The hydrography and water masses of the Natal Bight, South Africa

Abstract

The Natal Bight is an unusually wide coastal offset on South Africa's east coast along an otherwise uniformly narrow shelf. Interaction with the deep sea is limited by the strong Agulhas Current along the shelf edge. This semi-enclosed body of water has an unusual role in the local shelf ecosystem and also plays an important triggering role in major perturbations to the trajectory of the Agulhas Current. We present the results of the first dedicated research cruise that has encompassed the hydrography of the whole Bight. Water in the Bight consisted of South Indian Subtropical Surface Water and Indian Tropical Surface Water. There was only a negligible effect of river runoff. The topographically induced upwelling cell at the northern end of the Bight was characterised by lower temperatures, higher salinities, higher nutrients and higher chlorophyll a values. This upwelled water dominated the northern part of the Bight on this occasion and there is substantial evidence that the upwelling cell supplies the bottom water for the whole bight. There was little comparable water exchange along the rest of the shelf edge. This cell therefore also controls the supply of nutrients to the whole Bight. The abnormal divergence of the Agulhas Current from the shelf edge near Durban during the cruise suggests the initial stages of a Natal Pulse. Surface temperature distributions and nitrate values indicate a cyclonic motion in this incipient pulse.