Carbon Isotope Composition and Its Variation of Particulate Organic Matter in River Waters from the Kuzuryu River System

Abstract

Radioactive and stable carbon isotopes (¹⁴C and ¹³C) can serve as powerful tools for studying the dynamics of organic matter in aquatic systems. This study considered the transport of particulate organic matter in a river system, a main river and its tributary with different watershed conditions, using carbon isotopic signatures. The Kuzuryu River system is located at Fukui and Gifu Prefecture in the central part of Japan, and consists of a main river, Kuzuryu River, and its main tributary, the Hino River. River research was conducted at a fixed station in the lower reaches of both rivers during June to October in 2010. Suspended solid samples were collected from 85 L to 141 L of river water using a continuous-flow centrifugation. δ¹³C of organic matter in riverine suspended solids had –26.3‰ to –24.0‰ for the Kuzuryu River and –27.0‰ to –26.1‰ for the Hino River. The Δ¹⁴C values of the Kuzuryu and Hino samples were –168‰ to –87‰ and –209‰ to –143‰, respectively. The Δ¹⁴C values of Hino River were depleted in ¹⁴C rather than the Kuzuryu River. However, the carbon isotopes have correlations with the water level for both rivers. The differences in the particulate organic matter characteristics are explained by the distribution of paddy fields and forests in each watershed, and the differences in the supply of suspended solids from their watershed due to rainfall.