Abstract: The accurate measurement of ammonium concentrations is fundamental to understanding nitrogen biogeochemistry in aquatic ecosystems. Unfortunately, the commonly used indophenol blue method often yields inconsistent results, particularly when ammonium concentrations are low. Here, we present a fluorometric method that gives precise measurements of ammonium over a wide range of concentrations and salinities emphasizing submicromolar levels. The procedure not only solves analytical problems but also substantially simplifies sample collection and preservation. It uses a single working reagent (consisting of orthophthaldialdehyde, sodium sulfite, and sodium borate) that is stable for months when stored in the dark. The working reagent and sample can be mixed immediately after sample collection and the reaction proceeds to completion within 3 h at room temperature. Matrix effects and background fluorescence can be corrected without introducing substantial error. This simple method produces highly reproducible results even at very low ammonium concentrations.

Introduction: Ammonium determination is one of the most common yet troublesome measurements made by limnologists and oceanographers. A recent intercomparison study incolving more than 100 laboratories worldwide demonstrated than the marine community as a whole connot measure ammonium concentrations accurately (Aminot et al. 1997), particularly at submicromolar concentrations, which are the norm in many systems. It is unlikely that the freshwater community can do any better. At a minimum, uncertainty in ammonium measurements hampers our ability to discern the details of nitrogen cycling in aquatic ecosystems; at worst, it leads to incorrect conclusions about ecosystem processes or invalid comparisons across sites. For example, errors in measurement of ammonium concentration are propagated throughout complex models of nitrogen cycling, making invalid subsequent inference.