Substantial open-ocean phytoplankton blooms to the north of South Georgia, South Atlantic, during summer

Substantial open-ocean phytoplankton blooms in the Antarctic Zone to the northwest of South Georgia, South Atlantic are described. Chlorophyll a, nutrient and physical oceanography data, collected between 2 and 5 January 1994 and again 1 mo later between 2 and 4 February along a 450 km transect comprising 14 stations, are presented. The transect crossed the Subantarctic and the Polar Front. During the January transect survey, in 2 locations to the south of the Polar Front, average surface mixed-layer chlorophyll a concentrations were >8 and >13 mg m-3, and were associated with silicate, nitrate and phosphate depletions (<2.0, 0.3 and >1.4 mmol m-3 respectively). One of the phytoplankton blooms was associated with the nearby Polar Front, but the origin of a bloom further south, well within the Antarctic Zone, was not clear. Phytoplankton production predicted by nutrient drawdown was far greater than the observed biomass on both surveys. If a common Antarctic Zone origin is accepted for the southern bloom, a decline in biomass of ~2.7 mol C m-2 occurred in the upper 50 m of the water column between the 2 surveys, which cannot be accounted for by zooplankton grazing. If the bloom had originated in Subantarctic water advected to the south of the Polar Front, initial nutrient concentrations would have been lower and consequently predicted production was closer to the observed biomass values. Nevertheless, the area may be one of intense but local carbon export.

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