An enzymatic method was developed for the determination of ammonia concentrations in lake water. Lake water samples containing ammonia were mixed with a glutamate dehydrogenase (GIDH), reduced nicotinamide adenine dinucleotide phosphate (NADPH) and 2-oxoglutarate. The decrease in the absorbance intensity caused by the disappearance of NADPH by this reaction was measured at 340 nm. There was a linear relationship (r= 0.9997) between peak height and ammonia concentration over the range 0#x2013;29 #x3BC;M. The detection limit was 0.29 #x3BC;M for a sample volume of 250 #x3BC;l. Interference of amino acids and urea at concentrations of 50 mg l#x2212;1was negligible. Good agreement was found between the enzymatic method and indophenol blue colorimetry.
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