Spatio-temporal changes in the macrozooplankton community in the eastern Indian sector of the Southern Ocean during austral summers: A comparison between 1996 and 2018–2019

Several large-scale studies have examined the spatial and temporal (seasonal and interannual) variability in macrozooplankton communities in the eastern Indian sector of the Southern Ocean. In this study, variability in these communities was analyzed using samples collected by the RMT8 during the KY1804 survey, conducted between 80° and 150° E during the austral summer of 2018–2019. Furthermore, these findings were compared with those of the BROKE survey conducted in 1996. Using cluster analysis, the macrozooplankton community was divided into six groups. In both surveys, the zooplankton communities varied between the southern and northern stations of the sampling areas, though their distribution patterns differed between the two years. During the KY1804 survey, Thysanoessa macrura was more prevalent in the western region, while Themisto gaudichaudii and chaetognaths were more abundant in the eastern region; Salpa thompsoni dominated in the eastern region during the BROKE survey. Water temperature had the strongest influence on the macrozooplankton community during the KY1804 survey, whereas salinity was the primary influencing factor during the BROKE survey. This difference may largely reflect differences in sampling timing and latitudinal coverage, though a southward shift in the southern boundary of the Antarctic Circumpolar Current between the surveys may also have contributed. Hydrographic changes over the two decades between 1996 and 2019 likely affected the macrozooplankton community in this region, though differences in spatial and temporal survey coverage complicate interpretation of the results.