The effect of dietary nitrogen content on trophic level 15N enrichment

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Abstract
Given the great potential value of stable isotopes in a variety of scientific investigations, surprisingly little attention has been paid to the underlying physiological and biochemical mechanisms that account for trophic increases in ;415N values. This has lead to a general call for controlled studies investigating the relationship between organismal diet and corresponding isotopic composition. We conducted a series of laboratory studies varying dietary nitrogen content and measuring corresponding variations in organismal ;415N values. Specifically, we investigated the relationship between the ;415N values of the anomopod crustacean, Daphnia magna, and the C:N ratio of its food, the green algae, Scenedesmus acutus. Daphnids were raised to a standard life stage on three types of S. acutus as food, which ranged in C:N (atomic) from 7.3 to 24.8. The average C:N of the daphnids was 6.0. 15N enrichment was found to be strongly linearly related to the C:N of the algae, ranging from nearly zero to approximately 6‰, which would normally be considered a span of almost two trophic levels. The d15N values of the daphnids and the diet‐tissue isotope fractionation factor, (Δdt = D. magna ;415N – S. acutus ;415N), were inversely related to the nitrogen content of the algae (R2 = 0.82 and 0.99 in two separate runs). To our knowledge, this is the first controlled study of nitrogen balance and d15N values in animals.

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