Isotope trophic-step fractionation of suspension-feeding species: Implications for food partitioning in coastal ecosystems

Pacific oysters (Crassostrea gigas) and blue mussels (Mytilus edulis) were reared on a microalgal (Skeletonema costatum) diet to estimate their diet-tissue isotopic fractionation (Δ). We established that the diet-tissue fractionation value for stable carbon (δ13C) and stable nitrogen (δ15N) were around + 2.0 and + 3.6 for 13C and 15N, respectively. Equilibrium isotopic signatures, as well as turnovers of carbon and nitrogen in whole tissues, were approximated by exponential decay models. Half-life values in bivalves were about two times longer for carbon (around 8.5days) than for nitrogen (14.5days). We then used species-specific fractionation values to re-estimate contributions of food sources to the diets of similar suspension-feeding species, for which standard fractionation values had been assumed in previous studies. We emphasize the importance of knowing specific isotopic fractionation values to understand inter-specific trophic relationships and food webs.