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

Abstract

The composition, biomass, distribution and predation impact of carnivorous zooplankton were investigated along a transect from SANAE to Cape Town during the second cruise of the South African Antarctic Marine Ecosystem Study (SAAMES II) in January–February 1993. The carnivore component of the pelagic community consisted mainly of six groups: amphipods, euphausiids, decapods, fish, chaetognaths and jellyfish. Amongst these, euphausiids (mainly Thysanoessa macrura and Nematoscelis megalops), chaetognaths ( Sagitta gazellae and Eukrohnia hamata) and myctophids were the most prominent groups throughout the transect. Jellyfish were abundant within the Marginal Ice Zone (MIZ), while mesopelagic/interzonal decapods exhibited a peak in biomass north of the Subtropical Convergence (STC). Generally, carnivores comprised 10 to 30% of total zooplankton biomass. In frontal regions, such as the Antarctic Polar Front (APF) and the STC, their contribution to total zooplankton biomass decreased to ≤6%. Carnivores overwhelmingly dominated the zooplankton stock between the Subantarctic Front (SAF) and the STC, accounting for 42 to 96% of the total. Gut content analysis showed that all the species investigated are opportunistic predators, generally consuming the most abundant mesozooplanktonic groups. Chaetognaths and euphausiids exhibited the highest impact on secondary standing stock along the transect. 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% ( x̄=12%) of the total stock, were found 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 ≤5% of the local mesozooplankton stock per day.