Zooplankton life under the perennial Antarctic Sea ice

Zooplankton was collected during the drift on the USA-Russia Ice Station We ddell 1 in the western Weddell Sea. Stratified tows up to 1,000 m depths we re performed under perennial ice at a distance of ca. 300 km from the margi nal ice zone from March to May 1992. Seasonal abundance, developmental stag e composition and vertical distribution in populations of large dominant ca lanoid copepod species were studied. The abundance of Calanoides acutus cha nged inconspicuously and averaged 1,260 ind. m(-2); its developmental stage composition was characterised by the dominance of copepodites V (CV) and h ardly changed in the course of observations. Calanus propinquus was at most stations in fairly low abundance and was represented by CV and adults; at the latest stations, it was not found. Rhincalanus gigas was also scarce; I II-IV stages were present, but the first of them only at the southernmost s tations. In contrast to this species, in the relatively abundant population of Metridia gerlachei (1,370 ind. m(-2) on average), a new generation was observed. There m was a clear deepening of the median depth of occurrence o f the above three species from March to early May, while in late May a shif ting to upper layers was observed which remained unexplained. The data obta ined were compared to the data collected in the ice-open eastern part of th e Weddell Sea in the corresponding season. It can be seen that the differen ces in populations dynamics of the four species under the perennial ice dep ends on their trophic mode, i.e. capability to store lipid reserves in summ er and/or to feed in winter, and with food spectra.