A comparative study of tar ball distribution and movement in two boundary current regimes

Abstract

During the period 1975 through 1978 sampling for tar balls was undertaken at 1473 oceanographic stations situated around southern Africa. While negligible concentrations of tar balls were measured at the majority of the stations, 21% of samples were in excess of 0.1 mg m⁻² with the highest recorded value being 232 mg m⁻². The extensive tar ball sampling programme undertaken around the Cape of Good Hope during 1977 and 1978 coincided with a study of surface currents in the area using plastic drift cards. Analysis of the drift-card returns from batch releases at stations where the tar ball concentrations exceeded 0.1 mg m⁻² has provided a conceptual image of the probable movement of the tar balls in the system and an indication of the vulnerability of some areas of the cost to chronic pollution.

The main features and conclusions are as follows:

• (a)

  Tar balls are moved southwards and westwards off the east coast from a probable formations zone in the western part of the Mozambique Channel north of 27°S by the swiftly flowing Mozambique/Agulhas Current.

• (b)

  Although the majority of the tar balls are probably carried offshore into the West Wind Drift area via the retroflexion of the Agathas Current south of South Africa, some tar balls appear to be advected onto the Agulhas Bank by the shear-edge eddies associated with the Agulhas Current.

• (c)

  As a result of oceanographic and meteorological influences, the breaches of northern Natal and the areas east of Cape Agulhas are very vulnerable to chronic tar ball pollution.

• (d)

  In view of the divergent nature of the Benguela Current system off the west coast and lower levels of tar balls in the region, beaches on this coast are not very vulnerable to pollution during summer, although during winter, when the upwelling relaxes, there is an increased risk.