Mesozooplankton community structure across the Antarctic Circumpolar Current to the north of South Georgia: Southern Ocean

During four summer seasons mesozooplankton community composition and structure in relation to water mass distribution were investigated along a 735-km transect running across the Antarctic Circumpolar Current (ACC) to the north-west of the island of South Georgia, Southern Ocean. Samples were obtained each year during December–January from the top 200 m of the water column at 22 stations spaced 35 km apart. Cluster analysis revealed four station groupings that were geographically consistent with the different water masses identified on the basis of temperature and salinity properties along the transect. A Sub-Antarctic Zone (SAZ) community characterised by low overall plankton abundance was present at the northernmost end of the transect on three of the four cruises, separated from a Polar Frontal Zone (PFZ) community by the Sub-Antarctic Front (SAF). The PFZ community lay between the SAF and the Polar Front (PF) and was characterised by highest overall abundance and little interannual variability. Two Antarctic Zone (AAZ) communities were found south of the PF that, although taxonomically similar, differed in overall abundance. Although there were significant differences in nutrients (e.g. silicate) and phytoplankton (Chl a) between the different water masses, these factors only weakly correlated with plankton community structure. Copepods were the largest contributors to total abundance within all station groupings (median percentage 83–90% of total) and pteropods were also proportionately abundant in the PFZ (11%). With the exception of pteropods (≤11%) and ostracods (≤3%) all other non-copepod taxa contributed <1% to total abundance. All station groups were characterised by varying proportions of a relatively small subset of species, many of which were present throughout the transect, for example, Oithona similis, Ctenocalanus spp., Euchaeta antarctica, and Rhincalanus gigas. Others were particularly characteristic of different station groups, thus Neocalanus tonsus, Clausocalanus ingens, and Calocalanus spp. were characteristic of the SAZ with few in the PFZ and none in the AAZ. Microcalanus pygmaeus was particularly abundant in the AAZ compared to other regions, as was the polychaete, Pelagobia longicirrata. Other taxa, although widespread, tended to be typical of particular water masses, for example, Calanus simillimus and Limacina helicina in the PFZ, Calanoides acutus, appendicularians and P. longicirrata in the AAZ, and ostracods and chaetognaths in the SAZ. The close physical and biological coupling observed across the ACC confirms the frontal zones and particularly the PF as features across which community properties change in the Atlantic sector of the Southern Ocean.