Primary Production and Carbon uptake dynamics in the vicinity of south Georgia - balancing carbon fixation and removal

Primary production was measured at a series of stations in austral summer 1996 in the vicinity of South Georgia. Five stations were occupied along a 700 km transect southeastwards towards the western tip of South Georgia. Three stations were located north of the Antarctic Polar Front (PF) with mean primary production of 0.41 g C m-2 d-1 compared to 0.54 g C m-2 d-1 for the stations south of the front. Three stations were in the shelf break and off-shelf region to the east of South Georgia where the lowest rates of primary production were recorded (mean 0.34 g C m-2 d-1). At a further 4 stations northwest of the island, primary production was significantly greater: 0.8 to 2.5 g C m-2 d-1 (mean 1.5 g C m-2 d-1). Photoinhibition was a marked feature of the production profiles, resulting in the underestimation of column production by 4 to 16%. An assessment is made of the primary production required to maintain a representative assemblage of metazooplankton around South Georgia composed of 30 g fresh mass (FM) m-2 krill and 20 g FM m-2 copepods. The total C requirement of both groups at maximum growth rate is estimated as 0.93 g C m-2 d-1, with 0.54 g C m-2 d-1 required to maintain basal metabolism (i.e. for zero growth). Local primary production rates are well in excess of the C demands for basal metabolism by the metazooplankton, and likewise the highest regional values exceed those for maximum metazooplankton growth. It is also estimated that the krill biomass required to sustain higher predators during their breeding season could be supported by local production at plausible maximum krill growth rate. In particular, high primary production found in this study to the northwest of South Georgia appears to be adequate to sustain maximum growth rate by krill when their biomass is 30 g FM m-2, when krill production will exceed predator removal. It is already well known that advection from higher latitudes is important in supplying energy to the South Georgia system. This study adds to the evidence that locally enhanced primary production is also important in supporting the local food web