Bivalve shells can potentially record the carbon isotopic signature of dissolved inorganic carbon (delta C-13(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 CO2 and external dissolved inorganic carbon (DIC). The incorporation of respired C-13- depleted carbon into the skeletons of aquatic invertebrates is well documented, and may affect the delta C-13 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 delta C-13 value of bivalve shells. In this study, it is assessed if the delta C-13 values of Mytilus edulis shells can be used as a proxy of delta C-13(DIC) as well as providing an indication of salinity. First, the delta C-13 values of respired CO2 (delta C-13(R)) were considered using the delta C-13 values of soft tissues as a proxy for delta C-13(R). Along the strong biogeochernical gradient of the Scheldt estuary (The Netherlands-Belgium), delta C-13(R) was linearly related to delta C-13(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 delta C-13 varied differently than delta C-13(DIC) and delta C-13(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 delta C-13 in these shells as a robust delta C-13(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 delta C-13(DIC) (> 2 parts per thousand) can be determined using M. edulis shell delta C-13. (c) 2006 Elsevier Ltd. All rights reserved.