Stable carbon isotopic characterization of DOC and its humic fraction in Lake Kasumigaura, Japan

Recent studies in Japan have reported that dissolved organic carbon (DOC) concentration in lake water showed a steady increase (SHIGA PREFECTURE 1996, JE A 1997). Dissolved organic carbon has the potential to form various disinfection by-products (DBP) such as trihalomethanes (THM), which are well-known cancerogenic substances, during water disinfection (LEENHEER & CROUE 2003). Furthermore, DOC influences phytoplankton activity in the lake environment (NAGAI et al. 2006). To maintain good quality drinking water and sound status of aquatic environments, it is urgent to resolve why DOC accumulates in lake water. Although the mechanism of increasing DOC concentrations in lake is still unclear, the accumulation of recalcitrant DOC (RDOC) should play an important role. Humic substances are typical RDOC, accounting for more than 30% of RDOC in Lake Kasumigaura (IMAI 2004). Stable carbon isotope measurements have been used to determine the source and cycling of DOC in the continental margins and ocean (DRUFFEL & WILLIAMS 1992, BAUER et al. 2001 ). In this study, we applied stable carbon isotope measurements (o C) to DOC of lacustrine environment, its humic fraction (HS) and RDOC in Lake Kasumigaura as well as to those derived from forest stream (FS) water samples. Because FS was dominated by HS, which accounted for more than 60% of the DOC (IMAI et al. 2001 ), it could be a substantial source of HS for Lake Kasumigaura. Our stable carbon isotopic characterization can lead to further understanding of DOC sources and its fate in Lake Kasumigaura.