ALBERT

Description: The composition and distribution of squid and fish collected by Rectangular Midwater Trawls in the upper 200 m were investigated during the BROKE-West (Baseline Research on Oceanography, Krill and the Environment-West) survey (January-March 2006) in CCAMLR Subdivision 58.4.2 of the Southern Ocean. A total of 332 individuals were collected, with the most abundant fish species being Pleuragramma antarcticum (34%), Notolepis coatsi (27%) and Electrona antarctica (26%); and the most abundant squid being Galiteuthis glacialis (64%). Abundances of all species were among the lowest recorded using this type of gear. Cluster analysis revealed two distinct communities: a notothenioid-dominated coastal community and an oceanic community dominated by mesopelagic fish and squid. Environmental factors related to this segregation were explored using Redundancy Analysis (RDA). The notothenioid P. antarcticum was associated with shallow areas with high chlorophyll a concentrations. Larval stages of E. antarctica, N. coatsi and G. glacialis were found over deeper water and were positively correlated with higher temperatures and a deeper-reaching mixed layer. Postmetamorphic stages of E. antarctica were caught mostly after sunset and were negatively correlated with solar elevation. The observation of higher densities in the eastern part of the sampling area reflects a temporal rather than a geographical effect. Samples of the three most abundant fishes, E. antarctica, P. antarcticum and N. coatsi, were analysed for gut content. All species fed on a variety of mesozooplankton including copepods, amphipods and euphausiids, which is consistent with previous reports on similar life stages. Mean body energy density was highest for E. antarctica (27 kJ g-1), while it was similar for P. antarcticum and N. coatsi (22 kJ g-1). The high energy content emphasizing the importance as a food resource for top predators in the Southern Ocean.

Description: The macrozooplankton and micronekton community of the Lazarev Sea (Southern Ocean) was investigated at 3 depth layers during austral summer, autumn and winter: (1) the surface layer (0–2 m); (2) the epipelagic layer (0–200 m); and (3) the deep layer (0–3000 m). Altogether, 132 species were identified. Species composition changed with depth from a euphausiid-dominated community in the surface layer, via a siphonophore-dominated community in the epipelagic layer, to a chaetognath-dominated community in the deep layer. The surface layer community predominantly changed along gradients of surface water temperature and sea ice parameters, whereas the epipelagic community mainly changed along hydrographical gradients. Although representing only 1% of the depth range of the epipelagic layer, mean per-area macrofauna densities in the surface layer ranged at 8% of corresponding epipelagic densities in summer, 6% in autumn, and 24% in winter. Seasonal shifts of these proportional densities in abundant species indicated different strategies in the use of the surface layer, including both hibernal downward and hibernal upward shift in the vertical distribution, as well as year-round surface layer use by Antarctic krill. These findings imply that the surface layer, especially when it is ice-covered, is an important functional node of the pelagic ecosystem that has been underestimated by conventional depth-integrated sampling in the past. The exposure of this key habitat to climate-driven forces most likely adds to the known susceptibility of Antarctic pelagic ecosystems to temperature rise and changing sea ice conditions.