Estimating carbon, silica and diatom export from a naturally fertilised phytoplankton bloom in the Southern Ocean using PELAGRA: A novel drifting sediment trap

During the austral summer of 2004–2005, a large multi-disciplinary research cruise investigated the development and
fate of a naturally iron-fertilised phytoplankton bloom in the Southern Ocean (Crozet Plateau). As part of this extensive
process study, a neutrally buoyant sediment trap (PELAGRA) was deployed to constrain the magnitude, composition, and
variability of upper-ocean particle export. In the productive regime north of the plateau we observed depth-normalised
(100-m) organic carbon fluxes between 11 and 440 mg C m2 d1
, and in the HNLC control region to the south similarly
normalised fluxes between 28 and 46 mg C m2 d1
. Mass balance calculations indicate that the high levels of carbon
export north of the plateau would need to be maintained for at least 30 days in order to account for estimated seasonal
depletion of dissolved silicic acid in surface waters. This would imply that the flux of organic carbon is E15 g C m2 for the
period of the bloom north of the plateau. A range of export ratios (proportion of surface production lost to downward
flux) was calculated using both contemporaneous and retrospective estimates of integrated production, and these highlight
the temporal decoupling between production and export. Calculated export ratios were at their highest north of the plateau
and correlate strongly with the selective export of large, heavily silicified diatoms, particularly Eucampia antarctica, relative
to the surface community structure. By normalising the molar elemental ratios measured in the exported particles to the
molar elemental ratios of the upper-ocean particle field we also observed a strong decoupling of Si:C and Si:N. This
suggests that the decoupling of the global silica and carbon cycles, which is well known as a defining feature of the
Southern Ocean, has its origins in the upper ocean.