Seasonal variations in microzooplankton grazing in the region of the Subtropical Convergence

Abstract

Microzooplankton grazing and community structure were investigated in the region of the Subtropical Convergence (STC) during three cruises of the South African Antarctic Marine Ecosystem Study (SAAMES) in austral summer (January/February 1993; December 1994/January 1995) and winter (June/July 1993). Chlorophyll a concentrations were consistently dominated by the 20 μm) to total chlorophyll a concentrations varied considerably between cruises. Microzooplankton communities were numerically dominated by protozoans comprising ciliates (aloricates and tintinnids) and dinoflagellates. Instantaneous growth coefficients of phytoplankton in the vicinity of the STC showed no seasonal trends. However, marked seasonal differences were observed in the size structure of the phytoplankton. The grazing impact of microzooplankton was highest when the 95% of the total. Under these conditions, the instantaneous grazing rates ranged between 0.15 and 0.66 d-1. These correspond to daily losses of 14 to 48% of the inntial standing stock and between 45 and 81% of the potential primary production. At stations where microphytoplankton contributed significantly (∼-20%) to total chlorophyll concentrations, the grazing coefficients were lower, ranging between 0 and 0.53 d-1. This corresponds to a loss of <41% of the initial standing stock, or between 0 and 56% of the potential production. Our data suggest that microzooplankton represent the main grazing sink for production when the <20 μm chlorophyll size-class dominates total chlorophyll. These facts suggest that the efficiency of the biological pump may vary over time.