The Marine Ecosystem of the Sub-Antarctic, Prince Edward Islands

Abstract

[from the Introduction]Straddled between the northern and southern boundaries of the Antarctic CircumpolarCurrent (ACC), Sub-antarctic islands are typically oceanic; experiencing moist, cool andwindy climates. They are classified as regions, in which the terrestrial and marineecosystems are relatively simple and extremely sensitive to perturbations. One suchexample are the Prince Edward Islands - the most southerly part of South Africa’s officialterritory. The islands are located in the Indian sector of the Southern Ocean atapproximately 46°50’S and 37°50’E (Figure 1). The nearest landfall is the Crozet IslandGroup 950 km to the east, while South Africa lies over 2 000 km northwest. The islandsconsist of Marion and Prince Edward Island (Figure 1 - insert), two volcanic outcropsapproximately 250 000 years old, but still active. Marion Island covers an area of 270 km2;whereas Prince Edward Island – 19 km to the north-east – is only about 45 km2 in extent.The islands rise steeply from a region of complex bottom topography with a shallowsaddle, between 40 and 200 m deep, separating Prince Edward from Marion Island.Intensive investigations carried out on the oceanic frontal systems south of Africa(Lutjeharms & Valentine, 1984; Duncombe Rae, 1989 a,b; Belkin & Gordon, 1996) haveshown that the Prince Edward Islands lie directly in the path of the ACC, sandwichedbetween the Sub-antarctic Front (SAF) and the Antarctic Polar Front (APF). As such, theseislands provide an ideal ecological laboratory for studying how shifts in atmospheric andoceanic circulation patterns in the Southern Ocean will increase the ease in which theseislands, their ecosystems and their ocean surrounds can be invaded by alien species(Smith, 2002).The Prince Edward Islands, like many other oceanic islands within the Southern Ocean, areseasonally characterised by vast populations of marine organisms and a diversity andabundance of seabirds that use the islands as breeding grounds (Bergstrom & Chown, 1999;& Bester, 2008). It is estimated that the islands support over 5 million breeding pairs oftop predators including flying seabirds, penguins and seals during the peak in breedingseason. The energy necessary to sustain these top predators is derived from the surrounding marine environment. Changes in the marine ecosystem in response to global climate changeare therefore, likely to dramatically influence the populations of top predators thatseasonally occur on the islands (Ryan & Bester, 2008).