Zooplankton of the northern Benguela region in a quiescent upwelling period

The general features of the horizontal and vertical distribution of the main zooplankton groups in the northern Benguela upwelling region during a period of abated upwelling in April 1986 are discussed. Three different types of water were detected: (i) an inshore strip of recently upwelled water; (ii) oceanic water over the slope; and (iii) Angola Current water penetrating from the north. Abundance of the most representative zooplankton groups, as well as of the total volume of zooplankton, was highest in the shelf region, particularly south of Walvis Bay. Zooplankton was most abundant in areas where phytoplankton concentrations were high and offshore transport was low. There were significant differences between the abundance levels of copepods, euphausiids, chaetognaths, and fish eggs and larvae in the different layers of the water column sampled, with the highest concentrations in the surface layers. The sharpest vertical gradients were observed at stations affected by Angolan water, where the thermocline was very strong. In contrast, inshore, where stratification was low, no such vertical gradients existed.