Diet-dependent delta C-13 and delta N-15 fractionation among sea urchin Lytechinus variegatus tissues: implications for food web models

Consumer-diet discrimination factors (Delta N-15 and Delta C-13) are often applied without corroboration from laboratory experiments. Deviations in Delta N-15 and Delta C-13 that may occur from different diet types were quantified by raising 120 sea urchins Lytechinus variegatus in laboratory tanks on 3 different diets: seagrass Thalassia testudinum, red foliose macroalgae (Grauteloupia sp. and Palmaria palmata) and a mixed diet specifically formulated for L. variegatus. Patterns of delta C-13 and delta N-15 and resulting fractionation factors were then determined in muscle, gonad, gut, test organic matrix and whole individuals. Tissue delta C-13 values showed a strong positive association with delta C-13 of diets and estimates of absorption efficiency (R-2 = 0.81). The seagrass diet consistently resulted in negative Delta C-13 values in all tissues (from -0.86% in muscle to -1.63 parts per thousand in gonad) and whole individuals (-1.19 parts per thousand), whereas macroalgal and formulated diets showed positive values (0.11 and 0.19%, respectively). Only individuals on the formulated diet clearly reached isotopic equilibrium for delta C-13, suggesting that other lower quality diets may have resulted in more continuous reallocation of internal resources. delta N-15 values increased as the nitrogen content of these diets decreased (3.18, 1.21 and 0.82 parts per thousand for seagrass, macroalgae and formulated diets, respectively). Overall differences in the biochemical composition of diets and a robust relationship between delta C-13 and delta N-15 signatures suggest that protein quantity and quality could be central in driving isotope fractionation. The influence of macrophyte material type in the diet can be stronger than that of trophic level for both delta C-13 and delta N-15, so further compound-specific isotope analyses are necessary to determine reliable values of Delta C-13 and Delta N-15 for ecological applications.