ORIGINAL PAPER E. A. Pakhomov á I. J. Ansorge á P. W. Froneman Variability in the inter-island environment of the Prince Edward Islands Southern Ocean) Received: 1 November 1999 / Accepted: 12 February 2000 Abstract Variability in the oceanographic parameters and macrozooplankton and micronekton composition, densities and distributional patterns were investigated during a repeat survey conducted between the Prince Edward Islands in April 1998. Results of this study demonstrated the occurrence of pronounced water pulses along the inter-island trench. The location of the Sub- antarctic Front to the north of the island plateau, through its interactions with the island group, appeared to have a marked eect on the mesoscale dynamics of physical and biological parameters between and around the islands. Seawater temperature and salinity accounted for >40% of the variation in the zooplankton distribution during the trench studies. A total of 41 macroplankton and micronekton taxa, consisting of subantarctic, sub- tropical and Antarctic species, were identi®ed. Numerical analyses revealed two major groupings of stations cor- responding to an oshore and inshore region. Although there was no evidence for quantitative dierences in macroplankton densities between the inshore surveys, oshore plankton biomass was at least three- to eightfold higher than during the trench surveys. The importance of water pulses in carrying stocks of large plankton between the islands appeared to be minimal, at least during the time when the investigation took place. Introduction The Prince Edward Islands, consisting of two small volcanic islands, Marion and Prince Edward, are located in the Indian sector of the Southern Ocean at 46°50¢S and 37°50¢E Fig. 1). Rising from a depth of 3000 m, the islands are separated by a shallow plateau approxi- mately 200 m deep. Previous investigations conducted around the Prince Edward Islands demonstrated that the island group lies directly in the path of the easterly- ¯owing Antarctic Circumpolar Current ACC) within the Antarctic Polar Front Zone APFZ). This zone is bounded to the north by the Subantarctic Front SAF) and to the south by the Antarctic Polar Front APF) Lutjeharms and Valentine 1984; Duncombe Rae 1989a; Lutjeharms 1990). As a consequence of the shallow to- pography, the islands act as an obstacle to the ACC, resulting in substantial modi®cations within the oceano- graphic and biological environment, particularly in the downstream region Ansorge et al. 1999; Froneman et al. 1999). Typical of many subantarctic oceanic islands, the Prince Edward Islands accommodate large, seasonal populations of predatory birds and mammals, suggest- ing an adequate supply of food Williams et al. 1979; Condy 1981; Perissinotto and McQuaid 1992). The mechanism that sustains the large numbers of predators on the islands has been termed a ``life-support system'' Perissinotto and McQuaid 1992). It is now recognized that the ``life-support system'' of the islands includes two important components: an inshore and oshore com- ponent Pakhomov and Froneman 1999a). The inshore component represents the trophic links between selected top predators and the rich benthic and demersal ®sh communities, while the oshore component establishes links between numerous penguins, ¯ying birds, seals and the open-ocean allochthonous zooplankton and nekton Perissinotto and McQuaid 1992; Pakhomov and Froneman 1999a, b). It has been suggested that the geographic location of the SAF in relation to the islands may substantially in¯uence the oceanological conditions within and around the island group Perissinotto and Duncombe Rae 1990; Ansorge et al. 1999; Froneman et al. 1999; Pakhomov and Froneman 1999b; Perissin- otto et al., in press). When the SAF lies far to the north E. A. Pakhomov &) á P. W. Froneman Southern Ocean Group, Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa e-mail: e.pakhomov@ru.ac.za I. J. Ansorge Department of Oceanography, University of Cape Town, Rondebosch 7701, Cape Town, South Africa Polar Biol 2000) 23: 593±603 Ó Springer-Verlag 2000