Hydrography of a shelf ecosystem inshore of a major Western Boundary Current

Abstract

The Agulhas Current, the largest Western Boundary Current in the Southern Hemisphere, strongly influences the oceanographic conditions of its adjacent shelf system. As a result of limited societal drivers, such as large-scale fisheries and marine mining, the South African southeast shelf, between Port Elizabeth and Port Shepstone, is one of the least studied and under-sampled shelf systems in Southern Africa. This study describes hydrographic conditions from the first two shelf-wide high-resolution surveys (January/February and July/August 2017) of this region. These data provide a baseline of environmental conditions and hydrographic processes across the shelf. The Agulhas Current, as well as cyclonic mesoscale eddies along the inshore edge of the Current, were observed to strongly drive conditions on the shelf. Substantial differences in the impacts of these cyclonic eddies were evident, with the smaller less intense January/February eddy inducing much stronger upwelling than the larger more intense July/August eddy. Indications of seasonality in temperature, salinity and chlorophyll-a were observed in the surface layers. Surface waters were 4 °C warmer in January/February than in July/August. Considerably lower salinities (33.77 g kg−1) were observed in January/February as a result of high rainfall and subsequent increased river outflow. Upwelling mechanisms such as divergence-induced upwelling, Ekman veering in the bottom boundary layer, Ekman pumping associated with cyclonic eddies, were observed to uplift colder, nutrient-rich water from deeper layers onto the mesotrophic shelf. These colder waters ultimately reached the surface as a result of wind-driven Ekman transport and vertical mixing. Tropical Surface Water, Subtropical Surface Water and South Indian Central Water were observed on the shelf. Red Sea Water as well as Antarctic Intermediate Water generally occupied the slope regions except where cyclonic eddies uplifted intermediate waters onto the shelf. The shelf-wide dissolved oxygen levels (>3 ml l−1) suggested that oxygen availability adequately facilitated the survival of the existing shelf biology.