BG Biogeosciences BG 1726-4189 Copernicus GmbH Göttingen, Germany 10.5194/bg-6-2397-2009 Dissolved iron (II) in the Baltic Sea surface water and implications for cyanobacterial bloom development Breitbarth E. 1 2 3 Gelting J. 2 Walve J. 4 Hoffmann L. J. 1 3 5 Turner D. R. 1 Hassellöv M. 1 Ingri J. 2 Department of Chemistry, Analytical and Marine Chemistry, University of Gothenburg, Kemivägen 10, 412 96 Gothenburg, Sweden Luleå University of Technology, Division of Applied Geology, 97187 Luleå, Sweden Department of Chemistry, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand Department of Systems Ecology, Stockholm University, 10691 Stockholm, Sweden Department of Plant and Environmental Sciences, University of Gothenburg, P.O. Box 461, 40530 Gothenburg, Sweden 04 11 2009 6 11 2397 2420 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.biogeosciences.net/6/2397/2009/bg-6-2397-2009.html The full text article is available as a PDF file from http://www.biogeosciences.net/6/2397/2009/bg-6-2397-2009.pdf

Iron chemistry measurements were conducted during summer 2007 at two distinct locations in the Baltic Sea (Gotland Deep and Landsort Deep) to evaluate the role of iron for cyanobacterial bloom development in these estuarine waters. Depth profiles of Fe(II) were measured by chemiluminescent flow injection analysis (CL-FIA). Up to 0.9 nmol Fe(II) L<sup>&minus;1</sup> were detected in light penetrated surface waters, which constitutes up to 20% to the dissolved Fe pool. This bioavailable iron source is a major contributor to the Fe requirements of Baltic Sea phytoplankton and apparently plays a major role for cyanobacterial bloom development during our study. Measured Fe(II) half life times in oxygenated water exceed predicted values and indicate organic Fe(II) complexation. Potential sources for Fe(II) ligands, including rainwater, are discussed. Fe(II) concentrations of up to 1.44 nmol L<sup>&minus;1</sup> were detected at water depths below the euphotic zone, but above the oxic anoxic interface. Mixed layer depths after strong wind events are not deep enough in summer time to penetrate the oxic-anoxic boundary layer. However, Fe(II) from anoxic bottom water may enter the sub-oxic zone via diapycnal mixing and diffusion.

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