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

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 δ¹⁵SN 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 δ¹⁵N values. Specifically, we investigated the relationship between the δ¹⁵N 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 δ¹⁵N values of the daphnids and the diet-tissue isotope fractionation factor, (Δ(dt) = D. magna δ¹⁵N - S. acutus δ¹⁵N), 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 δ¹⁵N values in animals.