Krill and zooplankton in the Western Prydz Bay region, September-November 1985

During the third Antarctic Division BIOMASS Experiment (ADBEX III) cruise (September to November 1985) to the western Prydz Bay region, Antarctica, a net sampling program was carried out using RMT 1+8 and ORI-2000 nets, in the pack-ice zone when the sea ice was near to its maximum northern extent. From these samples the composition and abundance of euphausiids and other zooplankton were investigated. A total of 75 species of zooplankton and ichthyoplankton were fully identified, as well as 15 other major taxa. The copepod Rhincalanus gigas was the most abundant zooplankter, with Thysanoessa macrura the most abundant euphausiid. The two chaetognaths Sagitta gazellae and Eukrohnia hamata were the predominant carnivores. Major zooplankton species appeared to be well established under the sea ice in October, compared with abundance estimates from the previous summer cruises (January to March). Early larval stages of Thysanoessa macrura and Euphausia frigida were abundant in October, a result of spawning as early as September and late stage furcilia larvae of Euphausia superba were also collected in late September-early October. A supplementary diving program found large numbers of krill under the sea ice off the Enderby Land coast showing that krill remain inshore during pack-ice development. The previous laboratory and field studies have shown that adult Euphausia superba either cease growing or shrink in size during winter due to the lack of food. Results from this study suggests that the smaller zooplankton may not find food limiting in winter