The autumn mesozooplankton community at South Georgia: biomass, population structure and vertical distribution

Mesozooplankton were sampled at shelf and oceanic stations close to South Georgia, South Atlantic during austral autumn 2004 with a Longhurst Hardy Plankton Recorder. Onshelf biomass ranged from 2.18 to 5.75 g DM m(-2) (0-200 m) and was dominated by the small euphausiid Thysanoessa spp. At the oceanic stations (10.57-14.71 g DM m(-2), 0-1,000 m) large calanoids, principally Rhincalanus gigas comprised similar to 47-52% of biomass. Here Calanus simillimus was still active and reproducing in surface waters (0-11.2 eggs fem day(-1) but R. gigas and Calanoides acutus were largely resident in the warm deep water and undergoing their seasonal descent. A comparison with spring and summer data indicated increased abundance and biomass from spring through to summer followed by a decline towards autumn particularly over the shelf. Autumn values in oceanic waters differed little from summer. Mesozooplankton biomass in the surface 200 m of the oceanic stations as a proportion of that found in the top 1,000 m ranged from 63 to 78% of the total in spring and 62-73% in summer, but was only 23-29% of the total in this study, following redistribution down the water column.