Abstract. In order to determine and understand the stable isotope fractionation of ¹⁸O and ¹⁵N manifested during assimilation of NO₃⁻ in marine macro-benthic algae, two species (Ulva sp. and Agardhiella sp.) have been grown in a wide range of NO₃⁻ concentrations (2–500 μM). Two types of experiments were performed. The first was one in which the concentration of the NO₃⁻ was allowed to drift downward as it was assimilated by the algae, between 24 hour replacements of media. These experiments proceeded for periods of between 7 and 10 days. A second set of experiments maintained the NO₃⁻ concentration at a low steady-state value by means of a syringe pump. The effective fractionation during the assimilation of the NO₃⁻ was determined by measuring the δ¹⁵N of both the (i) new algal growth and (ii) residual NO₃⁻ in the free-drift experiments after 0, 12, 24 and 48 h. Modelling these data show that the fractionation during assimilation is dependent upon the concentration of NO₃⁻ and is effectively 0 at concentrations of less than ~2 μM. The change in the fractionation with respect to concentration is the greatest at lower concentrations (2–10 μM). The fractionation stablizes between 4 and 6‰ at concentrations of between 50 and 500 μM. Although the δ¹⁸O and δ¹⁵N values of NO₃⁻ in the residual solution were correlated, the slope of relationship also varied with respect to NO₃⁻ concentration, with slopes of greater than unity at low concentration. These results suggest shifts in the dominant fractionation mechanism of ¹⁵N and ¹⁸O between concentrations of 1 and 10 μM NO₃⁻. At higher NO₃⁻ concentrations (>10–50 μM), fractionation during assimilation will lead to δ¹⁵N values in algal biomass lower than the ambient NO₃⁻ and ¹⁵N enrichments in the residual NO₃⁻.