Distribution of size fractionated dissolved iron in the Canary Basin

Ussher, Simon J., Achterberg, Eric P., Sarthou, Géraldine, Laan, Patrick, de Baar, Hein J.W. and Worsfold, Paul J. (2010) Distribution of size fractionated dissolved iron in the Canary Basin. Marine Environmental Research, 70 (1), 46-55. (doi:10.1016/j.marenvres.2010.03.001).

Abstract

The distribution of size fractionated dissolved iron (DFe, <0.2 ?m) species was determined in the
upper water column (0-150 m) of the Canary Basin (25 – 32 ºN and 18 – 24 ºW) on a research cruise in
October 2002. A DFe concentration gradient resulting from a decrease in both soluble iron (SFe,
<0.02?m) and colloidal iron (CFe, 0.02 – 0.2 ?m) was shown to extend from the coast of northwest
Africa into the oligotrophic gyre (varying from ~1 nM in the shelf region to 0.15 nM in the most off
shore waters). At the time of this study, the dominant dissolved Fe input to the region was deduced to
be the advection of shelf and upwelled waters rather than Saharan dust deposition.
SFe and CFe fractions had mean concentrations (± standard deviation) of 0.25 ± 0.11 and 0.21 ±
0.16 nM respectively (n = 58). Colloidal iron formed a highly variable fraction of DFe (ca. 0 – 80%,
mean of 42%) in the region but was less variable in the low iron, oligotrophic intermediate waters
(0.18 ± 0.06 nM, 31.7 ºN, 22.0 ºW, 0 – 1300 m depth). The high variability found at the most
productive near-shelf stations was driven by biological processing and mixing of different water
masses. In contrast, less variability between SFe and CFe at the remote off shore stations suggested
that vertical variations in the water column were controlled more by chemical partitioning and vertical
particle fluxes with evidence of preferential biological uptake and/or removal of SFe in the most
remote surface waters.

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