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. 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.

More information

Identifiers

Catalogue record

Export record