We surveyed Antarctic krill (Euphausia superba) under sea ice using the autonomous underwater vehicle Autosub-2. Krill were concentrated within a band under ice between 1 and 13 kilometers south of the ice edge. Within this band, krill densities were fivefold greater than that of open water. The under-ice environment has long been considered an important habitat for krill, but sampling difficulties have previously prevented direct observations under ice over the scale necessary for robust krill density estimation. Autosub-2 enabled us to make continuous high-resolution measurements of krill density under ice reaching 27 kilometers beyond the ice edge.

Oceanographic data collected to the north of South Georgia were examined for three consecutive summers (1996/97, 1997/98, 1998/99). The results show the existence of a shelf break front during each period. The most reliable means of defining the front was the potential density anomaly at the near‐surface potential temperature minimum. In each year, off‐shelf waters were separated from on‐shelf waters by water with a potential density anomaly between 27.22 and 27.29 kg m−3. During 1997/98, the near‐surface potential temperature minimum was much colder and much shallower than in other years and was consistent with waters originating from much further south than South Georgia; these differences were further evident at a single deep off‐shelf station. The oceanographic changes during 1997/98 were consistent with a mesoscale or large‐scale movement of the southern Antarctic Circumpolar Current front. Acoustically determined densities of Antarctic krill, Euphausia superba, at South Georgia showed consistent patterns between years. Densities were substantially higher over the shelf compared with off‐shelf, with the highest densities at the shelf edge; densities were also higher to the east of the island. During 1997/98, acoustic densities of krill were substantially higher than in other years. The coincidence of the elevated acoustic density and the cooler oceanographic conditions was explored. When data from all years were combined and analysed by Generalized Additive Model, an inverse relationship between acoustic density and temperature was apparent. Historical data were also examined and it was noted that the only other occurrence of such a high estimate of krill density at South Georgia, was when oceanographic conditions were also colder.

Predator data and exploratory fishing in the Scotia Sea have revealed the presence of cephalopod stocks in the Antarctic Polar Frontal Zone (PFZ). This is a vast, remote region where large epipelagic cephalopods aggregate into highly mobde schools making them difficult to locate and sample. We used satellite tagged predators and shipboard acoustics for coarse and fine scale location of cephalopod concentrations, and sampled them with commercial and scientific nets to determine the relationship between cephalopod distribution and mesoscale oceanographic features at the PFZ. Satellite tags were attached to 9 grey-headed albatrosses Diomedea chrysostoma, breeding at Bird Island, South Georgia, to monitor foraging at sea in January-March 1994. A foraging area at the PFZ, north of South Georgia, was located, an acoustic survey undertaken and a fixed station established where acoustic targets were found. A net survey was carried out with a commercial pelagic trawl, a rectangular midwater trawl 25 mZ (RMT25), a horizontal multiple plankton sampler and a neuston net. Acoustic layers were targeted and the RMT25 sampled 200 m layers to 1000 m in daylight and darkness. Cephalopods were sinlultaneously recovered from food samples fed to D chrysostoma chicks at Bird Island. Two CTD transects, approximately normal to the major current flow, were undertaken across the PFZ and remote-sensed sea-surface temperature images from NOAA polar orbiting satellites were obtained aboard ship. The pelagic trawl sampled a cephalopod community that closely resembled that exploited by D. chrysostorna. The largest and most conspicuous species was the ommastrephid squid Martialia hyadesi which is the most important cephalopod prey species. Net-sampled M. hyadesi had been feeding on crustaceans and mesopelagic fish. The cephalopod community was sampled in a feature, interpreted as a warm core ring, in an area characterised by mesoscale features associated with the bathymetry of the northern end of the Northeast Georgia Rise and near a gap in the Falkland Ridge. The association of these mesoscale features with the bathyrnetry suggests that they may be predictable foraging locations for the cephalopods and their predators.

No abstract

In December 2000 we conducted high‐resolution hydrographic and towed undulator transects across the South Georgia shelf and into the deep waters of the Georgia Basin. The Southern Antarctic Circumpolar Current Front (SACCF) was observed flowing northwestward close to the base of the slope at 53.7°S, with a second manifestation around 53.4°S having flow in the reverse direction. Both crossings had a significant barotropic component aligned with the flow. The region of the SACCF was characterized by large‐scale undulations of properties on horizontal scales of 5–10 km, similar to the local internal Rossby radius. We observe a distinct surface temperature gradient associated with the SACCF in the vicinity of South Georgia and demonstrate the usefulness of this by tracing the course of the front around the island with advanced very high resolution radiometer data. A distinct northward deflection of the front to the north of South Georgia is probably due to topographic steering by the North Georgia Rise. The signature of the SACCF is pronounced in the Circumpolar Deep Water, with strong isopycnal interleaving indicative of cross‐frontal mixing. The salinity of the Weddell Sea Deep Water is marginally higher between the two manifestations of the SACCF, reflecting the different pathways from its source regions in the Weddell Sea. Remarkably strong vertical gradients of potential temperature and salinity exist in the bottom 100 m of the water column (at depths of around 3500 m) because of the vertical juxtaposition of waters that have taken different routes around the Northeast Georgia Rise.