Horizontal and vertical distribution of zooplankton biomass in the southern Benguela, May 1983

Abstract

Zooplankton was sampled over four depth ranges with an obliquely towed, multiple, opening-closing RMT-1 × 6 net in May 1983 during an anchovy recruitment survey in the southern Benguela. This sampler allowed animals ranging in size from small copepods (> 200 μm diameter) to large euphausiids (> 3 500 μm diameter) to be collected quantitatively over the whole water column on the continental shelf. The stratification of biomass estimates into different layers is a significant improvement over existing data in the southern Benguela region. The discovery of subsurface peaks in the vertical distribution of zooplankton biomass at night allowed estimation of the zooplankton standing stock with more confidence than previously. The horizontal distributions of zooplankton biomass are, in general, consistent with patterns previously observed in the southern Benguela. Copepods were dominant inshore, whereas larger organisms such as euphausiids, chaetognaths and hyperiid amphipods were dominant farther offshore. The difference in the patterns of vertical distribution between inshore and offshore zooplankters is discussed in relation to predation pressure and population maintenance.