Carbon export from natural iron fertilisation in the Southern Ocean

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

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2008

Author dc:creator

• Morris, Paul James