CAN THE STABLE CARBON ISOTOPIC COMPOSITION OF BIVALVE SHELLS BE USED AS A PROXY OF δ13C-DIC AND PROVIDE AN INDICATION OF ESTUARINE SALINITY?

Bivalve shells can potentially record the stable carbon isotopic signature of dissolved inorganic carbon (δ¹³C-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 ¹³C-depleted carbon into the skeletons of aquatic invertebrates is well documented, and may affect the δ¹³C 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 δ¹³C value of bivalve shells. In this study, it is assessed if the δ¹³C values of Mytilus edulis shells can be used as a proxy of δ¹³C-DIC as well as providing an indication of salinity. First, the δ¹³C values of respired CO₂ (δ¹³C-R) were considered using the δ¹³C values of soft tissues as a proxy for δ¹³C-R. Along the strong biogeochemical gradient of the Scheldt estuary (The Netherlands–Belgium), δ¹³C-R was linearly related to δ¹³C-DIC (R² = 0.87), which in turn was linearly related to salinity (R² = 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 δ¹³C varied differently than δ¹³C-DIC and δ¹³C-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 the δ¹³C composition of these shells as a robust δ¹³C-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 δ¹³C-DIC (>2 per mil) can be determined using M. edulis shell δ¹³C.