Dissolved iron distribution and organic complexation in the coastal waters of the East China Sea

Abstract

The distribution of total dissolved iron and organic ligands in the coastal waters of the East China Sea, especially in the Yangtze River estuary, was investigated using competitive ligand equilibration–adsorptive cathodic stripping voltammetry (CLE–ACSV) with 2,3-dihydroxynaphthalene (DHN) as the competing ligand.The seasonal variation of total dissolved iron (D-Fe) and its organic ligands (Lt) were observed in spring and autumn. The average D-Fe concentration in both surface and bottom waters in spring was higher than that in autumn. With respect to the horizontal distribution of D-Fe and Lt content, the highest values were observed in the estuary stations located in the Yangtze River Estuary and the Hangzhou Bay area and the lowest values were observed in offshore stations. The average surface D-Fe concentration of estuary stations in spring and autumn was 39.4±26.6 (the standard deviation) nmol/L and 20.5±11.0nmol/L, respectively. The average bottom D-Fe concentrations of estuary stations in spring and autumn were 76.1±58.6nmol/L and 78.5±39.0nmol/L, respectively.Higher surface Lt concentrations were observed during the spring. The average surface Lt concentrations of estuary stations in spring and autumn were 40.0±26.8 and 23.2±7.8nmol/L, respectively. The concentration of D-Fe and Lt in the bottom water was higher than that observed in surface water and exhibited less seasonal variability. Strong ligands were observed in spring estuary surface water. The average surface logK'FeL values in spring and autumn were 12.9±0.8 and 11.7±0.5, respectively. The total ligand concentration in this study was generally in excess of the dissolved iron, particularly in autumn at offshore stations. Higher excess organic ligand concentrations were also generally observed in the upper water column compared to bottom waters in autumn.