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

We investigated predation rates of small copepods, primarily species of Oit hona, on microprotozooplankton and net growth rates of these prey at severa l locations in the Ross Sea, Antarctica, during an austral summer (January 1997; US JGOFS Process Study II). Ciliates, particularly non-loricate cilia tes, contributed substantially to the carbon ration of Oithona spp., averag ing 90% body Cd-1, while dinoflagellates were much less important (1% body Cd-1) despite the latter's higher abundances. We found no significant diffe rence in net growth rates among non-loricate ciliates, tintinnid ciliates a nd dinoflagellates when zooplankton predators > 64 mum were removed. The ov erall average growth rate for each protozoan taxon across the main transect line (76 degrees 30'S) was 0.1 d(-1) (rates ranged from - 0.5 to 1.0 d(-1) ). Our findings also suggest that copepod predation has a minimal impact on the regulation of protozoan abundances. We estimated that predation by Oit hona spp. could account for the removal of only 0.3-4.8% d(-1) of ciliate s tanding stocks, and even less (< 0.05-0.2% d(-1)) of the dinoflagellates. L ow mortality from predation may help explain the relatively abundant popula tions of microprotozooplankton in the Ross Sea despite their low average ne t growth rates. (C) 2000 Elsevier Science Ltd. All rights reserved.