Controls of primary production in two phytoplankton blooms
in the Antarctic Circumpolar Current
The Antarctic Circumpolar Current has a high potential for primary production and carbon sequestration
through the biological pump. In the current study, two large-scale blooms observed in 2012 during a
cruise with R.V. Polarstern were investigated with respect to phytoplankton standing stocks, primary
productivity and nutrient budgets. While net primary productivity was similar in both blooms, chlorophyll
a –specific photosynthesis was more efficient in the bloom closer to the island of South Georgia
(39 °W, 50 °S) compared to the open ocean bloom further east (12 °W, 51 °S). We did not find evidence
for light being the driver of bloom dynamics as chlorophyll standing stocks up to 165 mg m2 developed
despite mixed layers as deep as 90 m. Since the two bloom regions differ in their distance to shelf areas,
potential sources of iron vary. Nutrient (nitrate, phosphate, silicate) deficits were similar in both areas
despite different bloom ages, but their ratios indicated more pronounced iron limitation at 12 °W
compared to 39 °W. While primarily the supply of iron and not the availability of light seemed to control
onset and duration of the blooms, higher grazing pressure could have exerted a stronger control toward
the declining phase of the blooms.