Environment determines nitrogen content and stable isotope composition in the sporophyte of Undaria pinnatifida (Harvey) Suringar

Nitrogen content and δ15N (nitrogen stable isotope ratio) were measured in different parts of the sporophyte of Undaria pinnatifida (Harvey) Suringar from two bays in the northeastern Japan. There were clear differences between the thalli collected in winter and in summer: high nitrogen content and low average δ15N were observed for the thalli collected in winter, while the opposite pattern was found for the thalli from summer. In addition, the pattern of internal δ15N distribution in the thallus changed with season. It is possible that the cause for these seasonal differences in the algae was the seasonal change in environmental conditions, because in winter water is normally rich in nitrogen, while in summer it is poor. U. pinnatifida sporophyte may be useful as an indicator of nitrogen sources in coastal waters, but consideration must be given to the effect of isotope fractionation on δ15N of the plant, especially in winter. Potential may exist for the use of different parts of the thallus to indicate nitrogen sources at different periods, but more investigation is necessary to accomplish this.