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Abstract

Thirteen metazooplankton and three ciliate species were examined for their ability to prey on heterotrophic nanoflagellates (HNF). Grazing losses of two species of HNF, Bodo saltans (kinetoplastid) and Spumella sp. (chrysomonad), were estimated in batch experiments by the disappearance of HNF in the presence or absence of predators. Nearly all species examined caused mortality of HNF, but to different extents. Individual clearance rates ranged from a few mu l ind.(-1) h(-1) with ciliates and rotifers to a few ml ind.(-1) h(-1) with daphnids. Weight-specific clearance rates revealed that oligotrichous ciliates were the most efficient feeders on HNF, whereas differences among the filter-feeding metazoans (rotifers, calanoid copepods, cladocerans) were relatively slight. Cyclopoid copepods had the lowest weight-specific clearance rates and can be assumed to be inefficient grazers on HNF. Clearance rates of filter-feeders increased significantly with body length. These rates resemble the allometric relationships of herbivorous zooplankton feeding on phytoplankton. Although four grazer species had higher feeding rates on the bodonid than on the chrysomonad flagellates, the overall clearance rate: body length relationship was independent of the HNF species