model, which at 15 degrees C yielded maximum growth rates of 0.96, 1.87 and 0.33 d(-1) and th reshold concentrations of 61, 78 and 290 ng C ml(-1) for S. lacustris, B. p lanctonicum and H, bodamicum, respectively. Functional response patterns di ffered between species. In all investigated ciliates, growth rates reached a maximum earlier than ingestion rates, and there were no threshold concent rations for zero ingestion. Food selectivity depended on feeding mode. H. b odamicum was not able to ingest the non-motile diatoms. Both S. lacustris a nd B. planctonicum selectively preferred cryptophytes when offered a mixed diet. This effect was more pronounced in the raptorial feeder compared to t he filter feeder. Our results indicate that during the phytoplankton spring bloom in Lake Constance prostome and oligotrich ciliates mainly exploit cr yptophytes, and that the scuticociliate H. bodamicum, due to its slow growt h, is an inferior competitor during this season. The observed threshold con centrations suggest that during the rest of the year prostomes and oligotri chs must rely on small-scale patches of this food, whereas H. bodamicum, wi th maximum development in late summer and autumn, presumably consumes a muc h larger variety of prey.