A new method, based on the direct detection of iron-humic substance (HS) species by cathodic stripping voltammetry (CSV), is used to determine the iron binding capacity and complex stability of fulvic acid (FA), humic acid (HA), and the natural HS in the seawater. The FA binds 16.7 6 2.0 nmol iron (Fe) (mg FA) 21 , whereas the HA and the marine HS bind 32 6 2.2 nmol Fe (mg FA) 21 . The complex stabilities are (log K9 Fe9HS values) 10.6 for FA and 11.1 for HA and coastal HS. Measurements of coastal waters (Irish Sea) show that the HS occur in a widespread fashion, the HS concentration decreasing with increasing salinity and occurring at levels of 400 (at salinity 30) to 70 (at salinity 34) mg L 21 . A sample from the deep Pacific was found to contain 36 mg HS L 21 , amounting to 4% of the dissolved organic matter. Comparative measurement of the total iron complexing capacity by CSV with competitive ligand exchange showed that the natural HS can account for the entire ligand concentration in the shallow coastal and deep ocean waters tested. Measurements of the iron solubility showed that FA added to seawater and the HS in coastal waters maintain iron in solution at a level just below the iron binding capacity. The preliminary data for the open ocean indicate that the same may be true for HS in deep ocean waters. The data are consistent with a mechanism by which iron is transported from land to sea, associated with land-derived HS.Humic substances (HS) are ubiquitous hydrophobic components of the natural organic matter present in soil and aquatic environments that consist predominantly of polyphenols and benzoic/carboxylic acids (Buffle 1990). HS are operationally divided into humic acids (HAs) and fulvic acids ( Previous work has shown that HS in seawater can be determined at good sensitivity by cathodic stripping voltammetry (CSV), which takes advantage of adsorption of the complex on the mercury drop electrode as a concentration step prior to the CSV scan; bromate is added to enhance the sensitivity through a catalytic effect (Laglera et al. 2007). A very similar procedure (Obata and Van Den Berg 2001) is used to determine iron in seawater by means of CSV in the presence of a specific adsorptive ligand (2,3-dihydroxynaphthalene; DHN). The iron (Fe)-HS peak occurs at ,20.6 V, at nearly the same potential as the peak for Fe-DHN. The CSV response for FA (Suwannee River FA) and HA (SRHA) standards in ultraviolet (UV)-digested seawater in the presence of bromate was found to be the same as that for HS in natural waters (fresh, estuarine, and coastal waters), indicating that FA and HA are good model compounds for the natural HS.HS and iron are known to co-precipitate at the lowsalinity end of estuaries (Sholkovitz and Copland 1981), removing more than 99% of the dissolved iron and lowering its concentration from 0.5-10 mmol L 21 , which is typical for freshwaters (Nagai et al. 2007), to the 1-20 nmol L 21 range found in end-estuarine and coastal waters (Sanudo-Wilhelmy et al. 1996;Buck and Bruland 2007). The...

In nearly a dozen open‐ocean fertilization experiments conducted by more than 100 researchers from nearly 20 countries, adding iron at the sea surface has led to distinct increases in photosynthesis rates and biomass. These experiments confirmed the hypothesis proposed by the late John Martin [Martin, 1990] that dissolved iron concentration is a key variable that controls phytoplankton processes in ocean surface waters However, the measurement of dissolved iron concentration in seawater remains a difficult task [Bruland and Rue, 2001] with significant interlaboratory differences apparent at times. The availability of a seawater reference solution with well‐known dissolved iron (Fe) concentrations similar to open‐ocean values, which could be used for the calibration of equipment or other tasks, would greatly alleviate these problems [National Research Council (NRC), 2002[.

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