Winter-time dissolved iron and nutrient distributions in the Subantarctic Zone from 40-52S; 155-160E

Michael Ellwood¹, Philip W Boyd² and Philip Sutton³

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 National Institute for Water and Atmospheric Research (NIWA), Centre for Chemical and Physical Oceanography, Department of Chemistry, University of Otago, Dunedin, New Zealand.
3 National Institute for Water and Atmospheric Research (NIWA), Wellington, New Zealand.

In the Southern Ocean, mesoscale iron fertilization experiments have clearly demonstrated that iron plays a pivotal role in controlling primary production in polar and subpolar High Nitrate Low Chlorophyll (HNLC) waters.  There has been considerable debate about the relative magnitude of different iron sources to surface waters in these regions, such as upwelling, dust or entrainment from island and continental self sediments. However, despite the rapidly emerging field of iron biogeochemistry, there are few vertical profiles of dissolved iron concentration, and almost no winter iron data. During 2006 we generated the first comprehensive winter dataset for dissolved iron and nitrate distributions (0-1000 m depth) between 40 °S - 52 °S, which transects the Subantarctic zone (SAZ), west of New Zealand (Figure 1). 

Surface iron concentrations (<0.2 nmol Fe kg-1) were conspicuously low, i.e., probably biologically limiting even at winter-reserve levels, at frontal zones between 43 °S (Subtropical Front) and ~51 °S (Subantarctic Front) (Figure 2). A fivefold range in iron:nitrate molar ratios was observed along the transect, with Subtropical waters, where blooms occur, having the highest ratios in subsurface waters. The major wintertime supply of dissolved iron in the SAZ is from Ekman advection of waters from the south (but calculated source water dissolved iron is ~0.2 nmol Fe kg-1), suggesting that mixed-layer dissolved iron concentration is controlled by how long these southern waters remain at the surface (~3 years).