Nutrient enrichment of the Sydney continental shelf

Abstract

Nutrient-rich waters arrived at the continental shelf at Sydney in late January 1992 in two ways: as an intrusion from the nearby continental slope and as a cold upwelled plume originating several hundred kilometres farther north. With the former, an undercurrent flowed northward on the upper continental slope south of where the nearshore edge of a warm anticyclonic eddy separated from the shelf and curved out to sea. The undercurrent rose onto the floor of the shelf and spread shoreward at least to the 60-m isobath as an intrusion of slope water. The other source of nutrients, the upwelled plume from the north, probably resulted from the East Australian Current spreading onto the shelf and driving an Ekman bottom boundary layer shoreward, where it upwelled to the surface and was then advected southward. Very high values of fluorescence at 20-40 m depth in the plume suggested a significant phytoplankton bloom. The plume was not continuous at the surface for the final 100 km of its passage to Sydney, rather taking the form of 40-km-long 'slugs' moving at -0.3 m s^-1. It was, however, continuous beneath the surface. From Sydney it was carried out to sea around the perimeter of the anticyclonic eddy.