FEEDING RATES OF MACROZOOPLANKTON AND MICROZOOPLANKTON ON HETEROTROPHIC NANOFLAGELLATES

Thirteen metazooplankton and three ciliate species were examined for t heir ability to prey on heterotrophic nanoflagellates (HNF). Grazing l osses of two species of HNF, Bodo saltans (kinetoplastid) and Spumella sp. (chrysomonad), were estimated in batch experiments by the disappe arance of HNF in the presence or absence of predators. Nearly all spec ies examined caused mortality of HNF, but to different extents. Indivi dual clearance rates ranged from a few mu l ind.(-1) h(-1) with ciliat es and rotifers to a few ml ind.(-1) h(-1) with daphnids. Weight-speci fic clearance rates revealed that oligotrichous ciliates were the most efficient feeders on HNF, whereas differences among the filter-feedin g metazoans (rotifers, calanoid copepods, cladocerans) were relatively slight. Cyclopoid copepods had the lowest weight-specific clearance r ates and can be assumed to be inefficient grazers on HNF. Clearance ra tes 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 fee ding rates on the bodonid than on the chrysomonad flagellates, the ove rall clearance rate: body length relationship was independent of the H NF species.