Dissolved iron in the Australian sector of the Southern Ocean (CLIVAR SR3 section): Meridional and seasonal trends

We report measurements of dissolved iron (dFe, o0.4 mm) in seawater collected from the upper 300m of the water column along the CLIVAR SR3 section south of Tasmania in March 1998 (between 421S and 541S) and November–December 2001 (between 471S and 661S). Results from both cruises indicate a general north-to-south decrease in mixed-layer dFe concentrations, from values as high as 0.76 nM in the Subtropical Front to uniformly low concentrations (o0.1 nM) between the Polar Front and the Antarctic continental shelf. Samples collected from the seasonal sea-ice zone in November–December 2001 provide no evidence of significant dFe inputs from the melting pack ice, which may explain the absence of pronounced ice-edge algal blooms in this sector of the Southern Ocean, as implied by satellite ocean-color images. Our data also allow us to infer changes in the dFe concentration of surface waters during the growing season. South of the Polar Front, a comparison of near-surface with subsurface (150m depth) dFe concentrations in November–December 2001 suggests a net seasonal biological uptake of at least 0.14–0.18 nM dFe, of which 0.05–0.12nM is depleted early in the growing season (before mid December). A comparison of our spring 2001 and fall 1998 data indicates a barely discernible seasonal depletion of dFe ( 0.03nM) within the Polar Frontal Zone. Further north, most of our iron profiles do not exhibit near-surface depletions, and mixedlayer dFe concentrations are sometimes higher in samples from fall 1998 compared to spring 2001; here, the near-surface dFe distributions appear to be dominated by timevarying inputs of aerosol iron or advection of iron-rich subtropical waters from the north.