Carbon export from natural iron fertilisation in the Southern Ocean

Abstract

It has long been recognised that some oceanic regions have persistently low chlorophyll
levels, even though inorganic nutrients are plentiful. Studies have shown that these
high-nutrient, low-chlorophyll (HNLC) areas are depleted in iron, which is an essential
micronutrient for phytoplankton growth. In HNLC regions biological production can be
enhanced with artificial mesoscale iron fertilisation. However, the ability of artificially
induced phytoplankton blooms to efficiently sequester carbon to mesopelagic depths is
still an open question. SubAntarctic islands in the HNLC Southern Ocean are a natural
source of iron and thus fuel the annual phytoplankton blooms observed in their
proximity. One such bloom, tied to the Crozet Islands (52ºE, 46ºS), provided the
opportunity to examine particulate organic carbon (POC) export during the austral
summer of 2004/5. This work was imbedded into the multi-disciplinary CROZEX
project thus providing a rich context for data interpretation.
Based on satellite imagery, a high chlorophyll region (max = 4 µg l-1) north and
downstream of the Crozet Islands was distinguished from a low chlorophyll region
(typically 0.3 µg l-1) south and upstream of the islands. POC export estimates, obtained
with the naturally occurring particle reactive radionuclide tracer, 234Th, were initially
D15 mmol C m-2 d-1 in the high chlorophyll region, compared with D5 mmol C m-2 d-1 in
the low chlorophyll region. After a moderately small increase in chlorophyll in the
south (max = 0.7 µg l-1) the spatial variability in POC export was lost, resulting in
equally high levels of POC export (ca. 20 mmol C m-2 d-1) throughout the study area.
After comparing the daily rates of POC export with temporally integrated new
production calculated from nitrate budgets, a different spatial pattern emerged. New
production (NP) presented consistently higher values in the north, when compared to
the south. Two hypotheses were formulated to explain this, 1) dissolved organic matter
(DOM) and suspended particulate organic matter (sPOM) produced from NP was stored
in the mixed layer with this effect relatively greater in the north, 2) the export event in
the north was longer resulting in greater seasonal POC export. Investigation of the
DOM pool revealed that DOM accounted for 46±7% of NP and was consistent across
the whole study area. In contrast, sPOM accumulated at differential rates of 18±7% in
the north and 0±7% in the south. This suggested that differential storage of sPOM was
responsible for the lack of a latitudinal gradient in POC export after the relatively small
increase in chlorophyll in the south. After investigating the second hypothesis, the daily
rates of POC export were scaled to seasonal integrals using a silicon budget, which
allowed the formulation of a seasonal carbon budget. This revealed that over the timescale
of the study the magnitude of NP and POC export were not the same with this
difference greatest within the northern high chlorophyll region. This was the result of
relatively greater storage of sPOM in the north and had the effect of reducing the
amount of easily exportable POC to mesopelagic depths. Thus both hypotheses
contributed to better understanding carbon export in the Crozet region.

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