MAGIC: A sensitive and precise method for measuring dissolved phosphorus in aquatic environments

A simple and straightforward method has been developed for the precise determination of nanomolar concentrations of soluble reactive phosphorus (SRP) and total dissolved phosphorus (TDP) in marine and freshwater environments. The method relies on the quantitative removal of P from solution by in vitro formation of brucite Mg(OH) 2 , initiated by the addition of NaOH. The precipitate is collected by centrifugation and either dissolved in HCl for SRP determinations or hydrolyzed (8 M HCl, 80°C, 1 h), dried, and ashed (450°C, 3 h) for TDP determinations. Both procedures are completed by reaction with the standard Murphy‐Riley molybdenum blue reagents. This mag nesium‐ i nduced c oprecipitation (MAGIC) procedure can be used to effect a 100‐fold concentration, thereby providing a method which can reliably detect 1 nmol SRP liter ‒1 of seawater. MAGIC can also be used to determine P in Mg 2+ ‐amended freshwater samples. The method is highly reproducible; typical estimates of precision for triplicate SRP determinations in the 10–100 nM range are from 1 to 3%. Field trials of the MAGIC procedure in the North Pacific Ocean reveal mixed‐layer SRP concentrations from 30 to 100 nM, with substantial temporal variability. The method is versatile and has many other potential applications.