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A detailed, multi-disciplinary investigation of the Angola–Benguela Frontal Zone (ABFZ) was carried out in the austral autumn of 2004. Both satellite remote monitoring and hydrographic observations demonstrate an unusually stable state of affairs before and during this survey, suggesting that it was representative of the frontal system during undisturbed – or average – conditions at the time. Sea surface temperatures from satellite were mirrored by distributions of near surface salinities and nutrients. Upwelled water was Eastern South Atlantic Central Water (ESACW) at the two southern transects, and thermocline water in the north. Between the thermocline and the level of the Antarctic Intermediate Water (AAIW), South Atlantic Central Waters (SACW) were oxygen-deficient and nutrient-rich along the two northern transects, and also within gyral disturbances at the offshore parts of the southern transects. In the southwest ESACW occurred, and Angola Current water covered the offshore and upper part of the northernmost transect. Seaward of the upwelling front the depth of the thermo- and halocline decreased toward the north, and their vertical gradients increased. The predominant meridional geostrophic flow was northward, except for the area within and north of the ABFZ. Isopleths indicate a poleward undercurrent along the shelf edge. This undercurrent was shallow at the two northern transects, and at shelf-edge depth in the south. The undercurrent's direction stayed constant during the cruise and transported oxygen-deficient, nutrient-rich SACW as well as tropical fish larvae – adapted to low oxygen conditions – southwards to at least 22° S. This is in contrast to earlier theories of spawning here. Geostrophy and distribution patterns of indicator species indicate transport offshore within and along the ABFZ. Except for the undercurrent and gyral disturbances, the offshore ABFZ separates the tropical fish larval community from temperate and Southern Ocean ones. These gyral disturbances had an anticyclonic surface expression, and on the basis of water mass, nutrients, and biological composition they were most likely spawned from the undercurrent. A comparison with earlier surveys made during a Benguela Nino showed that the ABFZ responded hydrographically and faunistically to that warm event by a southward displacement of at least four degrees of latitude and the intrusion of tropical, shallow-living fish larvae into Namibian waters. Historical abundance data of fish larvae for species of commercial importance were in part overestimates, and some historical cumulative distributions seem to have been mistaken for “higher abundances” in the past. When compared with our autumnal data, historical data do not indicate any decadal decline. |
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