A simple and precise method for measuring ammonium in marine and freshwater ecosystems

The accurate measurement of ammonium concentrations is fundamental to under standing nitrogen biogeochemistry in aquatic ecosystems. Unfortunately, the commonly used indophenol blue method often yields inconsistent results, pa rticularly when ammonium concentrations are low. Here, we present a fluorom etric method that gives precise measurements of ammonium over a wide range of concentrations and salinities emphasizing submicromolar levels. The proc edure not only solves analytical problems but also substantially simplifies sample collection and preservation. It uses a single working reagent (cons isting 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 ammoniu m concentrations.