Some characteristics of the zooplankton distribution in the Prydz Bay region of the Indian sector of the Antarctic Ocean in the summer of 1983/84

During the SIBEX I cruise of the R. V. KAIYO MARU of the Fisheries Agency in the 1983/84 season, zooplankton samplings were conducted in the Prydz Bay region in the Indian Ocean sector of the Antarctic Ocean. The sampling scheme consisted of two legs, i.e. December 4-24,1983 (Leg I) and January 19-February 2,1984 (Leg II), and a total of 63 stations were occupied. At the six stations between 62° and 64°S, zooplankton samplings were repeated in both December and January. The vertical hauls with a WP-2 net were made at all stations usually from 100m to the surface and from 500m to the surface. In the catches of 100-0m layer in December, it was shown that the abundance of the total zooplankton in terms of both individual number and wet weight per m^2 was larger in the Subantarctic and the Antarctic Convergence regions where chl. a was remarkably high. This high abundance is mainly due to the ontogenetic migration of the major copepod species from deeper layers. In January, both zooplankton abundance and chl. a were more abundant at the southernmost station (Stn. 194,69°S), which was probably due to the anomalous hydrographic situation of Prydz Bay by earlier ice melt. The zooplankton at this station was represented by the major large-sized copepods (Calanoides acutus, Calanus propinquus and Metridia gerlachei) of the more advanced developmental stages. The individual number and wet weight of total zooplankton in Januray at the repeated sampling stations became 64.5 and 38.8% of those in December, respectively, and this accompanied a decrease in all taxonomic groups. In Copepoda occupying more than 60% of the total zooplankton abundance throughout this investigation, there was observed a distinct decrease in their wet weight in January showing 35.1% of the weight in December while the individual number showed a little decrease during the same period. This was due to a large decrease in the abundance of the copepodite IV-VI stages of the major large-sized copepods, especially, C. acutus and Rhincalanus gigas. On the contrary, there were very little changes in total zooplankton abundance for the 500-100m layer. In this study, it was shown that the geographical and seasonal changes in the abundance of C. acutus, R. gigas, C. propinquus and M. gerlachei were related to their growth and ontogenetic migration, which largely influenced the abundance of total zooplankton in the 100-0m layer during the austral summer