| Summary: | During Polarstern Cruise ANT-XXIV/2, a contribution to the IPY project SCACE, we were able to observe biogeochemical changes that were associated with a phytoplankton bloom which developed in the eastern (8°W to 14°E) Weddell Gyre from December 2007 to January 2008 during the seasonal sea ice melt. At its culmination in January the bloom, as revealed by satellite imagery, covered an area of more than half a million square kilometres, the size of the North Sea. The bloom appeared to be triggered by the release of melt water and its subsequent warming, which lead to the formation of shallow, less than 30 m deep, mixed layers which help to keep phytoplankton cells closer to the well-illuminated surface. Within the bloom the concentration of chlorophyll-a (Chl) exceeded 2.5 mg m-3, five times the background concentration. Concomitant with the increase of Chl in the mixed layer there was an increase in the concentrations of dissolved oxygen and of particulate organic carbon (POC), while at the same time dissolved inorganic carbon (DIC) and nitrate (NO3) concentrations decreased – changes that can be used for tentatively estimating carbon budgets. Changes in DIC were strong enough to reverse the air-sea difference in CO2 partial pressures from an ocean source to a sink. Although the quantitative uncertainties associated with the assessment of the carbon budgets are rather large, primarily due to assumptions that have to be made with regard to the integration depths, the results confirm that ice edge blooms, which form in the Antarctic Divergence, play an important role in the global carbon cycle by taking up dissolved inorganic carbon that is upwelled together with the deep water masses and that otherwise would be released in part to the atmosphere.
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