Contrasting primary production regimes around South Georgia, Southern Ocean: large blooms versus high nutrient, low chlorophyll waters

During the austral summer of 2002, a large and persistent phytoplankton bloom was detected with SeaWiFS imagery in the Georgia Basin to the north-west of South Georgia, while waters to the east of the island were relatively unproductive. A British Antarctic Survey research cruise in January 2002 confirmed this west/east difference with production values of up to 2.5 g C m(-2) d(-1) and chlorophyll a (chl a) values up to 15 mg m(-3) at stations to the northwest of the island and 0.17 g C m(-2) d(-1) and 1.3 mg chl a m(-3) to the northeast. These differences were not attributable to light limitation as mixed layer depth never exceeded critical depth. Instead, substantial nutrient depletions to the northwest of the island compared with the northeast suggested a difference in nutrient use between the two regions. The exceedingly high nutrient depletions (to < 6.0 and 0.3 mmol m(-3) for NO3-N and PO4-P, respectively) measured to the northwest were associated with an anticyclonic eddy situated over the Northwest Georgia Rise. Furthermore, differences in NO3-N:PO4-P depletion ratios suggested a greater ability in the northwest phytoplankton to utilise NO3-N, and a greater dependence on NH4-N at the northeast stations. Three distinct station groups were identified around the island based on watermass and size-fractionated chlorophyll. To the east, waters were characterised by a high proportion of microplankton and low NO3-N:PO4-P depletion ratios, to the west, by either a high proportion of microplankton and high NO3-N:PO4-P depletion ratios, or a high proportion of nanoplankton and moderate NO3-N:PO4-P depletion ratios. We consider this to be indicative of greater Fe availability, promoting NO3-N use, to the northwest of South Georgia. However, an absence of microplankton over the western shelf regions may be due to size selective grazing by krill. Our field data, in conjunction with SeaWiFS imagery, indicated that the Georgia Basin phytoplankton most likely originated upstream of South Georgia. Subsequent interactions with the Northwest Georgia Rise and South Georgia's south-western shelf promoted increased growth that converged to the west of the island to form a large bloom in the Georgia Basin