Increasing iron concentrations in surface waters – a factor behind brownification?

Abstract

Abstract. Browning of inland waters has been noted over large parts of the Northern hemisphere and is a phenomenon with both ecological and societal consequences. The increase in water color is generally ascribed to increasing concentrations of dissolved organic matter of terrestrial origin. However, oftentimes the increase in water color is larger than that of organic matter, implying that changes in the concentration of organic matter alone cannot explain the enhanced water color. Water color is known to be affected also by the quality of organic matter and the prevalence of iron. Here we investigated trends in water color, organic matter and iron between 1972 and 2010 in 30 rivers draining into the Swedish coast (data from the national Swedish monitoring program), and performed a laboratory iron addition experiment to natural waters, to evaluate the role of iron and organic matter in determining water color. By comparing the effect of iron additions on water color in the experiment, to variation in water color and iron concentration in the monitoring data, we show that iron can explain a significant share of the variation in water color (on average 25 %), especially in the rivers in the north of Sweden (up to 74 %). Furthermore, positive trends for iron are seen in 27 of 30 rivers (21-468 %) and the increase in iron is larger than that of organic matter, indicating that iron and organic matter concentrations are controlled by similar but not identical processes. We speculate that increasing iron concentrations can be caused by changes in redox conditions, that mean that more anoxic water with high concentrations of soluble FeII are feeding into the surface waters. More studies are needed about why iron is increasing so strongly, since both causes and consequences are partly different from those of increasing organic matter content.

DOC-dynamics in a small headwater catchment as driven by redox fluctuations and hydrological flow paths – are DOC exports mediated by iron reduction/oxidation cycles?

“…In many studies on DOC, there is little information available for iron in the discharge. While Kritzberg and Ekström (2012) did observe parallel increases of iron and DOC, they hypothesised that DOC and iron do not necessarily have identical mobilisation processes, as the ratios of iron and DOC varied among catchments. Interestingly, in the catchment under study here, iron to DOC ratios did not change significantly since 1987 (not shown), indicating a parallel dynamics.…”

“…While the effect of acid and/or sulfate deposition on catchment DOC export has been demonstrated for many sites and may be responsible for a large part of the observed long-term trends (e.g., reviewed in Clark et al, 2010), the process based understanding from this study adds another potential factor to cause increased DOC exports from catchments. Due to the tight coupling of DOC and iron dynamics and due to the high release of DOC under reducing conditions in wetlands (Zak and Gelbrecht, 2007;Grybos et al, 2009), an increase in iron reducing activity or an increase in reducing conditions and associated increases in pH causing higher DOC mobility and higher microbial activity in general would also explain rising DOC and iron concentrations in catchment exports in a long-term perspective (Kritzberg and Ekström, 2012). Higher microbial activity and higher iron reduction could e.g., be caused by increased temperatures due to climate change (e.g., temperature relationships reported in Winterdahl et al, 2011;Köhler et al, 2008), as depicted for the site under study here in Fig.…”

Decadal oscillations of the aquatic chemistry of river waters in Latvia

“…strong correlation with features of covariation between fluxes of TOC and Total Iron (R = 0.91) demonstrating similar sources of both substances (Kritzberg and Ekström, 2012). Meanwhile, in the Lielupe basin the correlation is much less pronounced (R = 0.32), indicating difference of sources.…”

“…Although it has been difficult to find out a single factor causing brownification, there is an implicit consensus that the direct factor causing this process is the increased concentrations of terrestrially derived organic matter (OM) in the water (Kritzberg and Ekström, 2012).…”

“…However, it is necessary to admit that the increase in water color cannot be explained only by changes in the quantity and quality of OM, iron also must be mentioned as a potential source of color (Canfield et al, 1984;Kritzberg and Ekström, 2012;Ekström, 2013). Considering low solubility of iron compounds it becomes evident that iron compounds in surface waters exist in form of complexes with OM.…”