Plankton and nekton community structure in the vicinity of the South Sandwich Islands (Southern Ocean) and the influence of environmental factors

The South Sandwich Islands (SSI) are a biologically productive archipelago situated in the eastern Scotia Sea to the south of the eastward flowing Antarctic Circumpolar Current (ACC). The islands support important populations of higher predators, including several penguin species, seals and humpback whales. Despite this, the plankton ecology of the region has been little studied and information on mesoscale structure and environmental forcing of plankton ecology is particularly limited. We conducted a comprehensive oceanographic and net sampling campaign during the CCAMLR Area 48 Survey (January and February 2019), incorporating phytoplankton, mesozooplankton and macrozooplankton/nekton. Satellite chlorophyll-a (chl-a) data showed the development of a large bloom that was initiated two months prior to our study period at the south-eastern edge of the archipelago and propagated northwards along the eastern side, limited to the east by mesoscale features associated with the southern boundary of the ACC (SB). Multivariate cluster analysis revealed distinct mesoscale structure within the plankton community, with four spatially defined groups of phytoplankton and macrozooplankton/nekton, and three cluster groups of mesozooplankton. North of the SB, we found some spatial congruence between the three plankton assemblages, with a distinct, spatially coherent, cluster in each, corresponding to a warmer water community. Here, biomass was dominated by mesozooplankton, particularly calanoid copepods Rhincalanus gigas, Calanus propinquus, C. simillimus and Euchaetidae. The corresponding phytoplankton community was dominated by small diatoms, particularly Thalassionema spp., Pseudo-nitzschia spp., Fragilariopsis spp. and Chaetoceros spp., whilst Themisto gaudichaudii, Euphausia triacantha and myctophids were the major contributors to the macrozooplankton/nekton community. South of the SB, there was some spatial congruence between phytoplankton and macrozooplankton/nekton community structure on the western side of the archipelago, as well as on the eastern side that corresponded to the location of the bloom, but less association with mesozooplankton structure. Macrozooplankton/nekton structure was strongly driven by environmental conditions 1–2 months prior to the survey, including sea-ice distribution, surface phytoplankton concentration and productivity, whilst mesozooplankton was more tightly coupled to in-situ prevailing conditions such as surface temperature and integrated chl-a. Top-down pressure between trophic levels may have also had an influence on spatial patterns although direct evidence is lacking. Antarctic krill (Euphausia superba) was found with relatively low biomass at our net sampling sites (median biomass of 0.04 mg m−3 or