Protozooplankton community structure and grazing impact in the eastern Atlantic sector of the Southern Ocean in austral summer 1998

The protozooplankton community structure and grazing impact within the winter ice-edge zone (WIE), spring ice-edge zone (SIE) and in the vicinity of the Antarctic Polar Front (APF) in the eastern Atlantic sector of the Southern Ocean was investigated in austral summer (December 97—January 98). Total chlorophyll-a (Chl-a) concentrations in the vicinity of the SIZ and APF were always >0.5 μg l while at stations within the SIE total pigment concentrations were always <0.3 μg l−1. Protozooplankton communities throughout the investigation were dominated by nanoflagellates with densities varying between 2050 and 2750 ind. l−1 in the SIE, between 2525 and 3100 ind. l−1 in the WIE and between 2750 and 3350 ind. l−1 in the region of the APF. The >20 μm fraction was always dominated by ciliates comprising aloricates and tintinnids and dinoflagellates. The contribution of the larger components to the total was, however, always <40% of the total. Protozooplankton biomass estimated from biovolume:carbon ratios ranged from 0.4 to 1.5 μg C l−1. Protozooplankton herbivory throughout the study was highest within the <20 μm chlorophyll fraction. Instantaneous grazing coefficients of protozooplankton within this chlorophyll fraction ranged between 0.04 and 0.28 d−1 corresponding to a grazing impact equivalent to up to 24% (range 4–24%) of the chlorophyll standing stock. These rates are equivalent to a community ingestion rate of between 0.55 and 1.89 μg C l−1 or between 13% and 35% of the daily production within the <20 μm Chl-a fraction. Protozooplankton were unable to control the growth of the larger microphytoplankton. The highest impact of the protozooplankton was recorded in the WIE where small phytoplankton cells comprised >60% of total pigment. These results suggest that the grazing impact of the protozooplankton was largely mediated by the contribution of the small phytoplankton cells to total pigment.