Dissolved Fe(II) in a river-estuary system rich in dissolved organic matter
Introduction
Fe(II) has been reported to account for a large fraction of dissolved Fe in a range of both fresh and marine waters (Hong and Kester, 1985, Landing and Westerlund, 1988, Sarthou et al., 2011). The dominant source of Fe(II) in surface waters is normally photochemical (Collienne, 1983, Croot et al., 2008, Roy et al., 2008). Despite its presence in marine and fresh surface waters, and its greater solubility than Fe(III) (Shaked and Lis, 2012), little information is available on the distribution and speciation of dissolved Fe(II). As formation of Fe(II) is generally represented as a poorly characterised solubilisation process (Emmenegger et al., 2001), this mechanistic gap in our knowledge may impede the development of better global ocean models (Tagliabue and Voelker, 2011).
During estuarine mixing, dissolved Fe is known to be strongly non-conservative (Boyle et al., 1977). However, few studies have distinguished between the estuarine mixing of dissolved Fe(II) and total dissolved Fe. Hong and Kester (1985) demonstrated that oxidation of Fe(II) is not a significant contributor to dissolved Fe removal at low salinities. Given the importance of humic materials in initiating flocculation and the significant difference in the metal-ligand binding preferences of the two redox states of Fe (Sholkovitz et al., 1978), the question is therefore raised as to whether the ratio of dissolved Fe(II):Fe(III) present in riverine waters is maintained during estuarine mixing and delivery to the ocean. If so, estuaries would be a source of Fe(II) to coastal waters.
Measuring the concentration of Fe(II) during estuarine mixing in a high DOM estuary will establish whether or not Fe(II) is removed similarly to Fe(III) in the presence of high concentrations of natural organic compounds. Coastal waters are known to contain organic material that can stabilise higher Fe(II) concentrations than would be possible inorganically under oxic conditions (Zhuang et al., 1995, Rose and Waite, 2003a, Okada et al., 2005). If estuaries are a source of such material we would expect to see a relatively high fraction of dissolved Fe present as Fe(II) throughout estuarine waters. The main objective here was therefore to use the Beaulieu as a model of a high DOM river to investigate the forms of dissolved Fe present and to see how Fe(II) concentration changes during estuarine mixing.
Section snippets
The River Beaulieu
The River Beaulieu drains the New Forest (Hampshire, UK) and thus contains water primarily from exposed heathland and some mixed coniferous/deciduous forest. The river water has a near neutral pH (6.57.8), high dissolved organic carbon (DOC, 250–1800 μM) and high total dissolved (<0.45 μm) Fe (18 μM) content (Holliday and Liss, 1976, Moore et al., 1979, Jones, 1993, Fang, 1995, Smith, 1995).1
Sample collection
All sampling apparatus and
Temporal and spatial variation in riverine water
Dissolved oxygen in 10 °C river water averaged 370 ± 20 μM (calculated saturation at 10.0 °C is 352 μM). Diurnal changes in irradiance, DFe and Fe(II) concentrations (17 December 2012), as determined manually and using an in situ analyser, are shown in Fig. 4. DFe and Fe(II) concentrations determined manually throughout the day had mean values (and standard deviations) of 20.1 ± 0.40 μM and 3.96 ± 0.87 μM respectively. Over the sampling period, pH increased from 6.4 to 6.6 and water level fell
Temporal and spatial variation at King's Hat
Two features of Fig. 4 are particularly striking. First, there is a very large difference between reported dissolved Fe(II) and DFe using the two sampling and analysis methods. Second, there is an apparent absence of a diurnal sinuous trend in Fe(II). Such a trend has been reported in both fresh and marine surface waters (Johnson et al., 1994, Emmenegger et al., 2001).
Although the trend (Fig. 4) observed throughout the day by both measurement methods is similar, manually collected samples
Conclusions
Five transects measuring dissolved Fe are now reported for the Beaulieu (15 January 2013; 5 February 1974 (Holliday and Liss, 1976); 17 March, 13 May and 22 September 1994 (Fang, 1995)) providing snapshot images of an estuarine Fe distribution that varies hourly and only the transect reported here distinguishes between Fe(II) and DFe. We demonstrate that Fe(II) decreases as a fraction of DFe with increasing salinity. A range of DFe removal factors are reported for the Beaulieu estuary (60–90%),
Acknowledgements
The authors wish to thank the Gillings Foundation for studentship support (MJH), the crew of RV Bill Conway, Dr Gary Fones (University of Portsmouth) for ICP-MS data and Mr Mario Esposito (University of Southampton) for analysis of DOC samples. Two anonymous reviewers are thanked for comments that improved the manuscript.
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