Iron biogeochemistry in (sub-) Polar waters

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

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2009

Author dc:creator

• Nielsdóttir, Maria Chun