Fe (III) speciation in the high nutrient, low chlorophyll Pacific region of the Southern Ocean

Fe speciation was measured with competitive ligand equilibration adsorptive cathodic stripping voltammetry [Gledhill, M., Van den Berg, C.M.G., 1994. Determination of complexation of iron (III) with natural organic complexing ligands in sea water using cathodic stripping voltammetry. Mar. Chem., 47, 41-54.] in the Pacific part of the Southern Ocean between 58 degrees and 68 degrees 30'S along the 90 degrees W meridian. The conditional stability constant(K' with respect to [Fe3+]) was between 10(20.6) and 10(21.6) when one organic ligand was detected. The ligand concentration ([L-t]) varied between 2.2 and 12.3 equivalents of nM Fe (nEq of Fe). The ligand concentration was at least 6 times, and generally more than 10 times, that of the total dissolvable Fe concentration. At one station a depth profile was sampled where below 200 m depth, two organic ligands were measured with K-1' = 10(21) and K-2'=10(22.4). Organic complexation of Fe was similar to results found elsewhere [(Gledhill, M., Van den Berg, C.M.G., 1994. Determination of complexation of iron (III) with natural organic complexing ligands in sea water using cathodic stripping voltammetry. Mar. Chem., 47, 41-54.); (Van den Berg, C.M.G., 1995. Evidence for organic complexation in seawater. Mar. Chem., 50, 139-159.); (Rue, E.L., Bruland, K.W., 1995. Complexation of iron (III) by natural organic ligands in the Central North Pacific as determined by a new competitive ligand equilibration/adsorptive cathodic stripping voltammetric method. Mar. Chem., 50, 117-138.); (Rue, E.L., Bruland, K.W., 1997. The role of organic complexation on ambient iron chemistry in the equatorial Pacific Ocean and the response of a mesoscale iron addition experiment. Limnol. Oceanogr., 42, 901-910.)] judging from the overall organic alpha value (K' *[L-t]) 10(12.4)-10(13.9). The lower values of organic alpha were within one order of magnitude of our choice of the inorganic alpha (10(11.9), [Millero, F.J., Yao, W., Aicher, J., 1995. The speciation of Fe (II) and Fe (III) in natural waters. Mar. Chem., 50, 21-39.]) in which case the organic and inorganic ligands could compete effectively for Fe. Different values of organic alpha and the occurrence of two organic ligand classes were consistent with differences in hydrography. South of the Polar Front, the least organic complexation occurred (organic alpha = 10(12.4), organic complexation around 80%), where the highest chlorophyll a concentrations were measured. (C) 1998 Elsevier Science B.V. All rights reserved.