Humic substances (HS) are the primary constituents of dissolved organic matter (DOM) and play pivotal roles in aquatic systems. Optical indices of DOM, such as specific UV absorbance (SUVA254), the fluorescence index (FI) and biological index (BIX), have gained wide interest because of their ease of use. In this study, we explored the relationship between HS and the indices in the Trat River Basin (eastern Thailand) from headwaters to the river mouth through the distinct dry and rainy seasons to examine whether changes in index values reflect variability in the relative contribution of HS to DOM, or %HS. The results show that %HS and the indices did not exhibit significant linear relationships (FI and BIX, P > 0.05), or the relationships changed seasonally (SUVA254). However, analyzing the indices versus %HS did show clear DOM composition changes by season with more humic-like or terrestrial material in the rainy season. Relationships between DOM and dissolved iron (dFe) concentrations were also explored. Separating the relationships of DOM versus dFe into HS versus dFe and non-HS versus dFe provides us the opportunity to better understand which fraction contributes more to dFe mobilization. The results indicate stronger positive linear relationships between HS and dFe concentrations independent of river tributary. Overall, this study highlights the importance of quantifying HS for the study of DOM dynamics or compositional changes along a river transect as well as for DOM-induced iron mobilization.
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