Microprotozooplankton distribution and their potential grazing impact in the antarctic circumpolar current

The abundance and composition of microphyto- and microprotozooplankton were studied in late October 1992 along a meridional transect (6°W) extending from the ice edge (56°S) in the southern Antarctic Circumpolar Current (ACC) to the Polar Front region (PFr) between 51 and 47°S. Microphytoplankton standing stocks were low in the southern ACC, but attained bloom proportions in the PFr where diatoms dominated. Microprotozooplankton standing stocks varied from 197 to 434 mgC m−2 in the southern ACC and 282 to 665 mgC m−2 in the PFr. The composition of the ciliate community and the size structure of microprotozooplankton assemblages differed significantly between the southern ACC and the PFr, whereas there was no significant difference between the ice edge and the open southern ACC water. In the southern ACC, 45% of microprotistplankton standing stocks was composed of heterotrophs (18% flagellates and 27% ciliates), with an important fraction of protozoa larger than 40 μm. In the PFr, heterotrophs r represented only 16% of microprotistplankton standing stock (12% flagellates and 4% ciliates) with the smaller size fraction (20–40 μm) largely dominating. Grazing by microprotozooplankton, estimated by converting counts into ingestion rates using literature values, ranged from 11 to 54% and 4 to 56% of primary production in the southern ACC and PFr, respectively. These grazing estimates and the importance of larger protozoa (> 40 μm) in the southern ACC suggest that they can play a significant role in controlling blooms of diatoms. Further, factors other than food supply seem to determine both the size structure and composition of microprotozooplankton assemblages.