Zooplankton dynamics in the eastern Atlantic sector of the Southern Ocean during the austral summer 1997/1998—Part 1: Community structure

Zooplankton abundance, biomass and community structure in the top 300 m layer were studied along the 6°E meridian between 49 and 65°S during a Scandinavian/South African Joint Global Ocean Flux Study (December 1997–January 1998) onboard the S.A. Agulhas. Along the transect, total abundance and biomass of zooplankton ranged from 8 to 384 ind m−3 and 1.4 to 64.1 mg DW m−3, respectively. The general pattern in zooplankton distribution observed was an increase in density between Spring Ice Edge and the Antarctic Polar Front (APF) regions. Copepods numerically dominated zooplankton along the entire transect accounting for 68–97% of the total abundance and 24–90% of the total zooplankton standing stock. Other important contributors to total biomass, particularly within the Winter Ice Edge (WIE) and APF regions, were cnidarians and ctenophores (up to 50%), pteropods (up to 35%), tunicates (up to 30%), chaetognaths (up to 25%) and euphausiids (up to 23%). Numerical analyses identified three major zooplankton groupings, coinciding with the three regions of investigation. A dramatic change in the zooplankton composition occurred at the APF region as several copepod and euphausiid species were found on either side of the front only. A second change in the species composition was observed at the WIE, emphasizing the ecological importance of the frontal zone associated with the winter ice edge.