Zooplankton structure and grazing in the Atlantic sector of the Southern Ocean in late austral summer 1993 - Part 2. Biochemical zonation

The composition, biomass, distribution and grazing impact of dominant compo nents of the meso- and macrozooplankton community were investigated along a transect occupied in the Atlantic sector of the Southern Ocean during the second cruise of the South African Antarctic Ecosystem Study conducted in l ate austral summer (Jan.-Feb.) 1993. Total zooplankton abundance along the transect ranged from 6 to 161 ind. m(-3), and biomass levels ranged from 4. 2 to 80.5 mg DWt m(-3). Elevated abundances and biomass were recorded at s tations occupied in the vicinity of the Marginal Ice Zone (MIZ), the Antarc tic Polar Front (APF) and Subantarctic Front (SAF). Throughout the investig ation mesozooplankton, comprising mainly copepods, dominated numerically an d by biomass accounting for > 80% of the total. South of the APF, the copep ods Calanus propinquus, Calanoides acutus and Metridia gerlachei numericall y dominated zooplankton counts, while north of the front the zooplankton co mmunity was dominated by the copepods Calanus simillimus, Rhincalanus gigas and Lucicutia ovalis. Grazing impact of the six most abundant zooplankton taxa, accounting for up to 90% of all zooplankton counted at each station, demonstrated large spatial variability ranging from 0.01 to 18% of the chlo rophyll (chl-n) standing stock or up to 89% of the daily phytoplankton prod uction (DPP). The highest grazing impact along the transect was recorded wi thin the Polar Front Zone where zooplankton removed between 4 and 18% of th e chl-a standing stock or between 53 and 89% of DPP daily. Outside this reg ion, zooplankton grazing generally corresponded to < 5% of the integrated c hl-a or < 20% of the daily DPP. The implications of the spatial differences in grazing impact on the efficiency of the biological pump are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.