Physical-biological coupling in the waters surrounding the Prince Edward Islands (Southern Ocean)

The results of a macro-scale oceanographic survey conducted in the upstream and downstream regions of the Prince Edward Islands in austral autumn (Apr il/May) 1989 are presented. During the investigation, the Subantarctic Fron t, upstream of the islands, was shown to lie initially south at 46 degrees 38'S, while downstream, the front remained in a northern position of approx imately 46 degrees S. Surface expressions of the front show that the Subant arctic Front forms a zonal band, while the subsurface expressions (200 m) s how a distinct meander in both regions. In the upstream region of the islan ds, the northern branch of the Antarctic Circumpolar Current, the Subantarc tic Front, influenced by the shallow bathymetry, was deflected around the n orthern edge of the islands. Water masses in this region were shown to modi fy gradually from Subantarctic Surface Water (7 degrees C, 33.75) to Antarc tic Surface Water (5 degrees C, 33.70) as the Polar Frontal Zone was crosse d. Downstream of the islands a wake was formed resulting in the generation of broad, cross-frontal meanders. As a consequence, warm Subantarctic Surfa ce Water from north of the Subantarctic Front was advected southwards acros s the Polar Frontal Zone, while cooler waters, which had been modified in t he transitional band of the Polar Frontal Zone, were advected northwards. I n the downstream region a warm eddy consisting of Subant-arctic Surface Wat er was observed. Its generation is possibly due to baroclinic instabilities in the meandering wake. Zooplankton species composition and distribution p atterns during the investigation were consistent with the prevailing oceano graphic regime. Four distinct groupings of stations were identified by nume rical analysis. These corresponded to stations found north of the Subantarc tic Front, within the warm eddy, located in the Polar Frontal Zone, and tho se stations associated with the meander. The groupings were separated by th e Subantarctic Front, which appears to represent an important biogeographic boundary to the distribution of warm-water zooplankton species. Warm eddie s in the downstream region of the islands may represent an effective mechan ism for transporting warm water species across the Subantarctic Front.