Multivariate analyses of the macrozooplankton community and euphausiid larval ecology in the Prydz Bay region, Antarctica

Macrozooplankton data from the Prydz Bay region, collected between 1981 and 1991, and representing the months of September to March, were analysed using duster analysis and non-metric multidimensional scaling to define the communities within the region, their distribution patterns, indicator species and species affinities. Four communities were identified. Three of these communities were dominated respectively by Euphausia crystallorophias (neritic), copepods-chaetognaths (main oceanic), and salpshyperiids-small euphausiids (northern oceanic community). The fourth community was characterized by the high abundance and dominance of the Antarctic krill, E. superba, and also by the paucity of zooplankton. This community was mainly located along the outer shelf edge, usually between the main oceanic and neritic communities. Chlorophyll a abundance and temperature explained much of the variation in community distribution patterns. The annual recession of ice, pack-ice cover, and salinity also explained some of the variation. Water circulation seemed to have only a clear effect on community distributions at the end of summer in March, when distributional patterns exhibited a distinct longitudinal zonation compared to the strong latitudinal pattern seen through most of the summer. In January 1984, 1985 and March 1987, net sampling surveys were carried out on the distribution and abundance of euphausiid larvae, primarily E. superba, in the Prydz Bay region. E. superba occurred in low abundance in January, probably due to sampling preceding the main spawning period, but occurred in very high abundance in the east of the region in March 1987. Thysanoessa macrura occurred throughout the study area in consistently high abundance. E. crystallorophias was marginally more abundant within its restricted range. Distinct north-south variations in larval age and developmental stages of T. macrura were observed indicating regional differences in spawning. Euphausia frigida was mainly confined to the upper 200 m of the Antarctic Circumpolar Current. E. superba larvae produced north of the shelf break, between 70 to 83° E, moved north-east into the Antarctic Circumpolar Current. Larvae originating on the shelf moved rapidly west in the East Wind Drift. E. crystallorophias had the same westward dispersion, but some larvae appeared to return eastward via the Prydz Bay Gyre and remain in the region. However, the data indicate that most E. superba larvae, providing they survive injurious cold temperature and food deprivation, will leave the area, i.e. Prydz Bay krill may not be a self maintaining stock.