ABSTRACT

Stable isotope ratio measurements of dissolved inorganic carbon (δ¹³C_DIC) 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 δ¹³C_DIC in this environment. In our study, specimens of the bivalve mollusk Corbicula fluminea (the Asian freshwater clam) were reared under laboratory conditions and δ¹³C_shell and δ¹³C_DIC values were measured. It was found that the δ¹³C_shell 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 (C_M), δ¹³C_food was used as a proxy for the δ¹³C of metabolic carbon. C_M 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 δ¹³C_food and δ¹³C_DIC. Despite the significant incorporation of metabolic carbon into shell carbonate, our experimental results showed that δ¹³C_shell was highly correlated with δ¹³C_DIC, following this relationship: δ¹³C_DIC = 0.86 δ¹³C_shell + 1.57 (R² = 0.99). A shell collected in 2007 from a river monitored for δ¹³C_DIC shows good agreement between back-calculated and measured δ¹³C_DIC values for the summer of 2006. We suggest that the δ¹³C_shell of Corbicula fluminea is a promising qualitative proxy of large-scale variations in δ¹³C_DIC in the river environment.