Abstract: A method is provided for measuring the concentration of nutrients such as nitrite and nitrate in aqueous media, e.g. the upper layers of the ocean where the concentrations of such nutrients are relatively low (i.e. in the tens of nanomoles per liter or less). The method is believed to be capable of such measurement capabilities at near real time sampling rates. The measurement of nitrite includes (a) treating an aliquot of the aqueous medium with a reagent that acidifies the aliquot, converts nitrite in the aliquot to nitrosium ion, and reacts with the nitrosium ion to yield a chemical species which fluoresces and (b) fluorometrically analyzing the resulting aliquot/reagent mixture to determine the concentration of such chemical species in the mixture, and thus to determine the concentration of nitrite in the aliquot and the aqueous medium from whence it came. The reagent is preferably an acidified solution in which the active ingredient is aniline.