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

Abstract

The composition, biomass, distribution and grazing impact of dominant components 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 late 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 stations occupied in the vicinity of the Marginal Ice Zone (MIZ), the Antarctic Polar Front (APF) and Subantarctic Front (SAF). Throughout the investigation mesozooplankton, comprising mainly copepods, dominated numerically and by biomass accounting for >80% of the total. South of the APF, the copepods Calanus propinquus, Calanoides acutus and Metridia gerlachei numerically dominated zooplankton counts, while north of the front the zooplankton community 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 chlorophyll (chl- a) standing stock or up to 89% of the daily phytoplankton production (DPP). The highest grazing impact along the transect was recorded within the Polar Front Zone where zooplankton removed between 4 and 18% of the chl- a standing stock or between 53 and 89% of DPP daily. Outside this region, zooplankton grazing generally corresponded to <5% of the integrated chl- 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.