2020 年 54 巻 2 号 p. 71-79
Stable isotope ratio measurements of dissolved inorganic carbon (δ13CDIC) provide a useful indication of carbon sources, sinks and fluxes in river ecosystems. Because of the incorporation of dissolved inorganic carbonate during shell growth, shells of aquatic mollusks have the potential to record DIC information and to be a proxy for δ13CDIC in this environment. In our study, specimens of the bivalve mollusk Corbicula fluminea (the Asian freshwater clam) were reared under laboratory conditions and δ13Cshell and δ13CDIC values were measured. It was found that the δ13Cshell values of C. fluminea reared in three tanks which were provided with isotopically distinct foods differed significantly and that all values were more negative than would be obtained under equilibrium fractionation conditions; this is probably because of metabolic effects. In order to estimate the percentage of metabolically-derived carbon in the shell carbonate (CM), δ13Cfood was used as a proxy for the δ13C of metabolic carbon. CM values in the three different tanks were 35%, 49% and 44%, respectively. These proportions are larger than those reported in previous studies. We suggest that the most probable cause is the auto-correlation between δ13Cfood and δ13CDIC. Despite the significant incorporation of metabolic carbon into shell carbonate, our experimental results showed that δ13Cshell was highly correlated with δ13CDIC, following this relationship: δ13CDIC = 0.86 δ13Cshell + 1.57 (R2 = 0.99). A shell collected in 2007 from a river monitored for δ13CDIC shows good agreement between back-calculated and measured δ13CDIC values for the summer of 2006. We suggest that the δ13Cshell of Corbicula fluminea is a promising qualitative proxy of large-scale variations in δ13CDIC in the river environment.