Zooplankton structure and grazing in the Atlantic sector of the Southern Ocean in late austral summer 1993 - Part 1. Ecological zonation

The composition, distribution, abundance, biomass and size structure of mes ozooplankton, collected using Bongo nets in the top 300 m layer along a tra nsect between the Antarctic continent and Cape Town, were investigated duri ng the second South African Antarctic Marine Ecosystem Study (SAAMES II) in Jan.-Feb. 1993. Small (<10 mm) and medium (20-50 mm) size groups of zoopla nkton consistently dominated across the Southern Ocean. The highest zooplan kton densities were recorded at the Antarctic Polar Front (APF) and at the Subtropical Convergence (STC). Minor peaks in zooplankton densities were ob served in the southern vicinity of the Subantarctic Front (SAF) and APF. El evated zooplankton stocks were also found within the Marginal Ice Zone (MIZ ) and the Polar Frontal Zone. The lowest densities were recorded in the per manently open zone (MIZ-APF) and in the Subantarctic zone (SAF-STC). Copepo ds were generally important along the entire transect and formed the bulk o f zooplankton stock within the MIZ and in the Polar Frontal Zone (APF-SAF), accounting for at least 40-95% of total abundance and biomass. Euphausiids were also a prominent group along the transect. Their contribution was hig hest (up to 80% of total biomass) between the MIZ and the APF, mainly becau se of the occurrence of swarms of the Antarctic krill Euphausia superba. Tu nicates, Pyrosoma sp. and Salpa fusiformis, were found in great numbers onl y in the region of the STC and further north, while Salpa thompsoni was abu ndant at the southern boundary of the APF. Chaetognaths dominated samples n umerically and by mass in the Subantarctic Zone. Results obtained from clus ter and ordination analyses show that zooplankton community structure was w ell correlated with the position of various biogeographical zones separated by the main frontal systems of the Southern Ocean. Two major groupings of stations, separated by the SAF, were identified in these analyses. This fro nt separated the Antarctic and the subantarctic/subtropical assemblages, co nfirming its important role as a biogeographical boundary. (C) 2000 Elsevie r Science Ltd. All rights reserved.