Trophic typology of coastal ecosystems based on delta C-13 and delta N-15 ratios in an opportunistic suspension feeder

Our objective was to use stable carbon (C) and nitrogen (N) isotope ratios of the adductor muscles of cultured Crassostrea gigas to typify the trophic state of temperate coastal ecosystems. Young oysters were introduced on a regional geographical scale in 8 locations along the coast of Normandy (France) and sampled after 9 mo in cultivation. Food sources were also investigated using a mixing model based on food source isotopic composition data previously obtained. To strengthen the interpretation of trophic ecosystem functioning, values of stable isotope ratios were combined with environmental variables in a principal component analysis (PCA). Isotopic values of adductor muscles varied significantly between -19.94 and -17.26 parts per thousand for delta C-13 and between 7.73 and 12.14 parts per thousand for delta N-15. PCA discriminated 2 groups of coastal ecosystems that differed in coastal hydrology, inputs of nutrients, and size of their respective watersheds. Our results suggest that isotopic signature differences between these 2 spatial groups appeared too important to be due to (1) variations in the isotopic ratios of food sources and (2) differing trophic step fractionation between locations, These differences are more probably linked with differences in oyster diets. Finally, we conclude that cultured C. gigas is a useful spatial bio-indicator of coastal ecosystem trophic functioning in temperate ecosystems and an interesting biological model for the determination of isotopic baselines.