What's Hiding Behind Ontogenetic δ13C Variations in Mollusk Shells? New Insights from the Great Scallop (Pecten maximus)

Mollusk shells contain geochemical information about environmental conditions that prevailed at the time of formation. We investigated ontogenetic and seasonal variations of delta C-13 in calcitic shells of Pecten maximus. Ontogenetic variations of delta C-13(shell) in three large specimens collected in Norway, France, and Spain exhibited a similar linear decrease with increasing shell height. We removed this linear drift (detrending). These three residual time series displayed variations that could be linked to environmental fluctuations. To check it, we reanalyzed the isotopic datasets of Lorrain et al. (Journal of Experimental Marine Biology and Ecology 275:47-61, 2002, Geochimica et Cosmochimica Acta 68:3509-3519, 2004), who worked on three scallops harvested in 2000 in the bay of Brest (France), a well-monitored ecosystem. Lowest values of delta C-13(shell detrended) were recorded in all shells in late spring-early summer, most likely reflecting corresponding variations in food availability. Our results indicate that ontogenetic and seasonal variations of delta C-13(shell) cannot be used as a proxy for past delta C-13(DIC) variations but should be considered as promising tools for ecophysiological studies.