Iron biogeochemistry in (sub-) Polar waters

• Main Author: Nielsdóttir, Maria Chun
• Format: Thesis
• Language: English
• Published: 2009
• Subjects: Austral; Scotia Sea; Southern Ocean; North Atlantic; Ocean Island
• Online Access: https://eprints.soton.ac.uk/145859/; https://eprints.soton.ac.uk/145859/1/Nielsdottir_2009_PhD.pdf

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 ...