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

Abstract

One assumption underlying the use of stable nitrogen isotopes (δ15N) for determining nitrogen (N) sources is that the δ15N of primary producers reflects N sources in a predictable manner. To test this assumption, we conducted two experiments. First, we varied δ15N with constant concentration of NO3− or NH4+ to determine whether either nutrient is preferentially selected by the macroalga Enteromorpha intestinalis (L. Link) and if isotopic ratio affected selectivity. Tissue δ15N increased with δ15N supplied for both NO3− and NH4+ but sequestering of 15NH4+ was more rapid than for 15NO3−; in addition, some evidence suggested that high relative abundance of 15N may have decreased NO3− uptake. Second, we held δ15N constant and varied concentrations of either NO3− or NH4+ to determine whether fractionation is concentration dependent. Uptake of N was described by a Michaelis‐Menten equation for both NO3− and NH4+, with higher Vmax and K1/2 for NH4+ than for NO3−. There was no relationship between N concentration and tissue δ15N for either NO3− or NH4+; therefore, no selection for 14N over 15N occurred. This study demonstrated that accumulation of 15N in macroalgal tissue was predictable over a range of water δ15N 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 δ15N depending on the primary inorganic form of N available.