VARIATION IN INTERNAL δ15N AND δ13C DISTRIBUTIONS AND THEIR BULK VALUES IN THE BROWN MACROALGA PADINA AUSTRALIS GROWING IN SUBTROPICAL OLIGOTROPHIC WATERS1

The utility of δ¹⁵N measurements in Padina australis Hauck as a probe for its external nitrogen (N) sources was tested by monitoring the bulk values of chemical components δ¹⁵N, δ¹³C, and N and carbon (C) contents] and their internal distributions during a 12 d incubation in a controlled environment. Under the saturated conditions of isotopically heavier nitrate than that of original algal tissue, the bulk δ¹⁵N in P. australis was enriched, but less than what was predicted from a simple mixing model, signaling possible isotopic discrimination during N assimilation and subsequent N efflux from the cells. The enhanced N content (%), which occurred simultaneously with this δ¹⁵N shift, was a useful signal indicating this phenomenon. Bulk δ¹⁵N was enriched, especially around the meristem, in tissues growing under conditions of higher irradiance and temperature, probably due in part to dissolved organic nitrogen (DON) excretion. The δ¹³C enhancement in bulk algal tissues, also associated with high photosynthetic activity, may be an additional signal indicating this unbalanced internal δ¹⁵N distribution. However, in summer and winter environmental conditions with periodic nitrate supplies simulating typical fringing reef waters, the difference in measured algal bulk δ¹⁵N from theoretical predictions was within ±1.0‰. This difference is very small compared with the variation in δ¹⁵N in possible N sources in coastal areas. In the field, therefore, δ¹⁵N in Padina can be used effectively to trace N sources in both space and time after determining algal N content and δ¹³C to determine whether large alterations occur in algal δ¹⁵N.