Abstract
Concentrations of dissolved organic matter (DOM), dissolved humic substances (DHS), and DOM-complexed Fe (DOM-Fe) in Yamakuni and Oita Rivers, Japan, of which headstream is near in location, flow pass length is similar, but watershed soil type differs, were investigated. Soil organic matter level was higher in black Andosols distributing 67% of the watershed area of the Oita River than in Cambisols covering 90% of the watershed area of Yamakuni River. However, the DOM concentration in the Yamakuni River (0.44 - 1.62 mg·C·L-1) tended to be higher than that in the Oita River (0.13 - 1.37 mg·C·L-1). DHS accounted for 49% - 80% of DOM in both rivers. Fe and DOM-Fe concentrations showed a trend to increase toward downstream but decrease at the estuary in both rivers. DOM-Fe accounted for 26% - 90% and 55% - 93% of dissolved Fe in the Yamakuni and Oita Rivers, respectively. Correlation analysis suggested that the DOM-Fe concentration in the river water was controlled by the capacity for supplying Fe ions rather than that for supplying DOM. Although the ability to form a complex with Fe was suggested to be greater in the DOM in the Oita River than that in the Yamakuni River, the DOM-Fe concentration at the estuary was similar in the two rivers. Thus, the effect of soil organic matter level in the watershed area on the supply of Fe or DOM-Fe to the estuarine ecosystem was not significant.
Highlights
Dissolved organic matter (DOM) in aquatic environments has a number of biogeochemical functions, including supplying energy and nutrients to aquatic biota [1,2,3]
In the Oita River, both the DOC and dissolved humic substances (DHS) concentrations tended to increase from the headstream toward the estuary
Similar trends were observed in the Yamakuni River, the DOC and DHS concentrations at Point 3 (1.38 - 1.49 and 0.74 - 1.07 mg·C·L−1, respectively) while greater than those at Point 5 in the Oita River
Summary
Dissolved organic matter (DOM) in aquatic environments has a number of biogeochemical functions, including supplying energy and nutrients to aquatic biota [1,2,3]. Trace elements such as Fe and Cu are supplied from sediment and terrestrial sources, the chief component of which is soil in the watershed area. Ferric ions in a water body can remain dissolved by forming water-soluble complexes with DOM, which act as the Fe source for aquatic biota [4,5,6]. Dissolved HS (DHS) in water are classified as humic and fulvic acids on the basis of their differences in solubility at low pH. Fulvic acids, which are soluble at any pH, are a major DOM component in the aquatic environment [2,7]. It is considered that the DHS significantly contribute to the maintenance of ecosystems in estuarine and coastal areas through the delivery of Fe and other nutritional elements
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