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

• Main Authors: Atkinson, A., Whitehouse, M.J., Priddle, J., Cripps, G.C., Ward, P., Brandon, M.A.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Inter-Research 2001
• Subjects: Antarctic; The Antarctic; Weddell; Antarc*; Antarctica; Euphausia superba; Copepods
• Online Access: http://nora.nerc.ac.uk/id/eprint/18364/; http://www.int-res.com/articles/meps/216/m216p279.pdf

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–3) may be linked to enhanced supply of iron or reduced forms of nitrogen (up to ~3 mmol ammonium m–3). Although macronutrients are generally not limiting in the Antarctic Zone, silicate concentrations of <1 mmol m–3 are growth-limiting at South Georgia in some summers. The growth season 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 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 ...