Understanding the dynamics of δ 13C and δ 15N in soft tissues of the bivalve Crassostrea gigas facing environmental fluctuations in the context of Dynamic Energy Budgets (DEB)

Abstract

We studied the dynamics of stable isotopes δ 13C and δ 15N of an opportunistic suspension feeder the Pacific oyster ( Crassostrea gigas) to better understand the factors that influence the trophic enrichment (trophic-shift, Δ) between primary producers and consumers. Most of the previous studies on this topic do not quantify mass fluxes or isotopic discrimination phenomena in the organism, which are two pillars in isotope ecology. We used a Dynamic Energy Budget (DEB) approach ( Kooijman, 2010) to quantify i) the fluxes of elements and isotopes in C. gigas soft tissues and ii) the impact of the scaled feeding level, the organism mass and the isotopic ratio of food on the “trophic-shift” Δ, and isotope turnover in tissues. Calibration and parametrization modeling were based on data from the literature. We showed that a five-fold increase in scaled feeding level leads to a decrease of the trophic-shift value of 35% for carbon and 43% for nitrogen. This can be explained by the molecule selection for the anabolic and/or catabolic way. When f increases due to the reserve dynamic formulation in the standard DEB model, the half-life of the isotopic ratio t δ 1/2 in tissues also decreases from 13.1 to 7.9 d for δ 13C and from 22.1 to 10.3 d for δ 15N. Organism mass also affects the trophic-shift value: an increase of the individual initial mass from 0.025 g to 0.6 g leads to an enrichment of 22% for δ 13C and 21% for δ 15N. For a large individual, these patterns show that a high structural volume has to be maintained. Another consequence of the mass effect is an increase of the half-life for δ 13C from 6.6 to 12.0 d, and an increase of the half life for δ 15N from 8.3 to 19.4 d. In a dynamic environment, the difference in the isotopic ratios between the individual tissues and the food (δ 13C W –δ 13C X ) exhibits a range of variation of 2.02‰ for carbon and 3.03‰ for nitrogen. These results highlight the potential errors in estimating the contributions of the food sources without considering the selective incorporation of isotopes. We conclude that the Dynamic Energy Budget model is a powerful tool to investigate the fate of isotopes in organisms.