Abstract Bivalve shells can potentially record the carbon isotopic signature of dissolved inorganic carbon (d 13 C DIC ) in estuarine waters, thereby providing information about past estuarine biogeochemical cycles. However, the fluid from which these animals calcify is a 'pool' of metabolic CO 2 and external dissolved inorganic carbon (DIC). The incorporation of respired 13 Cdepleted carbon into the skeletons of aquatic invertebrates is well documented, and may affect the d 13 C record of the skeleton. Typically, less than 10% of the carbon in the skeleton is metabolic in origin, although higher amounts have been reported. If this small offset is more or less constant, large biogeochemical gradients in estuaries may be recorded in the d 13 C value of bivalve shells. In this study, it is assessed if the d 13 C values of Mytilus edulis shells can be used as a proxy of d 13 C DIC as well as providing an indication of salinity. First, the d 13 C values of respired CO 2 (d 13 C R ) were considered using the d 13 C values of soft tissues as a proxy for d 13 C R . Along the strong biogeochemical gradient of the Scheldt estuary (The NetherlandsBelgium), d 13 C R was linearly related to d 13 C DIC (r 2 = 0.87), which in turn was linearly related to salinity (r 2 = 0.94). The mussels were highly selective, assimilating most of their carbon from phytoplankton out of the total particulate organic carbon (POC) pool. However, on a seasonal basis, tissue d 13 C varied differently than d 13 C DIC and d 13 C POC , most likely due to lipid content of the tissue. All shells contained less than 10% metabolic carbon, but ranged from near zero to 10%, thus excluding the use of d 13 C in these shells as a robust d 13 C DIC or salinity proxy. As an example, an error in salinity of about 5 would have been made at one site. Nevertheless, large changes in d 13 C DIC (>2&) can be determined using M. edulis shell d 13 C
