Zooplankton communities at the sea surface of the eastern Indian sector of the Southern Ocean during the austral summer of 2018/2019

Knowledge on the distribution of zooplankton in the many unique habitats of the Southern Ocean is essential for understanding food web dynamics, assessing the impacts of environmental change and for managing the exploitation of marine living resources. Variation in the distribution of zooplankton may occur in the horizontal as well as the vertical plane, and the latter may show a diel cycle (diel vertical migration or DVM). Conventional sampling methods, including several types of nets and acoustics, often undersample or ignore the top 10 m of the water column. The surface waters may, however, host a specific zooplankton community and therefore be an important foraging ground for higher trophic level predators. In order to investigate the importance of the surface waters for understanding the distribution of species and potentially improving abundance estimates, the upper two meters of the water column were sampled in the eastern Indian sector of the Southern Ocean using a Surface and Under Ice Trawl (SUIT). Findings were compared to the zooplankton community structure in the epipelagic (15–200 m). Results showed that the surface zooplankton community could largely be divided into two regions. The surface community of the western side of the sampling area hosted large numbers of Antarctic krill, Euphausia superba, which were only present in low densities in the epipelagic depth layer. Densities of Limacina helicina were also relatively high in the west. The copepod Calanus propinquus and the amphipod Themisto gaudichaudii were present in relatively large numbers throughout the sampling area. T. gaudichaudii was the dominant species of the surface in the eastern side of the sampling area in the absence of Antarctic krill. Apart from cirripedia nauplii, no species were uniquely found in the surface water compared to the 15–200 m depth layer. Surface water sampling revealed patterns in vertical distribution and DVM, and showed that these patterns changed between the first and second half of the expedition. This could partially be explained by environmental variables but was likely also a result of sampling time and location, and associated variation in the size and ontogeny of species. Results revealed the impact of undersampling the surface layer regarding knowledge on distribution and vertical migration patterns of zooplankton species.