Nutrient enrichment off Port Stephens: the role of the East Australian Current

Abstract

In January 1997, a bloom of the dinoflagellate Noctiluca scintillans occurred off Port Stephens, on the New South Wales (NSW) central coast. The nutrient enrichment that presumably caused the bloom was apparently unrelated to local winds. The purpose of this study is to investigate the role of the East Australian Current (EAC) in promoting nutrient-rich slope water into the euphotic zone off Port Stephens. To this end, a regional model of the NSW coast is presented and a new mechanism for upwelling is proposed. The simulations indicate that uplifting of slope water results from the interaction of the EAC with the continental shelf topography near Laurieton, located to the north of Port Stephens. The uplifted water is consequently advected along the shelf to Port Stephens, where the EAC separates from the coast. As a result of this divergence, the uplifted slope water is upwelled to the surface, and outcrops over the shelf. In situ velocity and temperature measurements are presented as evidence for the upwelling mechanism and the proposed path of the upwelled slope water is inferred by modelling the dispersal of a passive concentration tracer injected to the north of the upwelling region. The proposed upwelling mechanism may provide insight into topographically induced upwelling in other western boundary current regions.