Iron concentrations are increasing in surface waters from forested headwater catchments in eastern Finland Sakari Sarkkola a, ⁎, Mika Nieminen b , Harri Koivusalo c , Ari Laurén a , Pirkko Kortelainen d , Tuija Mattsson d , Marjo Palviainen e , Sirpa Piirainen a , Mike Starr e , Leena Finér a a Finnish Forest Research Institute, P.O. Box 68, FI-80101 Joensuu, Finland b Finnish Forest Research Institute, P.O. Box 18, FI-01301 Vantaa, Finland c Aalto University School of Engineering, Department of Civil and Environmental Engineering, P.O. Box 15200, FI-00076 Aalto, Finland d Finnish Environment Institute, P.O. Box 140, FI-00251 Helsinki, Finland e University of Helsinki, Department of Forest Sciences, P.O. Box 27, FI-00014 Helsinki, Finland HIGHLIGHTS • Both Fe and TOC are increasing in forested streams in eastern Finland. • Water brownification during the last 15–20yr relates to changes in both TOC and Fe. • A change in redox conditions is a plausible driving force behind increasing Fe trends. • Future research should integrate water quality monitoring with catchment soil studies. abstract article info Article history: Received 27 February 2013 Received in revised form 14 June 2013 Accepted 16 June 2013 Available online 9 July 2013 Editor: Eddy Y. Zeng Keywords: Fe concentration TOC concentration Brownification Peatland Time series Trend analysis Observations of increased water colour have been made in lakes and rivers all across the northern mid-latitudes of Europe and North America, particularly during the last 10–20 yr. This water browning or brownification has been attributed to the increased organic carbon concentrations due to climate change and decreased acid atmospheric deposition. Given that iron (Fe) may also increase water colour, the contribution of Fe to water brownification has received small attention. Our aim was to study the temporal trends of Fe in forested headwa- ter catchments in eastern Finland, where an increasing air temperature and total organic carbon (TOC) trend had been observed in an earlier study. We found a statistically significant increasing trend also in stream water Fe concentrations and a strong correlation between the trends of TOC and Fe. The average increase in TOC and Fe concentrations between 1995 and 2006 was 0.5 mg l -1 yr -1 (2.5%), and 34.6 μgl -1 yr -1 (3.5%), respectively. These results indicate that the increased water colour or brownification in Northern Europe may not only be due to increased concentrations of organic matter but also increased concentrations of Fe. The change in precip- itation and temperature conditions, particularly during late autumn and early winter periods, appeared to be the main environmental factor behind increasing Fe trends. The strong correlation between the trends of Fe and TOC indicated that the increased Fe-organic matter complexation is the mechanism behind increasing Fe trends, but further research is needed to assess the chemical forms of increased Fe that coupled with increased TOC concen- trations would enhance water brownification. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Water colour has been observed to have increased in lakes and rivers across the northern mid-latitudes both in Europe and North America, particularly during the last 10–20 yr (e.g. Evans et al., 2005; Haaland et al., 2010). Several studies have been carried out to elucidate whether the cause of this widespread phenomenon, known as water browning or brownification, is global climatic warming (Eikebrokk et al., 2004; Erlandsson et al., 2008) or the decrease in acidic atmospheric, primarily sulphate deposition (Clark et al., 2005; Vuorenmaa et al., 2006; Monteith et al., 2007; de Wit et al., 2007; Löfgren and Zetterberg, 2011). Changes in land manage- ment practices have also been suggested as plausible contributors to an increased water colour (Siepak, 1999; Worrall et al., 2004). For a long time it has been known that the colour of water corre- lates strongly with total (TOC) and dissolved (DOC) organic carbon and during the last 15 yr the increased mobility and transport of TOC and DOC as the driving force for water browning has been the subject of substantial research activity (Freeman et al., 2001; Monteith et al., 2007; Erlandsson et al., 2008; Sarkkola et al., 2009; Zhang et al., 2010). Given that the relationship between iron (Fe) Science of the Total Environment 463–464 (2013) 683–689 ⁎ Corresponding author. Tel.: +358 40 801 5409. E-mail address: sakari.sarkkola@metla.fi (S. Sarkkola). 0048-9697/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2013.06.072 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv