Antarctic mesozooplankton community structure during BROKE-West (30°E–80°E), January–February 2006

The distribution of mesozooplankton, based on the catch by a RMT1 net, in the upper 200 m of the South Western Indian Ocean sector (30°E–80°E) of the Southern Ocean was examined during the large-scale BROKE-West survey in summer 2006. Multivariate analyses revealed four groups of zooplankton that could be broadly linked to oceanographic features. The first group, occurring south of the southern boundary of the Antarctic Circumpolar Current (sbACC) and west of 40°E, was associated with waters from the eastern limb of the Weddell Gyre. The group was typified by moderate abundance (mean: 11,251 ind. 1000 m−3) and included foraminiferans, appendicularians and small copepods. Group 2 was composed of stations found in the deeper, warmer waters lying between the sbACC and the southern ACC front (sACCf), along with a few stations north of the sACCf. Sites in this group exhibited the highest mean zooplankton abundance (81,750 ind. 1000 m−3) and comprised large numbers of small copepods and appendicularians. Sites in group 3 fell south of the sbACC, were situated east of 50°E, and generally located near the Antarctic slope current (ASC), a strong and narrow jet of westward flowing water. Typical species for this group included Euphausia crystallorophias, Fritillaria spp. and Metridia gerlachei. Finally, group 4 represented neritic sites in the far southwestern corner of the survey region. Abundances were lowest (mean: 2588 ind. 1000 m−3) and the assemblage was dominated by E. crystallorophias, Neogloboquadrina pachyderma and Fritillaria spp. The four groups differed more by abundance than by composition. The suite of environmental variables that best correlated with patterns in the species data included chlorophyll a concentration, proximity to the ASC and length of time without an ice cover, all features which indicated that large scale oceanographic processes were underpinning the patterns in mesozooplankton distribution.