COMPARATIVE STRUCTURE OF THE MACROZOOPLANKTON MICRONEKTON COMMUNITIES OF THE SUBTROPICAL AND ANTARCTIC POLAR FRONTS

The composition, distribution, abundance, biomass and size-structure of macroplankton/micronekton communities at the Subtropical Convergence (STC) and the Antarctic Polar Front (APF) regions were investigated during the South African Antarctic Marine Ecosystem Study (SAAMES II, January-February 1993; SAAMES III, June-July 1993). A total of 115 and 32 macroplankton/micronekton species were found in the epipelagic zones of the STC and APF, respectively. Cluster analysis based on species composition indicated the occurrence of 3 different plankton communities: one in the STC region, another to the north of the APF and the third to the south of the APF. Although the APF and the STC were investigated in different seasons, average abundance and biomass were similar in both regions. Tunicates, euphausiids, decapods and myctophiid fishes dominated the total stock of the 2 frontal regions, in terms of both abundance and biomass. Both fronts exhibited considerable fluctuations in the abundance and biomass levels which appeared to covary with the spatial distribution of the phytoplankton stock in the area. The dominant size-classes were composed of a small group of 5 to 30 mm siphonophores, tunicates and euphausiids and a larger group of 40 to 80 mm euphausiids, chaetognaths, vertically-migrating decapods and myctophiid fishes.