BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-7-2489-2010 Fractionation of iron species and iron isotopes in the Baltic Sea euphotic zone Gelting J. 1 Breitbarth E. 1 2 3 Stolpe B. 2 4 Hassellöv M. 2 Ingri J. 1 Division of Applied Geology, Luleå University of Technology, Sweden Department of Chemistry, University of Gothenburg, Gothenburg, Sweden Department of Chemistry, University of Otago, Dunedin, New Zealand Department of Marine Science, University of Southern Mississippi, Hattiesburg, USA 23 08 2010 7 8 2489 2508 Copyright: © 2010 J. Gelting et al. 2010 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/7/2489/2010/bg-7-2489-2010.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/7/2489/2010/bg-7-2489-2010.pdf

To indentify sources and transport mechanisms of iron in a coastal marine environment, we conducted measurements of the physiochemical speciation of Fe in the euphotic zone at three different locations in the Baltic Sea. In addition to sampling across a salinity gradient, we conducted this study over the spring and summer season. Moving from the riverine input characterized low salinity Bothnian Sea, via the Landsort Deep near Stockholm, towards the Gotland Deep in the Baltic Proper, total Fe concentrations averaged 114, 44, and 15 nM, respectively. At all three locations, a decrease in total Fe of 80–90% from early spring to summer was observed. Particulate Fe (PFe) was the dominating phase at all stations and accounted for 75–85% of the total Fe pool on average. The Fe isotope composition (&delta;<sup> 56</sup>Fe) of the PFe showed constant positive values in the Bothnian Sea surface waters (+0.08 to +0.20&permil;). Enrichment of heavy Fe in the Bothnian Sea PFe is possibly associated to input of aggregated land derived Fe-oxyhydroxides and oxidation of dissolved Fe(II). At the Landsort Deep the isotopic fractionation of PFe changed between −0.08&permil; to +0.28&permil; over the sampling period. The negative values in early spring indicate transport of PFe from the oxic-anoxic boundary at &sim;80 m depth. The average colloidal iron fraction (CFe) showed decreasing concentrations along the salinity gradient; Bothnian Sea 15 nM; Landsort Deep 1 nM, and Gotland Deep 0.5 nM. Field Flow Fractionation data indicate that the main colloidal carrier phase for Fe in the Baltic Sea is a carbon-rich fulvic acid associated compound, likely of riverine origin. A strong positive correlation between PFe and chl-<i>a</i> indicates that cycling of suspended Fe is at least partially controlled by primary production. However, this relationship may not be dominated by active uptake of Fe into phytoplankton, but instead may reflect scavenging and removal of PFe during phytoplankton sedimentation.

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