Korb, Rebecca, Mick Whitehouse and Pete Ward

British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB9 0ET, UK, Email: rk@bas.ac.uk

SeaWIFS in the Southern Ocean: spatial and temporal variability in phytoplankton biomass around South Georgia

In contrast to the High-Nutrient-Low-Chlorophyll conditions that pervade over much of the Southern Ocean, the waters surrounding the sub-Antarctic island of South Georgia have long been identified as being rich in phytoplankton. During the austral summer, chlorophyll a concentrations frequently exceed 10 mg m^-3, and macronutrients are occasionally depleted to growth limiting concentrations (e.g., <1mmol m^-3 Si(OH)₄- Si). Secondary production also greatly exceeds that which is typical for the Southern Ocean and in addition, the area supports a commercial krill fishery and huge numbers of breeding marine birds and mammals. Over the last 20 years, shipboard measurements have been made by the British Antarctic Survey (BAS) to investigate the causes of the island's richness. In recent years the use of SeaWiFS images have provided an invaluable tool in this investigation, and has enhanced our ability to examine interannual and spatial variability in this dynamic system.

In this study we compare SeaWiFS data with ship-collected surface and depth-integrated data gathered over three summer seasons. We document the spatial and temporal extent of surface phytoplankton around South Georgia - blooms can occur anytime between October and March but not usually continuously. From shipboard sampling alone, it was previously believed that phytoplankton biomass was greatest at the north-western shelf region, though the satellite data has revealed blooms occurring to the south-west, north-central and occasionally to the east of South Georgia. It is also evident that blooms are not confined to the island's neritic areas. Some blooms are undoubtedly delivered to the island's system by major frontal jets such as the SACCF, while others are generated at the island itself. Here we speculate on the factors responsible for South Georgia's enhanced phytoplankton productivity and the causes of its variability. Given this revised view of primary production around South Georgia we discuss the potential consequences for secondary production.