title: Iron as a source of color in river waters.
abstract: Organic chromophores of total organic carbon (TOC) and those of iron (Fe) contribute to the color of water, but the relative contributions of colored organic carbon (COC%) and Fe (Fe%) are poorly known. In this study, we unraveled Fe% and COC% in 6128 unfiltered water samples collected from 94 Finnish river sites of contrasting catchment properties. According to regression analysis focusing on TOC alone, on average 84% of the mean TOC consisted of COC, while 16% was non-colored or below the color-detection limit. COC and Fe were much more important sources of color than phytoplankton (chlorophyll a as a proxy) or non-algal particles (suspended solids as a proxy). When COC and Fe were considered as the only two sources of color, COC% ranged from 16.8% to 99.5% (mean 71%) and Fe% from 0.5% to 83.2% (mean 29%). Similar Fe% and COC% values were obtained when color was estimated from the absorption coefficients of COC and Fe at 490 nm. Fe% increased as a function of the concentration of Fe and was well predicted by the TOC-to-Fe mass ratio. In 608 samples with TOC-to-Fe ratios of <4.5, Fe dominated the color. TOC-to-Fe ratios varied widely within most sites, but in relation to hydrology. In catchments with a peatland coverage of >30%, peak flow exported elevated amounts of TOC relative to Fe and resulted in a high COC%. Base flow, instead, mobilized elevated amounts Fe relative to TOC and resulted in a high Fe%. In a catchment covered with 31% of agricultural fields, peak flow transported eroded soil particles high in Fe and thus resulted in a high Fe%, while during base flow the water was high in COC%. This study demonstrated that Fe% and COC% vary widely in river water depending on the catchment properties and hydrology.
authors.affiliation: Department of Environmental Sciences, University of Helsinki, Viikinkaari 1, P.O. Box 65, FI-00014 Helsinki, Finland. Electronic address: yihua.y.xiao@jyu.fi.
authors.forename: Yi-Hua
authors.initials: YH
authors.lastname: Xiao
authors.affiliation: Finnish Environment Institute (SYKE), Mechelininkatu 34a, FI-00260 Helsinki, Finland. Electronic address: antti.raike@ymparisto.fi.
authors.forename: Antti
authors.initials: A
authors.lastname: Räike
authors.affiliation: Department of Food and Environmental Sciences, University of Helsinki, Latokartanonkaari 11, FI-00410 Helsinki, Finland. Electronic address: helina.hartikainen@helsinki.fi.
authors.forename: Helinä
authors.initials: H
authors.lastname: Hartikainen
authors.affiliation: Department of Biological and Environmental Sciences, University of Jyväskylä, Survontie 9, P.O. Box 35, FI-40014 Jyväskylä, Finland. Electronic address: anssi.vahatalo@jyu.fi.
authors.forename: Anssi V
authors.initials: AV
authors.lastname: Vähätalo
keywords: Absorption coefficient; Iron; Total organic carbon; Water color