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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:500748 |
| Date | January 2008 |
| Creators | Morris, Paul James |
| Publisher | University of Southampton |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://eprints.soton.ac.uk/66262/ |
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