Macrozooplankton diversity and distribution off South Orkney Islands

Zooplankton organisms are considered ecological bio-indicators used to monitor the sustainability and health of marine ecosystems. Climate monitoring shows that the western part of the Atlantic sector of the Southern Ocean is undergoing warming over the second half of the twentieth century. This study describes the macrozooplankton composition in the upper 200 m water column of the South Orkney Islands shelf, slope, and offshore areas during the austral summer of February 2016. In total, 24 taxa were identified (15 determined to species level) from 22 sampling stations. The dominating species in catch weight were Euphausia superba (43.4 %) and Salpa thompsoni (42.7 %). Lower species diversity was associated with the offshore stations while higher species diversity was found on the shelf and slope areas. Spatial distribution of biota was examined in relation to temperature, salinity, bathymetry, and surface chlorophyll a using a non-metric multidimensional scaling (NMDS) analysis. A hierarchical clustering revealed five distinct groups: two dominated by S. thompsoni, linked to offshore waters, two groups with mixed characteristics, greater diversity and located over the shelf and slope, and a group dominated by E. superba. This study establishes a baseline for long-term monitoring of macrozooplankton at the South Orkney Islands, aiding future assessments of species composition and distribution in response to environmental change.