ROLE OF MICROPLANKTON IN THE DIET AND DAILY RATION OF ANTARCTIC ZOOPLANKTON SPECIES DURING AUSTRAL SUMMER

Predation rates of the 9 most abundant Antarctic meso- (4 copepods) an d macrozooplankton (3 euphausiids, 1 hyperiid and 1 salp) species on m icroplankton (20 to 200 mu m) were estimated using in vitro incubation s during the fourth cruise of the South African Antarctic Marine Ecosy stem Study (SAAMES IV) to the ice-edge region of the Lazarev Sea durin g austral summer (Dec/Jan) 1994/1995. Chlorophyll a concentrations dur ing the incubations ranged between 0.187 and 1.410 mu g l(-1) and were dominated by ice-associated chain-forming microphytoplankton (>20 mu m) of the genera Nitzschia and Chaetoceros. The microplankton assembla ges were entirely dominated by protozoans comprised of ciliates and di noflagellates. Densities of protozoans ranged from 1375 to 2690 cells l(-1) Based on previously published results, meso- and macrozooplankto n species generally consumed >120% of their minimum daily ration, i.e. minimum carbon uptake (MCU), when offered microplankton. Exceptions w ere Euphausia crystallorophias and Vibilia antarctica for which microp lankton carbon contributed 68 and 30% of MCU, respectively. Microplank ton carbon contributed between 17 and 24% of the total carbon requirem ents for the 4 copepod species examined and between 21 and 73% for the macrozooplankton. The daily rations of juveniles were, however, twice those of the adults,suggesting that the relative importance of microz ooplankton to the daily ration of macrozooplankton shifts with life st age. Carnivory by metazoan grazers may, therefore, potentially reduce the high grazing impact of microzooplankton on the local phytoplankton stock.