Structure and grazing of the microzooplankton communities of the Subtropical Convergence and a warm-core eddy in the Atlantic sector of the Southern Ocean

The structure and grazing dynamics of microzooplankton were investigated at 15 stations during the SAAMES III cruise to the region of the Subtropical Convergence (STC) and across a warm-core eddy in subantarctic waters during austral winter (June/July) 1993. Microzooplankton abundance co-varied with the combined concentration of the nano- and picophytoplankton size fractions. Nanoflagellates dominated numerically at all stations while the >20 mu m fraction was generally dominated by ciliates (oligotrichs and tintinnids). Mixotrophs comprised between 0 and 5% of total chlorophyll concentration. Production in the region showed a weak seasonal trend with the exception of stations in the vicinity of the STC. Instantaneous growth and grazing coefficients exhibited clear spatial trends, with the highest rates recorded at the edge of the eddy and in the region of the STC. Instantaneous grazing rates at stations at the edge of the eddy and at the STC varied from 0.347 to 0.701 d(-1), equivalent to a loss of 30 to 51% of the initial standing stock and between 56 and 69% of the potential primary production. In the warm-core eddy, subantarctic and Agulhas waters, instantaneous grazing rates ranged from 0.281 to 0.433 d(-1). This is equivalent to a loss of 24 to 35% of the initial standing stock and between 59 and 83% of the potential primary production. Size selectivity experiments suggest that microzooplankton preferentially graze on the pico- and nanophytoplankton size fractions. The results of this study show that the bulk of photosynthetically fixed carbon is channelled into the microbial loop during austral winter. This implies that the carbon pump is relatively inefficient during winter and that atmospheric CO2 drawdown via sinking of organic matter into deep water may be very limited in this area.