COMPARATIVE SIZE-SPECIFIC GRAZING RATES IN-FIELD POPULATIONS OF CILIATES AND DINOFLAGELLATES

We compared specific feeding rates of ciliates and thecate and gymnodinoid dinoflagellates on the natural food spectrum applying a dual-label isotope method (Dare 1978, Helgolander wiss Meeresunters 31:241-248; Lessard & Swift 1985, Mar Biol 87:289-296) in coastal waters off Oregon, USA. Ciliates had the highest size-specific clearance rates on heterotrophic food sources with an average of 3.9 x 10(5) body volumes (BV) h(-1). Gymnodinoid dinoflagellates strongly selected for phototrophic food sources and showed the highest size-specific clearance rates on phototrophs with an average of 2.2 x 10(5) BV h(-1). The range of clearance rates on photosynthetic organisms in gymnodinoids overlapped with that in the ciliates. We also present evidence for bacterivory in gymnodinoid dinoflagellates with a mean of 2.4 x 10(4) BV h(-1), comparable to bacterivory by thecate dinoflagellates, but more than an order of magnitude lower than the mean found for the ciliates studied. Our data indicate that in gymnodinoid dinoflagellates, small cells have higher size-specific clearance rates than larger cells. The composition as well as the size distribution of the grazer populations need to be considered when comparing the grazing impact of ciliates and dinoflagellates on prey populations in the field.