Predation impact of carnivorous macrozooplankton in the vicinity of the Prince Edward Island archipelago (Southern Ocean) in austral autumn 1998

The composition, biomass, feeding and predation impact of carnivorous macrozooplankton (>2 cm) on mesozooplankton (0.2–2 cm) in the sub-Antarctic waters of the southwest Indian Ocean were investigated at 19 stations in austral autumn (April/May) 1998. Zooplankton abundances and biomass ranged from 6.9 to 95.2 ind m−3 and between 1.8 and 18.1 mg Dwt m−3, respectively. Throughout the investigation, mesozooplankton comprising mainly copepods numerically and by biomass dominated net samples. Among the copepods, Calanus simillimus, Clausocalanus brevipes, Ctenocalanus vanus, and Oithona spp. dominated. The carnivore component of the macrozooplankton consisted mainly of five groups: decapods, amphipods, chaetognaths, euphausiids and gelatinous zooplankton. Among these, chaetognaths (Eukrohnia hamata and Sagitta gazellae) and euphausiids (Nematoscelis megalops and Euphausia longirostris) were the most prominent. Collectively, the carnivorous macrozooplankton comprised between 11% and 72% of total zooplankton biomass. Total predation impact of the carnivorous macrozooplankton varied considerably but generally accounted for <5% (range 0.7–44%) of the total mesozooplankton standing stock. Tentative calculations suggest that carnivorous macrozooplankton may contribute, via vertical migrations and production of fast sinking faecal pellets, to a downward flux of carbon equivalent to up to 9% of the total mesozooplankton stock per day within the Antarctic Polar Frontal Zone. As a consequence, carnivorous macrozooplankton may increase the localised efficiency of the biological pump.