South Georgia, Antarctica: a productive, cold water, pelagic ecosystem

The South Georgia region is characterised by high biomass and productivity of phytoplankton, zooplankton and vertebrate predators, Important commercia l fisheries have been based at the island since the late 1700s, initially e xploiting seals and whales, and currently taking krill Euphausia superba an d finfish. Despite studies dating from the beginning of the last century, t he causes of the high productivity remain unclear. The island lies within t he Antarctic Zone of the Antarctic Circumpolar Current, to the south of the Polar Front. The offshore waters to its north and east are affected by a n orthwards deflection of the Southern Antarctic Circumpolar Current Front, t ogether with waters from the Weddell-Scotia Confluence, Despite a retentive circulation over the shelf, the high productivity of phytoplankton and cop epods is widespread, occurring far downstream and possibly extending to the Polar Front. High phytoplankton concentrations (> 20 mg chlorophyll a m(-3 )) may be linked to enhanced supply of iron or reduced forms of nitrogen (u p to similar to3 mmol ammonium m(-3)). Although macronutrients are generall y not limiting in the Antarctic Zone, silicate concentrations of < 1 mmol m (-3) are growth-limiting at South Georgia in some summers, The growth seaso n is long and blooms of > 2 mg chl a m(-3) occur for 4 to 5 mo. Biomass of krill plus net-caught zooplankton in summer is around 15 to 20 g dry mass m (-2), equally dominated by krill and copepods. This greatly exceeds typical values for Antarctica, and is high compared to productive northern shelves , Zooplankton, and in particular krill, appear to have a pivotal role in re gulating energy flow in this food web, through selective grazing and possib ly also through nutrient regeneration. Abundances of krill and copepods are negatively related across a wide range of scales, suggesting direct intera ction through competition or predation. Evidence suggests that when phytopl ankton stocks are low, energy flow through krill is maintained by their fee ding on the large populations of small copepods. Metazoans and higher preda tors at South Georgia can feed across several trophic levels according to p rey abundance, and they regenerate substantial quantities of reduced nitrog en. Therefore we suggest that these groups have a controlling influence an lower trophic levels, both stabilising population sizes and maintaining hig h rates of energy flow. Hydrography, nutrient concentrations, phytoplankton , copepod, and krill biomasses fluctuate between years. Periodically (once or twice a decade), shortages of krill in summer result in breeding failure s among many of their predators, This appears to be a downstream effect fro m wider scale, Scotia Sea phenomena, although the processes involved are un clear. The elevated biomass and energy flows at South Georgia appear to be caused by locally enhanced growth rates; there is no evidence so far for an y physical concentration mechanism. Even for krill, which do not breed ther e, local growth rates are probably of a similar order to the biomass remove d by their main land-based predators in summer. Thus the transfer of energy to higher predators depends on local enhancement of fluxes through the foo d web as well as the supply of plankton to the island by the Antarctic Circ umpolar Current.