Carbon isotope composition and its variation of particulate organic matter in river waters from the Kuzuryu river system

The carbon isotope compositions of organic matters (δ¹³C, Δ¹⁴C) are effective tracers for investigating the dynamics of organic matters in the aquatic environment. In this study, the authors collected 85 to 141 liters of river water from the Kuzuryu River main stream located in Fukui and Gifu prefectures and the downstream of the Hino River, major tributary of the Kuzuryu River, at one point each, four times during the period from June to October 2010. They separated suspended particles using a continuous centrifuge, and measured δ¹³C and Δ¹⁴C of the organic matters. As a result, the δ¹³C values of particulate organic matters in river water showed a different range from -26.3 ‰ to -24.0 ‰ in the Kuzuryu River and -27.0 ‰ to -26.1 ‰ in the Hino River. The Δ¹⁴C values were -168 ‰ to -87 ‰ in the Kuzuryu River and -209 ‰ to -143 ‰ in the Hino River, revealing that the particulates in the Kuzuryu River is apparently composed of new organic matters. In addition, δ¹³C and Δ¹⁴C against the concentration of particulate organic matters were plotted in different regions on the chart, but a correlation was observed with the river water level. From these findings, it is considered that the difference in carbon isotope compositions between the two rivers is due to be the result of reflecting the difference in the characteristics of organic matters supplied from each basin to the main river and the difference in runoff behavior due to rainfall. (A.O.)