Dissolved iron (II) in the Baltic Sea surface water and implications for cyanobacterial bloom development

Abstract

Abstract. 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 −1 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 −1 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.Correspondence to: E. Breitbarth (ebreitbarth@chemistry.otago.ac.nz)

Fractionation of iron species and iron isotopes in the Baltic Sea euphotic zone

“…We conclude three major findings from our study: (Breitbarth et al, 2009) are orders of magnitude lower than this number. In spite of this, a strong correlation was observed between chl-a and PFe.…”

“…Complexes between metals and fulvic acids may pass through the diffusive gel and are thus measured by the DGT technique (Scally et al, 2006), however it is not clear if other Fe complexes may be retained by the membrane. Breitbarth et al (2009), show that the DGT labile fraction is very similar to the Fe(II) concentration at Gotland Deep during the summer of 2007, which invokes that DGT's may under sample the actual soluble Fe fraction that may be larger than the Fe(II) pool.…”

“…Estimations of Fe(II) half-lives range in the same order of magnitude (∼20-120 s, data not shown). However as shown by Breitbarth et al (2009) actual Fe(II) half-lives may far exceed predicted values and together with overall higher Fe concentrations could result in significant Fe(II) concentrations during sunlit days in the Bothnian Sea. Probably a large portion of the oxidized Fe is recycled back to Fe(II), but if for example 10% of all Fe(II) goes to formation of PFe, this process will contribute to more than 10 nanomolar PFe per day.…”

“…Studies have shown that nonbiogenic oxidation of Fe(II) generates Fe-oxyhydroxides enriched in 56 Fe (Bullen et al, 2001). If the levels of Fe(II) at the other Baltic Sea stations are similar to the standing stock of 0.25-0.5 nM at the Gotland Deep station (Breitbarth et al, 2009), then the redox cycle of Fe in the euphotic zone may affect the isotopic composition of the suspended matter as much as the riverine input. Estimations of Fe(II) half-lives range in the same order of magnitude (∼20-120 s, data not shown).…”

“…In the Baltic Proper surface water a high persistent standing stock of Fe(II) recently was measured in the summer (Breitbarth et al, 2009). Studies have shown that nonbiogenic oxidation of Fe(II) generates Fe-oxyhydroxides enriched in 56 Fe (Bullen et al, 2001).…”