Structure and grazing impact of the protozooplankton community in the waters surrounding the Prince Edward Islands (Southern Ocean)

The structure and grazing dynamics of the protozooplankton community at ten stations in the upstream and downstream region of the Prince Edward Islands (46 degrees 38'S, 37 degrees 57'E) were investigated during early austral autumn (April/May) 1997. Total chlorophyll-a (chl-a) concentrations throughout the investigation were <0.6 mu g l(-1) and were always dominated by nano- and picophytoplankton (<20 mu m). Although microphytoplankton (>20 mu m) generally contributed <10% of the total chi-a, its contribution to total was significantly higher at downstream stations (F = 1.03; P < 0.05). Nanoflagellates (<20 mu m) numerically dominated the protozooplankton community while the >20 mu m fraction was dominated by dinoflagellates. Instantaneous growth rates of phytoplankton at upstream stations ranged from 0.01 to 0.18 day(-1) and between 0.01 and 0.19 day(-1) at downstream stations. Instantaneous grazing rates of protists on phytoplankton upstream of the islands ranged from 0.03 to 0.07 day(-1) which corresponds to a loss of up to 8% (mean = 5%) of the initial phytoplankton standing stock or up to 71% (mean = 37%) of the potential primary production per day. Downstream of the islands, although the grazing rates of the protists were within the same range (0.01-0.07 day(-1)), the grazing impact was lower, corresponding to up to 7% (mean = 3%) of the initial standing stock or up to 47% (mean = 25%) of the potential primary production per day. The spatial difference in the grazing impact of protozooplankton appears to reflect a shift in the size structure of the phytoplankton community resulting from the "island mass effect".