Temporal and spatial variations in copepod and euphausiid biomass off the southern and south-western coasts of South Africa in 1977/78

Abstract

On the basis of 519 night-time Bongo net (300 μm) samples taken monthly between August 1977 and August 1978 off the southern and south-western coasts of South Africa, the temporal and spatial patterns of copepod and euphausiids biomass were examined. The region was divided into three areas: St Helena Bay/Cape Columbine, Cape Peninsula and Agulhas Bank. Biomass (mg dry wt·m⁻²) was highest off the West Coast with similar proportions of copepods and euphausiids constituting 90–95 per cent of the total. Euphausiid biomass was significantly lower on the Agulhas Bank, where copepods contributed 68 per cent of the total, than on the West Coast. Monthly variations within each zooplankton group differed between the West Coast and the Agulhas Bank. Euphausiids did not respond to seasonal changes characteristic of the three areas. In the copepod group seasonality was found, principally in the Cape Peninsula area, and also in the St Helena Bay/Cape Columbine area where there was evidence of offshore displacement during the upwelling season (September-March). There was limited evidence of biomass being highest nearest the coast. Biomass variability (coefficient of variation) was highest nearest the coast and decreased markedly with distance offshore. Euphausiid biomass was generally more variable than copepod biomass.