Spatial and seasonal variations of iron speciation in surface waters of the Subantarctic front and the Otago Continental Shelf

Here we present the longest time series of iron speciation along a coastal shelf area. A time series of a surface transect across the Otago Continental Shelf and into the high-nutrient low-chlorophyll (HNLC) Subantarctic surface water (SAW) was undertaken between 2000 and 2008 to measure the dissolved iron (DFe) concentration, organic iron-complexation parameters, as well as macronutrient concentrations and general hydrographic data. The study area contains three distinct water types: 1) neritic water; 2) modified subtropical water (STW); and 3) Subantarctic surface water (SAW). Variations in nutrient concentrations in the study area indicated that SAW is the predominant source of nitrate and phosphate to the shelf. Dissolved iron (DFe) concentrations dropped seawards from several nanomolar to sub-nanomolar levels. The dissolved iron was fully complexed with strong organic ligands (L1) in all three water types, and the ligand concentrations decreased seawards. However the ratio between [L1]/[DFe], a measure of the ligand saturation, increased from < 2 in the neritic water to 10 in SAW. Seasonal trends in DFe and the L1 concentrations were only significant in SAW, where DFe was lowest in early spring and late autumn and highest in summer. The seasonal DFe maximum coincided with [L1] minimum, as well as minimum concentrations of macronutrients, indicating a shift from iron limitation in spring with potential siderophore production to probably silica limitation in summer. This study provides new insight on the driving forces of iron speciation in a coastal shelf area and HNLC waters.