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African Journal of Marine Science

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A demonstration of the hydrographic partition of the Benguela upwelling ecosystem at 26°40'S

CM Duncombe Rae

Abstract


Continuous CTD data from a series of recent cruises show that the distribution of the water mass characteristics in the central Benguela region from the Orange River mouth (28°38\'S) to Walvis Bay (22°57\'S) is discontinuous in the central and intermediate waters at about the latitude of Lüderitz (26°40\'S), Namibia. The central and intermediate water masses at the shelf edge and shelf break north of the Lüderitz upwelling cell have a high salinity relative to the potential temperature compared to similar waters south of the upwelling cell. It is shown that the feed waters for the wind-induced upwelling on the shelf to the north and south of the Lüderitz discontinuity are different in character and source. The distribution of the water masses shows that the shelf-edge poleward undercurrent provides lowoxygen water from different regions in the Atlantic Ocean to be upwelled onto the shelf. North of the Lüderitz upwelling cell, the central and intermediate waters come from the oxygen-depleted Angola Basin, whereas south of the discontinuity those waters are from the interior of the adjacent Cape Basin, which is less oxygen-deficient. This has implications for the dispersion of low-oxygen water and the triggering of anoxic events, and consequences for the biota on the shelf, including commercially important fish species.

Keywords: Benguela Current, environmental barrier, Lüderitz upwelling cell, South Atlantic, water masses
African Journal of Marine Science 2005, 27(3): 617–628



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