The influence of the Antarctic Circumpolar Current on the oceanographic setting of a sub-Antarctic island

The circulation in the Southern Ocean is dominated by the Antarctic Circumpolar Current (ACe), which is the only current that flows completely around the globe. The ACC is a complex system consisting of narrow regions of sharp horizontal density gradients, which extend through the entire water column as frontal bands separated by broad zones with less intense gradients. Meridional excursions from the otherwise zonaillow in the form of meanders or mesoscale eddies occur in regions of prominent bathymetric features, suggesting a close relationship between the level of mesoscale sea surface variability and bottom topography, The Prince Edward Islands (Marion Island and Prince Edward Island) lying within the Polar Frontal Zone (PFZ) at approximately 38 DE, 47 os form a very special terrestrial habitat and have, for this reason, been designated a declared nature reserve. The ecology of the islands is directly dependent on its ocean environment, With the advent of satellite altimetry it has become apparent that the Prince Edward Islands are located on the northern border of a region of remarkably high mesoscale variability. The source of this variability lies where the current jet associated with the ACC intersects the South-West Indian Ridge. From here newly generated eddies are steered by the bottom topography past the Prince Edward Islands. '[his discovery probably explains the high physical and biological variability found at the islands, and warrants further analysis

Dynamics of physical and biological systems of the Prince Edward Islands in a changing climate

Sub-Antarctic islands are classified as isolated, hostile, impoverished regions, in which the terrestrial and marine ecosystems are relatively simple and extremely sensitive to perturbations. They provide an ideal ecological laboratory for studying how organisms, ecological processes and ecosystems respond to a changing ocean climate in the Southern Ocean. 1hese islands are characterised by large populations of top predators and subsequently any changes in the oceanographic frontal dynamics associated with the Antarctic Circumpolar Current, either in the vicinity of these islands or further afield, may have strong implications on their foraging behaviour. The relatively easy accessibility of the Prince Edward Islands from South Africa and their location between the main frontal systems bordering the Antarctic Circumpolar Current enable high-resolution synoptic field studies to be undertaken. Such studies have provided information on the impact changes in the large-scale ocean dynamics have on the local marine ecosystems

The changing form of Antarctic biodiversity

Antarctic biodiversity is much more extensive, ecologically diverse and biogeographically structured than previously thought. Understanding of how this diversity is distributed in marine and terrestrial systems, the mechanisms underlying its spatial variation, and the significance of the microbiota is growing rapidly. Broadly recognizable drivers of diversity variation include energy availability and historical refugia. The impacts of local human activities and global environmental change nonetheless pose challenges to the current and future understanding of Antarctic biodiversity. Life in the Antarctic and the Southern Ocean is surprisingly rich, and as much at risk from environmental change as it is elsewher

Population Structure of Humpback Whales from Their Breeding Grounds in the South Atlantic and Indian Oceans

Although humpback whales are among the best-studied of the large whales, population boundaries in the Southern Hemisphere (SH) have remained largely untested. We assess population structure of SH humpback whales using 1,527 samples collected from whales at fourteen sampling sites within the Southwestern and Southeastern Atlantic, the Southwestern Indian Ocean, and Northern Indian Ocean (Breeding Stocks A, B, C and X, respectively). Evaluation of mtDNA population structure and migration rates was carried out under different statistical frameworks. Using all genetic evidence, the results suggest significant degrees of population structure between all ocean basins, with the Southwestern and Northern Indian Ocean most differentiated from each other. Effective migration rates were highest between the Southeastern Atlantic and the Southwestern Indian Ocean, followed by rates within the Southeastern Atlantic, and the lowest between the Southwestern and Northern Indian Ocean. At finer scales, very low gene flow was detected between the two neighbouring sub-regions in the Southeastern Atlantic, compared to high gene flow for whales within the Southwestern Indian Ocean. Our genetic results support the current management designations proposed by the International Whaling Commission of Breeding Stocks A, B, C, and X as four strongly structured populations. The population structure patterns found in this study are likely to have been influenced by a combination of long-term maternally directed fidelity of migratory destinations, along with other ecological and oceanographic features in the region

Mesoscale features and phytoplankton biomass at the GoodHope line in the Southern Ocean during austral summer

Two sets of high-resolution subsurface hydrographic and underway surface chlorophylla (Chl a) measurements are used, in conjunction with satellite remotely sensed data, to investigate the upper layer oceanography (mesoscale features and mixed layer depth variability) and phytoplankton biomass at the GoodHope line south of Africa, during the 2010–2011 austral summer. The link between physical parameters of the upper ocean, specifically frontal activity, to the spatially varying in situ and satellite measurements of Chl a concentrations is investigated. The observations provide evidence to show that the fronts act to both enhance phytoplankton biomass as well as to delimit regions of similar chlorophyll concentrations, although the front–chlorophyll relationships become obscure towards the end of the growing season due to bloom advection and ‘patchy’ Chl a behaviour. Satellite ocean colour measurements are compared to in situ chlorophyll measurements to assess the disparity between the two sampling techniques. The scientific value of the time-series of oceanographic observations collected at the GoodHope line between 2004 to present is being realised. Continued efforts in this programme are essential to better understand both the physical and biogeochemical dynamics of the upper ocean in the Atlantic sector of the Southern Ocean. Keywords: chlorophyll a, mixed layer depth, ocean frontsAfrican Journal of Marine Science 2012, 34(4): 511–52