SIMULTANEOUS MEASUREMENT OF FOOD PARTICLE SELECTION AND CLEARANCE RATES OF PLANKTONIC OLIGOTRICH CILIATES (CILIOPHORA, OLIGOTRICHINA)

Oligotrich ciliates are a major microzooplankton component in the nort hern Baltic Sea during summer. The use of wheat starch particles as a tracer-level food source enabled us to simultaneously evaluate in situ clearance rates and food particle size selection of natural ciliate a ssemblages. Average clearance rates of 9 ciliates of the genera Lohman niella, Strobilidium, Strombidium, Tintinnidium and Tintinnopsis on th eir most favored particle sizes ranged from 1.9 to 11.4 mu l cell(-1) h(-1). Particle size selectivity was studied by the use of chi(2)-base d selectivity index. Each species showed a specific particle size pref erence pattern. The optimal particle size varied from 1.4 mu m (Stromb idium sp. 20 mu m in size) to 9.8 mu m (Strobilidium sp. 40 mu m) Most species were able to ingest effectively nanoflagellate-size food, but only 2 of the species showed effective grazing on the smallest partic les, suggesting a possible ability to utilize bacteria-size prey. The overall ciliate food size spectrum covered the size range of the most abundant food items in the Baltic summer plankton. Among the ciliates investigated, there appears to be 2 different feeding strategies: spec ialistic and generalistic, where the ciliates either concentrate on fe eding on a narrow size range of food organisms, or use food particles of a wide size range, with little or no preferences within this range.