Increasing concentrations of iron in surface waters as a consequence of reducing conditions in the catchment area

• Published in: Journal of geophysical research. Biogeosciences 2016-02, Vol.121 (2), p.479-493
• Main Authors: Ekström, Sara M., Regnell, Olof, Reader, Heather E., Nilsson, P. Anders, Löfgren, Stefan, Kritzberg, Emma S.
• Format: Article
• Language: English
• Subjects: Air temperature; Anaerobic conditions; Aquatic environment; Biogeochemical cycles; Biogeochemistry; Biologi; Biology; Catchment areas; Catchments; Discharge; Dissolved organic matter; Dynamics; Earth and Related Environmental Sciences; Environmental Sciences; Freshwater; Geochemistry; Geokemi; Geovetenskap och miljövetenskap; Iron; Mercury (metal); Methylmercury; Microbial activity; Miljövetenskap; Natural Sciences; Naturvetenskap; Oceanografi, hydrologi, vattenresurser; Oceanography, Hydrology, Water Resources; redox dynamics; Rivers; Seasonal variations; Snowmelt; Sulfates; sulfur; Surface water; Trends; Water chemistry
• ISSN: 2169-8953; 2169-8961; 2169-8961
• DOI: 10.1002/2015JG003141

Recent studies report trends of strongly increasing iron (Fe) concentrations in freshwaters. Since Fe is a key element with a decisive role in the biogeochemical cycling of major elements, it is important to understand the mechanisms behind these trends. We hypothesized that variations in Fe concentration are driven mainly by redox dynamics in hydraulically connected soils. Notably, Fe(III), which is the favored oxidation state except in environments where microbial activity provide strong reducing intensity, has several orders of magnitude lower water solubility than Fe(II). To test our hypothesis, seasonal variation in water chemistry, discharge, and air temperature was studied in three Swedish rivers. Methylmercury and sulfate were used as indicators of seasonal redox changes. Seasonal variability in water chemistry, discharge, and air temperature in the Emån and Lyckeby Rivers implied that the variation in Fe was primarily driven by the prevalence of reducing conditions in the catchment. In general, high Fe concentrations were observed when methylmercury was high and sulfate was low, indicative of reducing conditions. The Fe concentrations showed no or weak relationships with variations in dissolved organic matter concentration and aromaticity. The seasonal variation in Fe concentration of the Ume river was primarily dependent on timing of the snowmelt in high‐ versus low‐altitude areas of the catchment. There were long‐term trends of increasing temperature in all catchments and also trends of increasing discharge in the southern rivers, which should increase the probability for anaerobic conditions in space and time and thereby increase Fe transport to the aquatic systems. Key Points Factors controlling variability in riverine iron concentrations is studied Results support that seasonal variation is driven by microbially mediated redox dynamics Long‐term trends may also be supported by increasing weathering