General biological features of the South Atlantic
Zooplankton Processes

Zooplankton studies in the Benguela Current have largely focused on the regions of the most intense fishing operations in the Southwestern Cape (33-35°S), and in the vicinity of Walvis Bay (23°S). Within these areas research has focused on the distribution and seasonal abundance of particular taxonomic groups (e.g., euphausiids, copepods, cladocerans, tunicates, chaetognaths), followed by more process-orientated and quantitative studies of life history strategies, biomass fluctuations and productivity estimates of the dominant species of copepods and euphausiids, which are the most important components relevant to fisheries. ( Gb28), ( Gb29)

Hutchings et al. (1991) described the biomass and production of zooplankton in the Benguela region, based on the limited measurements available. Highest biomass occurrs downstream from major upwelling sites both off Namibia (23-25°S; ( Gb28) and off the SW Cape coast (32-33°S; ( Gb29), on the inner to midshelf region. The mean biomass of zooplankton was estimated to be quite similar in the northern and southern Benguela, approximately 0.8 to 1.0 g C mö-2 in the upper 200 m. However, Timonin et al. (1992) concluded that moderate to high concentrations of deep-living Calanoides carinatus live at depths of 300-700 m, and total biomass in the offshore zone is therefore underestimated. In the extreme south and north, where warm water boundaries occur, biomass is reduced. Few seasonal studies are available; Andrews and Hutchings (1980) show a distinct but erratic seasonal cycle in zooplankton biomass in the southern Benguela at 34°S, with highest biomass occurring during the summer upwelling season ( Gb30). Seasonality is reduced slightly to the north (Pillar, 1987), while off Namibia Kollmer (1963) showed little seasonal variation in zooplankton biomass in a limited area close to Walvis Bay. Elsewhere in the Benguela region zooplankton surveys have been limited to irregular sampling on a seasonal basis, but usually with comparisons of “active upwelling” and “quiescent” conditions (e.g., Timonin et al., 1992).

The adaptations of the dominant copepod Calanoides carinatus and the euphausiid Euphausia lucens to the shelf circulation in the southern Benguela have been studied by Verheye et al. (1991, 1992) and Pillar (1987), Pillar and Stuart (1988), and Pillar et al. (1992). Similar studies of the dominant euphausiids Nyctiphanes capensis and Euphausia hanseni were conducted off Namibia by Barange and Pillar (1992). Laboratory rearing and shipboard measurements of grazing, egg production and moulting rates have been made for a number of copepod species and Euphausia lucens in the southern Benguela (Stuart and Pillar, 1988, 1990; Borchers and Hutchings, 1986; Attwood and Peterson, 1989; Peterson and Painting, 1990). A 30-day time series at a fixed location in St. Helena Bay in the southern Benguela (Bailey and Chapman, 1991) allowed the time scales relating to pulsed upwelling to be linked through the phytoplankton successional sequences (Mitchell-Innes and Walker, 1991; Pitcher et al., 1991) to the responses and behaviour of the dominant copepods (Armstrong et al., 1991; Verheye, 1991). Transect studies across the southern Benguela shelf sampled water of differing ages after upwelling have allowed primary production, phytoplankton community structure and cell size to be related to zooplankton grazing and production (Peterson et al., 1990; Walker and Peterson, 1991).