Twenty-five years of physical oceanographic research at the Prince Edward Islands

The Prince Edward Islands constitute the southernmost part of the territory of South Africa. Home to millions of nesting seabirds of a number of speci es as well as a haulout for certain threatened marine mammals, the islands have been declared a protected natural region. Surrounded as they are by va st tracks of ocean, it has long been recognized that the ocean environment of the Prince Edward Islands must be crucial to the ecosystem of the island s. Oceanic research on this environment has gone through a number of phases over the past 25 years. First,;severely limited observations suggested a r egion of upwelling in the lee of the islands. During the 1970s and 1980s, r egular observations in the general vicinity succeeded in locating the islan ds relative to the main oceanic fronts and for the first time described edd ies in the region. Subsequent observational programmes at the islands found no upwelling, but instead the recurrent presence of an eddy on the shelf b etween the islands. More regular and more extensive subsequent surveys have not found such an eddy to be a regular feature. Two oceanographic cruises covering large areas both upstream and downstream of the islands during the 1990s have shown that it is an unusually variable ocean region with both c yclonic and anticyclonic eddies of unknown origin. These mesoscale features have been linked to the primary productivity of the islands' surroundings. Satellite altimetry has now indicated that the origin of these eddies may be where the core of the Antarctic Circumpolar Current crosses the South-We st Indian Ridge, upstream of the islands. A study has therefore now been pr oposed to investigate this possible source of eddies. A successful project will give definitive answers to questions regarding the origin of the ocean ic variability at the Prince Edward Islands.