Iron represents an important control on primary production in high nutrient low chlorophyll (HNLC) regimes and has received considerably attention during the last two decades. This work has focussed on the biogeochemistry of iron in two oceanic environments; the high latitude North Atlantic and the Scotia Sea in the Southern Ocean. The mechanisms of iron supply and the biological response of resident phytoplankton communities to iron were addressed in both study areas. Two cruises to the high latitude North Atlantic Ocean (>55 °N) during late July-early September 2007 indicated that nitrate concentrations of 2 to 5 M persisted in the surface waters. The concentration of dissolved iron (dFe) in the surface waters was very low, with an average of 0.093 (<0.010-0.218, n=43) nM, and in situ chlorophyll concentrations were < 0.5 mg m-3. In vitro iron addition experiments demonstrated that the addition of iron increased photosynthetic efficiencies (Fv/Fm) and resulted in enhanced chlorophyll in treatments amended with iron when compared to controls. A number of phytoplankton taxa, including the coccolithophore Emiliania huxleyi, were observed to increase their net growth rates following iron addition. These results provide strong evidence that iron limitation within the post spring bloom phytoplankton community contributes to the observed residual macronutrient pool during summer. Low atmospheric iron supply and suboptimal Fe:N ratios in winter overturned deep water are suggested as proximal causes for this seasonal High Nutrient Low Chlorophyll (HNLC) condition, which represents an inefficiency of the biological (soft tissue) carbon pump. Large areas of the Southern Ocean are characterised as HNLC. Satellite chlorophyll data indicate that phytoplankton blooms occur in vicinity to Southern Ocean Island systems. The bloom associated with South Georgia has the largest spatial extent and duration (16-20 weeks). Detailed measurements were made on austral spring and summer cruises to the Scotia Sea during November – early December 2006 and January – February 2008. This work presents the first comprehensive study of seasonal variations in phytoplankton biomass and iron availability in the Scotia Sea. The drawdown of nitrate between the two seasons in the South Georgia bloom was 16 M indicative of substantial new production. Surface water concentrations of dissolved iron (dFe) were slightly higher during summer than spring (0.31 nM compared to 0.20 nM, with P>0.05). We suggest that the South Georgia bloom is sustained by a continuous benthic supply of iron from the South Georgia shelf. In addition, enhanced dFe (0.34 nM) was observed in a cryptophyte dominated bloom in the southern Scotia Sea in the vicinity of South Orkney Islands. The difference in the community composition between the two natural occurring blooms highlight that Southern Ocean island systems have individual characteristics and should be viewed independently.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:516186 |
| Date | January 2009 |
| Creators | Nielsdóttir, Maria Chun |
| Publisher | University of Southampton |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://eprints.soton.ac.uk/145859/ |
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