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

Abstract

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.

Distribution patterns of chaetognata, polychaeta, pteropoda and salpidae off south georgia and south orkney islands

“…Its absence around SG may be related to elevated chlorophyll-a concentrations usually present in the area (GILPIN et al, 2002;ATKINSON et al, 2001). High phytoplankton concentrations can cause the clogging of the salps' filter feeding system (HARBISON; MCALISTER, 1979) which may lead to the inability to feed.…”

“…Many studies have highlighted the high biological productivity around the South Georgia Islands (WARD et al, 2002(WARD et al, , 2003GILPIN et al, 2002;ATKINSON et al, 2001;WHITEHOUSE et al, 1996;ATKINSON;PECK, 1988) and, to a lesser extent, the South Orkney Islands (SIGLEO et al, 2000). Research has been focused mainly on euphausiids (particularly Euphausia superba) and copepods, (e.g.…”

First evidence of widespread active methane seepage in the Southern Ocean, off the sub-Antarctic island of South Georgia

“…South Georgia lies in the eastward flowing ACC, creating a morphological high in the largest meander modifying the Southern ACC 280 front 28,48 . Due to this particular hydrographic configuration, intensive and regular phytoplankton blooms develop in the area north and northwest of the South Georgia shelf 32 , leading to both a rich food web 49 , and a high carbon production, which is either exported 50 or ultimately deposited at the sea floor. Although there is no indication for deeply buried reservoirs of thermogenic gas fueling the gas emission sites investigated in this study, thermogenic gas migration through deep-rooted faults 285 cannot be entirely excluded as the fjords and connecting cross-shelf troughs may have established along lines of structural weakness that could have evolved in association to faults zones 26 .…”

Surface Circulation at the Tip of the Antarctic Peninsula from Drifters

“…It is now generally accepted that iron limits phytoplankton biomass in pelagic Antarctic waters, and there is increasing evidence that this is also true in coastal waters (Fitzwater et al 2000; discussion in Holm- Hansen et al 2004a). The export of iron-rich waters found near the Antarctic Peninsula may explain elevated chlorophyll levels-visible from satellite observations-that extend from this region to South Georgia Island (Falkowski et al 1998;Atkinson et al 2001;Korb et al 2004). Therefore, the pathways through which exchange occurs between the Weddell Sea and the ACC influence both the global overturning circulation and local biological dynamics.…”