EXPERIMENTAL EVIDENCE SUPPORTS THE USE OF δ15N CONTENT OF THE OPPORTUNISTIC GREEN MACROALGA ENTEROMORPHA INTESTINALIS (CHLOROPHYTA) TO DETERMINE NITROGEN SOURCES TO ESTUARIES¹

One assumption underlying the use of stable nitrogen isotopes (δ 15 N) for determining nitrogen (N) sources is that the δ 15 N of primary producers reflects N sources in a predictable manner. To test this assumption, we conducted two experiments. First, we varied δ 15 N with constant concentration of NO 3 − or NH 4 + to determine whether either nutrient is preferentially selected by the macroalga Enteromorpha intestinalis (L. Link) and if isotopic ratio affected selectivity. Tissue δ 15 N increased with δ 15 N supplied for both NO 3 − and NH 4 + but sequestering of 15 NH 4 + was more rapid than for 15 NO 3 − ; in addition, some evidence suggested that high relative abundance of 15 N may have decreased NO 3 − uptake. Second, we held δ 15 N constant and varied concentrations of either NO 3 − or NH 4 + to determine whether fractionation is concentration dependent. Uptake of N was described by a Michaelis‐Menten equation for both NO 3 − and NH 4 + , with higher V max and K 1/2 for NH 4 + than for NO 3 − . There was no relationship between N concentration and tissue δ 15 N for either NO 3 − or NH 4 + ; therefore, no selection for 14 N over 15 N occurred. This study demonstrated that accumulation of 15 N in macroalgal tissue was predictable over a range of water δ 15 N values and N concentrations, suggesting that E. intestinalis may be used to assess the availability of N sources to estuarine and coastal communities. However, caution must be used when interpreting tissue δ 15 N depending on the primary inorganic form of N available.