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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. [Copyright &y& Elsevier]

SeaWiFS was used to examine the spatial and temporal distribution of chlorophyll-a (chl-a) concentrations around the island of South Georgia during four summers between October 1997 and March 2001. A comparison was made between SeaWiFS and ship-collected (in situ) chl-a gathered during three of these seasons. A significant correlation was found between daily SeaWiFS and in situ match-ups (r2=0.62). At low chl-a concentrations (<1 mg m-3), SeaWiFS estimates were approximately 87% of in situ values, but only 30% at high chl-a concentrations (>5 mg m-3). Analysis of SeaWiFS data from two survey boxes (the Western and Eastern Core Boxes) sampled during cruises indicated significant differences between seasons and regions; the 1999 and 2000 seasons were nearly twice as productive as the 1998 and 2001 seasons. In addition, phytoplankton biomass was significantly higher in the Western (∼1.4 mg m-3) than in the Eastern Core Box (∼0.6 mg m-3). These seasonal and regional differences also were reflected in the distribution of copepods, one of the major zooplankton groups around South Georgia. SeaWiFS revealed patterns of growth outside of the survey boxes and cruise periods (January only). Blooms regularly occurred to the southwest and directly north of the island, and occasionally occupied the entire area between South Georgia and the Polar Front. In addition, blooms took place any time between October and March but not continuously. Primary production at South Georgia is mainly a local feature with little growth initiated upstream. However, chl-a may remain elevated downstream of the island (∼40°W) as part of a continuous feature along the Polar Front to the 0° meridian. The passage of the Antarctic Circumpolar Current over the ridge and shelf waters in the vicinity of South Georgia may contribute micronutrients such as iron, and could partially explain the elevated chl-a levels associated with the island. We also examine the role of temperature, light and grazing on controlling phytoplankton productivity in this region of the Southern Ocean. [Copyright &y& Elsevier]

Abstract: Diatom cells have distinctive optical characteristics, originating from their relatively large cell size, fucoxanthin content and silica cell wall. It has been proposed that diatom-dominated phytoplankton blooms can be identified by optical remote sensing and that specifically tuned chlorophyll and primary production algorithms should be applied in regions where these blooms are present. However there have been few studies on how the optical properties of diatom blooms change as they progress from active growth to senescence, and it is unlikely that measurements on laboratory cultures encompass the full range of physiological states found in natural waters. We have therefore examined the inherent optical properties (IOPs) of the waters around the island of South Georgia at the end of the spring diatom bloom. Considerable variability was found in the relationships between the inherent optical properties and analytically determined chlorophyll a concentrations even in the surface layer, which meant that the usual bio-optical assumptions for Case 1 waters did not apply. To account for this variability, phytoplankton absorption and scattering were modeled as a two-component mixture, with the components representing actively growing and senescent material. The specific inherent optical properties of the two components were derived by linear regression of total IOPs against chlorophyll concentration and a fraction of the suspended mineral concentration. These specific IOPs were used to develop radiative transfer models of diatom blooms in varying stages of growth and senescence. Remote sensing reflectances calculated using this technique confirmed the tendency of the standard algorithms employed in SeaWiFS, MODIS and MERIS data processing to under-estimate near-surface chlorophyll concentrations in diatom blooms. However the inclusion of increasing proportions of senescent material had a significant effect on algorithm performance only at chlorophyll concentrations below 10mgm−3. Optical depths predicted by the model around South Georgia were 9+/−2m at 512nm, indicating that a large fraction of the phytoplankton biomass was located below the depth from which the remote sensing signals originated. [Copyright &y& Elsevier]