Netherlands Polar Data Center

The distribution and biological availability of Fe is strongly controlled by its physical-chemical speciation within seawater, where colloids and Fe-organic complexes are dominant factors. In order to study the distribution and the biological availability of Fe the natural Fe organic complexes were determined in the surface waters of the Amundsen Sea (300 m). Methods Samples were collected using Go Flo bottles and filtered under ultra-clean conditions in flow benches (class 0). The concentration of iron binding ligands (organic compounds which strongly bind Fe) and their binding strength (conditional stability constant) were studied in 5 size classes here: unfiltered water, 0.2 μm filtered water, < 1000 kDa (Stereapore, Mitsubishi-rayon Co. Ltd, Nishioka and al., 2000, 2005), < 100 kDa and < 10 kDa ultra-filtrated water (Sartorius, Vivaflow 50, Schlosser and Croot, 2008). The left-over fraction from the ultra filtrations (retentates) were also analyzed for dFe and ligand characteristics to ensure a mass balance calculation and validate the ultrafiltration method. The dissolved iron concentrations in all the size fractions (and retentates) were measured (see dFe measurement section) using a chemo luminescence method (FIA) with acidified samples (pH 1.8). Total iron concentrations will be measured 6-12 months after the acidification of the unfiltered sample. Ligand characteristics were determined by using a complexing ligand titration with addition of iron (between 0 and 10 nM of Fe added) in buffered seawater (mixed NH3/NH4OH borate buffer, 5 mM). The competing ligand 'TAC' (2-(2-Thiazolylazo)-p-cresol) with a final concentration of 10 μM was used and the complex (TAC)2-Fe was measured after equilibration (> 15 h) by cathodic stripping voltammetry (CSV) (Croot and Johansson, 2000). The electrical signal recorded with this method (nA) was converted to a concentration of (TAC)2-Fe (nM). Subsequently, the ligand concentration and the binding strength were estimated using the non-linear regression of the Langmuir isotherm (Gerringa and al., 1995) and a newer "Leo" model currently built up (Gerringa et al, in prep). The voltammetric equipment consisted of a μAutolab potentiostat (Type II and III, Ecochemie, The Netherlands), a mercury drop electrode (model VA 663 from Metrohm). All equipment was protected against electrical noise by a current filter (Fortress 750, Best Power). Sampling statistics 26 stations were sampled on this cruise. These included 14 profiles and 5 stations where different size fractions were analyzed after 4 filtrations with different filter sizes (0.2 μm cut-off, 1000 kDa, 100 kDa and 10 kDa). In addition, 11 Fe/Ligand experiments were analyzed. Special attention was given to determine the iron binding ligands before and after incubation with and without artificial ligands in these experiments, to look at the response of algae and the change of the ligand characteristics during the incubations. Preliminary results An average ligand concentration of 0.789 nEq was found on the NBP0901 cruise, varying from 0.2 and 1.6 nEq of Fe. Highest ligand concentrations were found at 10 m depth followed by a minimum at 25 m. Concentrations increased with depth to become rather constant at 200 and 300 m. Low Fe binding strength of the ligands was found at 10 and 25 m in the polynya suggesting freshly produced ligands by organisms (phytoplankton or bacteria) or by a change in the ligand content and characteristics due to the biologic activity (the pool of strong binding ligands may be removed or used). The ratio ligand/dissolved iron (Fig. 38) clearly shows differences between the surface water and deeper samples. Very high ratios (10) were found in the surface waters of the Pine Island polynya due to the low dissolved iron concentration and high ligand concentrations. In the deep water (200 and 300 m) of the polynya and the circumpolar deep water upwelling in front of the PIG the ligand/dissolved iron ratio was close to 1 indicating a saturation of the ligands by iron and the possibility for iron to be removed from the water column by precipitation.