Predation impact of carnivorous macrozooplankton and micronekton in the Atlantic sector of the Southern Ocean

The composition, biomass, distribution and predation impact of carnivorous zooplankton were investigated along a transect from SANAE to Cape Town duri ng the second cruise of the South African Antarctic Marine Ecosystem Study (SAAMES II) in January-February 1993. The carnivore component of the pelagi c community consisted mainly of six groups: amphipods, euphausiids, decapod s, fish, chaetognaths and jellyfish. Amongst these, euphausiids (mainly Thy sanoessa macruva and Nematoscelis megalops), chaetognaths (Sagitta gazellae and Eukrohnia hamata) and myctophids were the most prominent groups throug hout the transect. Jellyfish were abundant within the Marginal Ice Zone (MI Z), while mesopelagic/interzonal decapods exhibited a peak in biomass north of the Subtropical Convergence (STC). Generally, carnivores comprised 10 t o 30% of total zooplankton biomass. In frontal regions, such as the Antarct ic Polar Front (APF) and the STC, their contribution to total zooplankton b iomass decreased to less than or equal to 6%. Carnivores overwhelmingly dom inated the zooplankton stock between the Subantarctic Front (SAF) and the S TC, accounting for 42 to 96% of the total. Gut content analysis showed that all the species investigated are opportunistic predators, generally consum ing the most abundant mesozooplanktonic groups. Chaetognaths and euphausiid s exhibited the highest impact on secondary standing stock along the transe ct. Total daily predation impact varied considerably along the transect but generally accounted for <5% of mesozooplankton stock. The highest impact, ranging from 6.5 to 20.5% ((x) over bar = 12%) of the total stock, were fou nd in the region between the SAF and the STC, in conjunction with the high densities of chaetognaths. Carnivores may potentially contribute a regional downward flux of faeces equivalent to less than or equal to 5% of the loca l mesozooplankton stock per day. (C) 1999 Elsevier Science B.V. All rights reserved.