Experimental evidence supports the use of δ¹⁵N content of the opportunistic green macroalga Enteromorpha intestinalis (Chlorophyta) to determine nitrogen sources to estuaries

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 NO ₃^- or NH₄⁺ to determine whether fractionation is concentration dependent. Uptake of N was described by a Michaelis-Menten equation for both NO₃^- and NH ₄⁺, with higher V_max and K_1/2 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.