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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-234
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-234
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Jul 2019

Research article | 03 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Organic Iron Complexes Enhance Iron Transport Capacity along Estuarine Salinity Gradients

Simon David Herzog1, Kristina Kvashnina2, Per Persson3,4, and Emma Kritzberg5 Simon David Herzog et al.
  • 1Department of Science and Environment, Roskilde University, DK-4000 Roskilde, Denmark
  • 2Centre for Environmental and Climate Research & Department of Biology, Lund University, 223 62, Lund, Sweden
  • 3The European Synchrotron, CS40220, 38043 Grenoble Cedex 9, France
  • 4Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, P.O. Box 510119, 01314, Dresden, Germany
  • 5Department of Biology/Aquatic Ecology, Lund University, SE-223 62, Lund, Sweden

Abstract. Rivers discharge a notable amount of Fe (1.5 x 109 mol yr−1) to coastal waters, but are still not considered important sources of bioavailable Fe to open marine waters. The reason is that the vast majority of riverine Fe is considered to be lost to the sediment due to aggregation during estuarine mixing. Recently however, several studies demonstrate relatively high stability of riverine Fe to salinity induced aggregation, and it has been proposed that organically complexed Fe (Fe-OM) can “survive” the salinity gradient, while Fe (oxy)hydroxides are prone to aggregation and selectively removed. In this study, we directly identified, by X-ray absorption spectroscopy, the occurrence of these two Fe phases across eight boreal rivers and confirmed a significant but variable contribution of Fe-OM in relation to Fe (oxy)hydroxides among river mouths. We further found that that Fe-OM was more prevalent at high flow conditions in spring than at low flow conditions during autumn, and that Fe-OM was more dominant in low-order streams in a catchment than at the river mouth. The stability of Fe to increasing salinity correlated well to the relative contribution of Fe-OM, i.e. confirming that organic complexes promote Fe transport capacity. This study suggests that boreal rivers may provide significant amounts of potentially bioavailable Fe to marine waters beyond the estuary, due to organic matter complexes.

Simon David Herzog et al.
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Simon David Herzog et al.
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Short summary
Fe concentrations in boreal rivers are increasing strongly in several regions in Northern Europe. This study focuses on was how Fe speciation and the interaction with organic matter affects stability of Fe across estuarine salinity gradients. The results confirm a positive relationship between the relative contribution of organically complexed Fe and stability. Moreover, organically complexed Fe was more prevalent at high flow conditions and more dominant further upstream in a catchment.
Fe concentrations in boreal rivers are increasing strongly in several regions in Northern...
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