THE SUMMER ZOOPLANKTON COMMUNITY AT SOUTH GEORGIA - BIOMASS, VERTICALMIGRATION AND GRAZING

Zooplankton abundance and biomass were determined during January 1990 at two stations to the north-west of South Georgia using a Longhurst H ardy Plankton Recorder (LHPR). At both shelf and oceanic station sites , zooplankton biomass, (excluding Euphausia superba), was found to be ca. 13 g dry mass m(-2). Copepods and small euphausiids dominated the catches. These estimates are over 4 times higher than values generally reported for the Southern Ocean and may reflect firstly, the high pro ductivity of the study area, secondly, the time of year, summer, when biomass for many species is maximal, and thirdly, the high sampling ef ficiency of the LHPR. Principal components analysis disclosed similari ties and differences between adjacent depth strata in terms of abundan ce, biomass and species composition. At both stations most variability occurred in the mixed layer (0-60 m) and thermocline (60-120 m) with depth horizons below this being more homogeneous. Diel migrations were observed for most taxa with abundance increasing in the mixed layer a t night. At the oceanic station, species and higher taxa belonging to the mesopelagic community were generally well spread throughout this d omain and, with the exception of Pleuromamma robusta and Metridia curt icauda, showed little evidence of migration. The grazing impact of the epipelagic community (copepods and small euphausiids) was estimated t o remove 3-4% of the microbial standing stock day(-1) and a conservati ve 25% and 56% of daily primary production at the oceanic and shelf st ations respectively.