The South Georgia region is characterised by high biomass and productivity of phytoplankton, zooplankton and vertebrate predators. Important commercial fisheries have been based at the island since the late 1700s, initially exploiting seals and whales, and currently taking krill Euphausia superba and finfish. Despite studies dating from the beginning of the last century, the causes of the high productivity remain unclear. The island lies within the 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 northwards deflection of the Southern Antarctic Circumpolar Current Front, together with waters from the Weddell-Scotia Confluence. Despite a retentive circulation over the shelf, the high productivity of phytoplankton and copepods is widespread, occurring far downstream and possibly extending to the Polar Front. High phytoplankton concentrations (>20 mg chlorophyll a m , 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 regulating energy flow in this food web, through selective grazing and possibly also through nutrient regeneration. Abundances of krill and copepods are negatively related across a wide range of scales, suggesting direct interaction through competition or predation. Evidence suggests that when phytoplankton stocks are low, energy flow through krill is maintained by their feeding on the large populations of small copepods. Metazoans and higher predators at South Georgia can feed across several trophic levels according to prey abundance, and they regenerate substantial quantities of reduced nitrogen. Therefore we suggest that these groups have a controlling influence on lower trophic levels, both stabilising population sizes and maintaining high 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 failures among many of their predators. This appears to be a downstream effect from wider scale, Scotia Sea phenomena, although the processes involved are unclear. 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 any physical concentration mechanism. Even for krill, which do not breed there, local growth rates are probably of a similar order to the biomass removed by their main land-based predators in summer. Thus the transfer of energy to higher predators depends on local enhancement of fluxes through the food web as well as the supply of plankton to the island by the Antarctic Circumpolar Current.

In a previous study at South Georgia, carnivory was invoked as a cause of high polyunsaturated fatty acid (PUFA) content of Antarctic krill, Euphausia superba. To examine this, krill were sampled and fed for 16 days exclusively on the locally abundant copepod Drepanopus forcipatus. After 16 days, the krill had increased their PUFA content from 28 to 54% of the total fatty acids. Concurrently, monounsaturated fatty acids (MUFAs) and saturated fatty acids (SFAs) decreased from 41 to 27%. Thus, the krill appeared to accumulate PUFAs as reflected in their diet of D. forcipatus, which also had a relatively high PUFA content (50%). Overall, the results support omnivorous feeding by krill at South Georgia during nonbloom periods. We propose that the ratio of PUFA to SFA content may be used to detect carnivory in the recent feeding history of krill and suggest that this may be an index which could be applied to other zooplankton.

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Euphausia superba (hereafter 'krill') and copepods are major zooplankton taxa in the Southern Ocean, but there is little information on how they interact. This paper investigates their coincidence across a wide range of temporal and spatial scales to examine whether copepod distribution is related to that of krill. During 2 summers of high krill abundance near South Georgia (1996 and 1997) copepod abundance was <40% of that during an abnormally low krill year (1994). No such depletion was found north of the Polar Front, where krill were rare. Analysis of 2 mesoscale data sets showed that krill, rather than food or environmental factors, were most strongly implicated in copepod distribution. An area of persistently high krill abundance just north of South Georgia was characterised by exceptionally few copepods. Fine-scale relationships between patches of krill and copepods were studied with a Longhurst Hardy Plankton Recorder. Within krill swarms copepod abundance was low, but more dispersed krill associated with high concentrations of copepods Copepods also appeared to Live deeper and to make more extensive vertical migrations when krill were present. The inverse relationship between krill and copepod abundances thus occurred repeatedly and across a wide range of scales. The facts that krill swarms are mobile and were unrelated to hydrography further suggest that the inverse relationship was caused by krill. This could arise from competitive exclusion, direct predation or both. Evidence for competition is that South Georgia copepods rely on high phytoplankton biomass for recruitment and krill can remove this. Predation is suggested by the fact that crustaceans were found in krill guts in this region during both summer and wlnter. During the 1996 summer, experirnentally derived predation rates on copepods, combined with krill bion~ass values, suggested a significant impact on small copepods. Therefore we suggest that copepod numbers can be controlled by a combination of competition and predation by krill.