Organic iron complexes enhance iron transport capacity along estuarine salinity gradients of Baltic estuaries

Simon David Herzog,Kristina Kvashnina,Emma Sofia Kritzberg,Per Persson

doi:10.5194/bg-17-331-2020

Abstract

Abstract. Rivers discharge a notable amount of dissolved Fe (1.5×109 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 particular and dissolved riverine Fe is considered to be lost to the sediment due to aggregation during estuarine mixing. Recently, however, several studies demonstrated 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 more 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 draining into the Baltic Sea and confirmed a significant but variable contribution of Fe-OM in relation to Fe (oxy)hydroxides among river mouths. We further found 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 upstream in a catchment than at the river mouth. The stability of Fe to increasing salinity, as assessed by artificial mixing experiments, correlated well to the relative contribution of Fe-OM, confirming that organic complexes promote Fe transport capacity. This study suggests that boreal rivers may provide significant amounts of potentially bioavailable Fe beyond the estuary, due to organic matter complexes.

Highlights

Iron (Fe) mobility from the lithosphere and pedosphere into the hydrosphere and biosphere is controlled by physical, chemical and biological processes
The results of the quantitative modeling of the extended X-ray absorption fine structure (EXAFS) spectra and linear combination fitting (LCF) analysis correlated, which is satisfying considering the potential sources of error of both analyses
Results showed a wide variation in the relative contribution of the Fe phases across river mouths, with some dominated by Fe-organic matter (OM), e.g., Lyckebyspring river samples, and some by Fehydroxide, e.g., Öre river

Summary

Introduction

Iron (Fe) mobility from the lithosphere and pedosphere into the hydrosphere and biosphere is controlled by physical, chemical and biological processes. While Fe is the fourth most abundant element in the Earth’s crust (Taylor, 1964), Fe concentrations in oxygenated aquatic systems are generally low (Johnson et al, 1997; Kraemer, 2004), but they can be higher during high flow conditions and in boreal waters with high dissolved organic carbon (DOC) concentrations (Kritzberg et al, 2014; Ekström et al, 2016). Several studies report rising Fe concentrations in surface waters, especially in northern Europe (Neal et al, 2008; Kritzberg and Ekström, 2012; Sarkkola et al, 2013; Weyhenmeyer et al, 2014; Björnerås et al, 2017), suggesting that Fe export from soils is increasing.

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