Mesozooplankton community structure and grazing impact in the region of the Subtropical Convergence south of Africa

Abstract

Mesozooplankton distribution and community structure in the region of the Subtropical Convergence (STC) south of Africa were investigated during the SAAMES III cruise in austral winter (June-July) 1993. Both the STC and an associated warm-core eddy (WCE) exhibited enhancements in zooplankton abundance, compared to the Subantarctic waters. Particularly, elevated zooplankton densities were found in the centre of the STC and in the region north of it as well as at the edge of the WCE. Copepods (mainly Pleuromamma abdominalis and Metridia lucens), euphausiids (Euphausia spinifera, E. similis and E. recurva), pteropods (Limacina spp.) and chaetognaths (Eukrohnia hamata and Sagitta spp.) dominated numerically and accounted for >60% of the mesozooplankton community. Both cluster and ordination analyses indicated the occurrence of two major plankton communities located north and south of the STC. These results confirm that the STC is one of the strongest regional zoogeographical barriers, but south of Africa, WCEs may represent the most important source of disturbance in the meso-/macroscale zonation of the zooplankton assemblages. The daily grazing impact of the epipelagic community was estimated to remove ~1-18% of phytoplankton standing stock and 20-165% of daily primary production. The highest rates of phytoplankton consumption were found in the centre of the STC and within the WCE, suggesting that the zooplankton assemblage transferred by the WCE may enhance substantially the mesoscale carbon flux in the Subantarctic zone south of Africa.