Distributions of dissolved and particulate iron in the sub-Antarctic and Polar Frontal Southern Ocean (Australian sector)
Bowie, Andrew R.
Remenyi, Tomas A.
Lam, Phoebe J.
Townsend, Ashley T.
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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).
Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 58 (2011): 2094-2112, doi:10.1016/j.dsr2.2011.05.027.
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