Zonal distribution and seasonal vertical migration of copepod assemblages in the Scotia Sea

Large, biomass-dominant Southern Ocean copepod species have been much studi ed, but small and mesopelagic species also play major roles in these ecosys tems. However, little is known of some basic aspects of their ecology. To a ddress this, the abundances of 23 copepod species and genera were analysed from 72 stations sampled during the Discovery Expeditions in the 1920s to 1 950s. Stratified net samples, usually to a depth of 1000 m, provided year-r ound coverage in the Scotia Sea from the Subantarctic Front to the Weddell- Scotia Confluence. Small copepods (Microcalanus pygmaeus, Ctenocalanus spp. , Oncaea spp. and Oithona spp.) formed similar to 75% of total copepod abun dance in the top 1000 m across all major zones. Oithona sop. composed simil ar to 40% of copepod numbers in the Polar Front area and to its south: furt her north their importance declined. All mesopelagic taxa except for the wa rmer-water species Metridia lucens and Pleuromamma robusta, extended throug hout the entire study area, with smaller regional differences than for the shallower-living species. The species showed a continuum of temperature ran ges, and there was no evidence that the Polar Front was a major biogeograph ic boundary to their distribution. Indeed, several important species, inclu ding Oithona spp. (mainly Oithona similis), Ctenocalanus spp., Metridia luc ens and Rhincalanus gigas reached maximum numbers in this area. Total copep od abundance was thus higher in the vicinity of the Polar Front than in any other region. Only two copepod families made pronounced seasonal vertical migrations: Eucalaniidae (Eucalanus longiceps and R. gigas) and Calaniidae (Neocalanus tonsus, Calanoides acutus, Calanus simillimus and Calanus propi nquus). Some evidence for a winter descent was found for Ctenocalanus spp. and some deeper-living groups: Euchaeta spp. and the Metridiidae, although their migrations were not so great as for the eucalanids and calanids.