Distributions of dissolved and particulate iron in the sub-Antarctic and Polar Frontal Southern Ocean (Australian sector)

Abstract

This paper presents iron (Fe) profiles in the upper 1000 m from nine short-term (transect) stations and three long-term (process) stations occupied in the Australian sector of the Southern Ocean during the SAZ-Sense expedition in austral summer (January–February) 2007. Strong vertical and horizontal gradients in Fe concentrations were observed between the 18 sampled profiles (i.e. 0.09–0.63 nmol/l dissolved Fe (dFe)). Average dFe concentrations in surface waters in the northern Sub-Antarctic Zone (SAZ-N) West (station P1) were 0.27±0.04 nmol/l. This is lower than in the SAZ-N East region (station P3 and around) where average dFe values in the mixed layer were 0.48±0.10 nmol/l. The Polar Front (PF) station (P2) exhibited the lowest average surface Fe values (i.e. 0.22±0.02 nmol/l). Iron concentrations in deep waters down to 1000 m were more uniform (0.25–0.37 nmol/l dFe), which is in accordance with values reported elsewhere in remote waters of the Southern Ocean, but lower than those observed in the North Atlantic and North Pacific basins. A strong decoupling was observed between dFe and nutrient cycles at all stations. Particulate Fe levels were generally very low for all SAZ stations (<0.08 – 1.38 nmol/l), with higher values observed at stations collected near Tasmania and in the SAZ-N East region. The intrusion of subtropical waters, enriched with Fe from sediments or dust further north, is thought to mediate Fe input to the SAZ-N and STZ areas, while input from below would be the main source of Fe in the PF region. We applied the tracer Fe* (Fe*= [dFe]-RFe:P × [PO4 3-], where RFe:P is the algal uptake ratio) to estimate the degree to which the water masses were Fe limited. In this study, Fe* tended to be negative and decreased with increasing depths and latitude. Positive Fe* values, indicating Fe sufficiency, were observed in the (near-)surface waters collected in the SAZ-N East and near continental sources, where primary production was higher and ultimately limited by the lack of macro-nutrients, not Fe. Micro-organisms residing in the SAZ-N West and PF on the other hand experienced negative Fe*, indicating a strong co-limitation by low silicic acid concentration and Fe supply (and light in the case of PF).