Iron biogeochemistry in (sub-) Polar waters

Abstract

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. Thiswork has focussed on the biogeochemistry of iron in two oceanic environments; the highlatitude North Atlantic and the Scotia Sea in the Southern Ocean. The mechanisms of ironsupply and the biological response of resident phytoplankton communities to iron wereaddressed 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 ?Mpersisted in the surface waters. The concentration of dissolved iron (dFe) in the surface waterswas very low, with an average of 0.093 (<0.010-0.218, n=43) nM, and in situ chlorophyllconcentrations were < 0.5 mg m-3. In vitro iron addition experiments demonstrated that theaddition of iron increased photosynthetic efficiencies (Fv/Fm) and resulted in enhancedchlorophyll in treatments amended with iron when compared to controls. A number ofphytoplankton taxa, including the coccolithophore Emiliania huxleyi, were observed toincrease their net growth rates following iron addition. These results provide strong evidencethat iron limitation within the post spring bloom phytoplankton community contributes to theobserved residual macronutrient pool during summer. Low atmospheric iron supply and suboptimalFe:N ratios in winter overturned deep water are suggested as proximal causes for thisseasonal High Nutrient Low Chlorophyll (HNLC) condition, which represents an inefficiencyof the biological (soft tissue) carbon pump. Large areas of the Southern Ocean arecharacterised as HNLC. Satellite chlorophyll data indicate that phytoplankton blooms occurin vicinity to Southern Ocean Island systems. The bloom associated with South Georgia hasthe largest spatial extent and duration (16-20 weeks). Detailed measurements were made onaustral spring and summer cruises to the Scotia Sea during November – early December 2006and January – February 2008. This work presents the first comprehensive study of seasonalvariations in phytoplankton biomass and iron availability in the Scotia Sea. The drawdown ofnitrate between the two seasons in the South Georgia bloom was 16 ?M indicative ofsubstantial new production. Surface water concentrations of dissolved iron (dFe) were slightlyhigher during summer than spring (0.31 nM compared to 0.20 nM, with P>0.05). We suggestthat the South Georgia bloom is sustained by a continuous benthic supply of iron from theSouth Georgia shelf. In addition, enhanced dFe (0.34 nM) was observed in a cryptophytedominated bloom in the southern Scotia Sea in the vicinity of South Orkney Islands. Thedifference in the community composition between the two natural occurring blooms highlightthat Southern Ocean island systems have individual characteristics and should be viewedindependently