Fig. 6.3.

(A) Compound-specific deuterium allocation into fatty acids of Daphnia via diet without labelling (clear bars), feeding deuterium-labelled diet (light grey bars) or raising Daphnia in deuterium-enriched ambient water (dark grey bars). Strict dietary controlled allocation of LIN and ALA can be seen, while deuterium was enriched in ARA and EPA, when Daphnia were raised in deuterium-enriched environmental water with poor diet quality (Scenedesmus) but not with high diet quality (Cryptomonas) in which case EPA is also strictly dietary controlled. (B) δ13C values of converted ARA and EPA were significantly depleted in 13C in the same experiment, showing that δ2H and δ13C are kinetically coupled in fatty acid synthesising processes. (C) Relationships between fatty acid mass fractions in seston and fractionation of hydrogen isotopes in fatty acids of zooplankton relative to seston in multiple Austrian eutrophic ponds; the isotopic fractionation of ²H in EPA decreased strongly with increasing dietary EPA mass fractions. Shaded backgrounds indicate standard errors. (D) In a corresponding multilinear regression model, using environmental water, zooplankton ALA (precursor), C16:1 (proxy for FA synthesis) and seston EPA δ2H values, a switch in Feldman’s relative importance, calculating their contribution to the model at the respective level, occurs at mass fractions between 0.3 and 0.5 mg/L dry weight in seston, indicating transition from conversion to dietary allocation at this threshold (Pilecky et al. 2022a). ALA, α-linolenic acid; EPA, eicosapentaenoic acid; LIN, linoleic acid. Image credit: original figure by the authors. Black and white version prepared by Belinda Cale.