The influences of diet and seawater dissolved inorganic carbon (DIC) on the carbon isotope composition of shell aragonite d13Cshell in the Manila clam Ruditapes philippinarum reared under laboratory conditions were investigated. Clams were exposed to two successive negative carbon isotope shifts: a first shift in diet (d13Cphytoplankton) and a second shift, 35 days later, in DIC (d13CDIC). Both successive shifts induced a decrease in d13Cshell. These results are the first to experimentally confirm an incorporation of respired carbon derived from food and carbon from DIC into shell carbonate of adult bivalves. Skeletal d13C responded to changes in the d13C of both diet and DIC in less than 7 days. Consequently, proxies based on d13Cshell may be used with high temporal resolution. Using d13Cphycoplankton as a proxy for the carbon isotope composition of respired carbon (d13CR) resulted in a rather constant percentage of metabolic carbon (CM) into the shell carbonate over time (close to 12%). However, an accurate estimation of d13CR is required in order to precisely estimate the percentage of metabolic carbon incorporated into the shell. Despite the significant incorporation of metabolic carbon into shell carbonate, our experimental results revealed that d13Cshell was highly correlated with d13CDIC (r2=0.77, p<0.0001). Thus it seems that d13Cshell is a promising proxy of large scale variations in d13CDIC and therefore of salinity in estuarine water.
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