Influence of isotope fractionation on the nitrogen isotope composition of the brown macroalga Undaria pinnatifida

Although the stable nitrogen isotope ratio (δ¹⁵N value) of macroalgae is used to indicate sources of dissolved inorganic nitrogen in coastal marine environments, little is known about the effects of isotope fractionation on the δ¹⁵N value. We compared the δ¹⁵N values of Undaria pinnatifida cultivated at an inner bay and an exposed site (sites A and B, respectively) in Shizugawa Bay on the northeastern Pacific coast of Japan. Values for algal growth indicators, including thallus length, area, and weight, were higher at site B than at site A. The δ¹⁵N value of U. pinnatifida was significantly lower at site B (4.7 ± 1.0‰; mean ± standard deviation) than at site A (6.7 ± 0.4‰). This difference in the δ¹⁵N value of U. pinnatifida could not be explained by the δ¹⁵N value of seawater NO₃ (source isotopic signature), which was similar at the two sites (7.5 ± 0.3‰). The pooled data from the two sites indicated that the δ¹⁵N value of U. pinnatifida decreased with increasing thallus size. The results suggest that the δ¹⁵N value of U. pinnatifida is influenced by isotope fractionation.