Major pelagic borders of the Benguela upwelling system according to euphausiid species distribution

Abstract

The distribution patterns of 21 species of euphausiids collected during eight research cruises extending from 17°30′ to 32°30′S and offshore to 135 miles were studied by means of Principal Component Analysis. The existence of possible environmental borders for zooplankton populations was explored and discussed in relation to the main hydrographic features of the Benguela upwelling system. Three main boundaries were identified: the shelf-break zone; the area between 19 and 20°S; and the northernmost extension of the Lüderitz upwelling cell (24°30′S). The first boundary separates inshore and shelf species from shelf-break and oceanic species, revealing the boundary between shelf and oceanic circulation. It is reinforced by the strong equatorward and poleward flows that occur over the shelf-break at different depths. The boundary at 19–20°S was related to the general surface, wind-driven flow of the Benguela and its interaction with the Angola Current. Knowledge of the temporal variability associated with this boundary may well be enhanced by monitoring the extension of the proposed indicator species, Euphausia americana and E. gibboides. The turbulent regime associated with the perennial activity of the Lüderitz upwelling cell defines the third main boundary zone. It partially separates inshore species of the northern and southern Benguela and reflects the relative dominance of cross-shelf or alongshore currents