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) around the Prince Edward Islands. On both occasions, substantial disturbances in the flow pattern of the Antarctic Circumpolar Current (ACC) were demonstrated particularly downstream of the islands. In this region, enhanced cross frontal and zonal mixing were generated by the intrusion of foreign water masses from outside the APFZ. This may have been promoted by the differences in the flow intensity of the ACC in the vicinity of the islands which is affected by the Subantarctic Front (SAF) location near the island plateau. Distribution of chl-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 identified during both surveys, quantitative distribution of the surface phytoplankton showed a good consistency with spatial patterns of water masses. Community structure and distribution of meso- and macrozooplankton/micronekton confirmed that extensive cross-frontal mixing occurred in the downstream region of the islands. While no substantial differences between the inter-island and offshore assemblages of mesozooplankton were found, a distinct inter-island macroplankton community was identified. Zooplankton community structure 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 the effect of the flow regime, in particular by the speed at which the incident current, the ACC, approaches and interacts with the island group.