Temporal variability in the physico-biological environment of the Prince Edward Islands (Southern Ocean)

Oceanographic environment, chlorophyll-a (chl-a), plankton composition and spatial dynamics were investigated during two large-scale surveys conducted in April/May 1989 and 1997 within the Antarctic Polar Front Zone (APFZ) ar ound the Prince Edward Islands. On both occasions, substantial disturbances in the flow pattern of the Antarctic Circumpolar Current (ACC) were demons trated particularly downstream of the islands. In this region, enhanced cro ss frontal and zonal mixing were generated by the intrusion of foreign wate r masses from outside the APFZ. This may have been promoted by the differen ces in the flow intensity of the ACC in the vicinity of the islands which i s affected by the Subantarctic Front (SAF) location near the island plateau . Distribution of chi-a was generally uniform throughout both surveys with values ranging from 0.1 to 0.2 and from 0.2 to 0.3 mg m(-3) during 1989 and 1997, respectively. Although a single phytoplankon community was identifie d during both surveys, quantitative distribution of the surface phytoplankt on showed a good consistency with spatial patterns of water masses. Communi ty structure and distribution of meso- and macrozoo-plankton/micronekton co nfirmed that extensive cross-frontal mixing occurred in the downstream regi on of the islands. While no substantial differences between the inter-islan d and offshore assemblages of mesozooplankton were found, a distinct inter- island macroplankton community was identified. Zooplankton community struct ure clearly indicated the intrusion of subtropical species into the region during the 1997 survey. The extent of modifications in the APFZ planktonic realms in the vicinity of the Prince Edward Islands may be influenced by th e effect of the flow regime, in particular by the speed at which the incide nt current, the ACC, approaches and interacts with the island group. (C) 20 00 Elsevier Science B.V. All rights reserved.