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

Thomas S. Adams and Robert W. Sterner

Limnol. Oceanogr., 45(3), 2000, 601-607 | DOI: 10.4319/lo.2000.45.3.0601

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 d15N 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 d15N values. Specifically, we investigated the relationship between the d15N 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, (Ddt = D. magna d15N - S. acutus d15N), 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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