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

Abstract

In HNLC oceanic regions, iron (Fe) controls phytoplankton productivity yet large gaps remain in our understanding of iron's biogeochemical cycle. Here we present the first comprehensive winter dataset for dissolved Fe (dFe) and nitrate (NO3) distributions (0-1000 in depth) between 40S-52S, which transects the Subantarctic zone (SAZ), west of New Zealand. Surface concentrations (<0.2 nmol Fe kg-1) were conspicuously low, i.e., probably biologically limiting even at winter-reserve levels, at frontal zones between 43S (Subtropical Front) and ∼51S (Subantarctic Front). A fivefold range in Fe:NO3 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 dFe in the SAZ is from Ekman advection of waters from the south (but calculated source water dFe is ∼0.2 nmol Fe kg-1), suggesting that mixed-layer dFe concentration is controlled by how long these southern waters remain at the surface (∼3 years).