Predation by Oithona spp. on protozooplankton in the Ross Sea, Antarctica

We investigated predation rates of small copepods, primarily species of Oithona, on microprotozooplankton and net growth rates of these prey at several locations in the Ross Sea, Antarctica, during an austral summer (January 1997; US JGOFS Process Study II). Ciliates, particularly non-loricate ciliates, contributed substantially to the carbon ration of Oithona spp., averaging 90% body Cd^-1, while dinoflagellates were much less important (1% body Cd^-1) despite the latter's higher abundances. We found no significant difference in net growth rates among non-loricate ciliates, tintinnid ciliates and dinoflagellates when zooplankton predators > 64 μm were removed. The overall average growth rate for each protozoan taxon across the main transect line (76°30'S) was 0.1 d^-1 (rates ranged from -0.5 to 1.0d^-1). Our findings also suggest that copepod predation has a minimal impact on the regulation of protozoan abundances. We estimated that predation by Oithona spp. could account for the removal of only 0.3-4.8% d^-1 of ciliate standing stocks, and even less (< 0.05-0.2% d^-1) of the dinoflagellates. Low mortality from predation may help explain the relatively abundant populations of microprotozooplankton in the Ross Sea despite their low average net growth rates.