ABUNDANCE AND FEEDING ECOLOGY OF LARGER PROTOZOOPLANKTON IN THE ICE EDGE ZONE OF THE WEDDELL AND SCOTIA SEAS DURING THE AUSTRAL WINTER

Biomasses, abundances and feeding ecology of larger (>50-mu-m diameter) protozooplankton were studied in the upper 210 m in the ice edge zone of the Weddell/Scotia Sea area in the austral winter of 1988. Sixty-liter water samples were taken at five depths at 17 stations, and organisms were concentrated by reverse-flow filtration. Mean abundances of the total assemblage of larger protozooplankton (radiolarians, formaminiferans, acantharians, the heliozoan Sticholonche, tintinnid and aloricate ciliates, and thecate and athecate dinoflagellates) ranged from 2040 to 3745 m-3 in the upper 210 m. Biomass ranged from 33 to 48-mu-g C m-3 in the upper 85 m, and from 32 to 54-mu-g C m-3 from 115 to 210 m. Phaeodarian radiolarians larger than 1.6 mm (sampled with plummet nets) contributed an additional 3-mu-g C m-3 in the upper 100 m and an additional 7-mu-g C m-3 from 100 to 200 m. These abundances and biomasses are lower than for other seasons in the Antarctic, but are comparable to abundances reported for several of these groups in lower latitude waters. We attribute the low winter abundances to slower growth and reduced food, rather than to increased mortality. The large protozooplankton are trophically diverse; in addition to heterotrophy on a variety of organisms. we found apparent evidence of mixotrophy and symbiosis in some of the groups. The large protozooplankton fed on both autotrophic and heterotrophic organisms in winter, although the biomass of smaller forms is dominated by heterotrophs. Feeding on detrital particles also was indicated by the presence of siliceous fragments in vacuoles. The larger protozooplankton in the winter ice edge zone may be important in reducing particle flux to the deep sea and as a food source for larger zooplankton, especially from the base of the euphotic zone to 210 m.