An overview of meso- and macrozooplankton studies (including Antarctic krill) in Division 58.4.1 from the multidisciplinary ecosystem survey by RV Kaiyo-maru in 2018/19 (KY1804 survey)

This document summarizes findings from studies on meso- and macrozooplankton conducted on data collected during an expedition on board RV Kaiyo-maru in CCAMLR division 58.4.1 in 2018/2019 (KY1804 survey). During the KY1804 survey, samples were collected from 15-200 m depth (epipelagic layer) using a Rectangular Midwater Trawl (RMT) 1+8 on a predetermined grid of stations. Target RMT stations were additionally performed at various depths aiming to identify species composition from acoustic targets. A Surface and Under Ice Trawl (SUIT) was used to sample the upper two meters of the water column (surface layer). Samples were used to investigate the meso- and macrozooplankton community structure in both depth layers, and horizontal variation herein. Comparing data from both nets allowed for the study of changes in vertical distribution and potential diel vertical migration strategies. Further in-depth investigations on the distribution of life stages were performed for large copepod species, larval stages of euphausiids and post-larval Antarctic krill (Euphausia superba). The epipelagic macrozooplankton community was dominated by the amphipod Themisto gaudichaudi, the krill Thysanoessa macrura and the gastropod Limacina helicina, while the mesozooplankton community structure was dominated by Oithona sp. and Calanus acutus. The zooplankton community at the surface was dominated by E. superba, T. gaudichaudi and Calanus propinquus. For both the 0-2 m and the 15-200 m depth layer there were marked differences in macrozooplankton community structure between the areas west and east of approximately 120˚E, corresponding to Leg 1 (mid-December-early January) and Leg 2 (late January-late-February) of the expedition, respectively. In addition, the southernmost stations of both depth layers showed a different community structure. Differences were likely related to environmental properties such as temperature and the recent presence of sea-ice, which also varied between legs. Particularly in the epipelagic the Antarctic Slope Front seemed to have a marked influence on macrozooplankton community structure. Additionally, several species seemed to shift their vertical distribution between the eastern and western side of the sampling area which, in combination with environmental properties, may be attributed to seasonal progression and ontogeny because of the difference in survey timing between the legs. The post-larval Antarctic krill distribution also differed between east and west, where the species was distributed relatively evenly across the sampling area in the west, while they concentrated in the slope area in the east. A marked part of the Antarctic krill population resided in the upper two meters of the water column, particularly in the western side of the sampling area. The krill here were mainly juveniles, indicating that part of the population may be overlooked by conventional nets and echosounders.