The hydrology off the South African south-western coast between Cape Point and Danger Point in 1975

Abstract

Monthly monitoring in 1975 of the hydrology of the south-western coast of South Africa revealed a strong seasonal cycle, with both upwelling and stratification most intense in summer and almost uniform conditions in winter. Uplift of cool, low-salinity Central Water (with typical t = 12°C, s = 35,0 × 10−3) to within 20–40 m of the surface occurred from January to April and from September to November, months in which SE winds were dominant. However, this water was observed to have upwelled to the surface only between February and April, and in winter it remained beneath 80 m. The upwelling response to wind was apparently not as rapid as off the Cape Peninsula to the west. Stratification was best developed offshore in summer, largely because of the uplift of cooler Central Water and the onshore advection of a layer of Subtropical Surface Water and Agulhas Bank water with salinities > 35,3 × 10−3. These conditions were concurrent with a subsurface front from Danger Point to Cape Point. This front, which would enhance westward flow at the surface, was on occasion modified by upwelling west of Danger Point and Cape Hangklip and by upwelling eddies in Sandown Bay and Walker Bay. Inorganic nutrients upwelled with the Central Water in spring and summer, but generally did not penetrate through the thermocline into the warmer, more-saline and nutrient-poor surface layer. In late autumn and winter, strong NW winds caused vigorous vertical mixing to 60–80 m deep, and nutrient concentrations in the upper layer increased. On the upper shelf maximum nutrient concentrations occurred from January to April, whereas on the lower shelf they occurred in April and May. These maxima were both largely concurrent with oxygen values of less than 3,5 mℓ·ℓ−1. Nutrient regeneration was indicated on the upper shelf, but on the lower shelf there was evidence of a continuation of the oxygen-poor undercurrent from the West Coast at a depth of 120–150 m. Very low oxygen content and associated nitrate-reduction were not encountered, probably owing to the limitations on the productivity of the region, and to the substantial vertical mixing in winter. © 1985 Taylor & Francis Group, LLC.