AbstractThe influence of acidity and alkalinity on the enzymatic hydrolysis of amide‐N in the aquatic system was investigated by determining the stability of L‐asparaginase (EC [Enzyme Commission] 3.5.1.1). L‐glutaminase (EC 3.5.1.2), and amidase (EC 3.5.1.4), and urease (EC 3.5.1.5) with respect to pH in three freshwater lakes. The water samples were first incubated at the indicated buffer pH (1–13) for 24 h; then each enzyme was assayed at its optimal pH (pHopt) under standardized conditions. L‐Asparaginase retained essentially 100% of its activity within the pH range of 6–7 in Lake Evans and Lake Matthews, and 5–7 in Lake Perris. L‐Glutaminase was active between pH 6 and 7 in the water samples surveyed. Amidase remained completely active at pH ranges of 4–8 in Lake Evans, 4–7 in Lake Matthews, and 5–8 in Lake Perris. Urease remained active over a broad pH range of 5–9 in Lake Evans, 5–8 in Lake Matthews, and 4–8 in Lake Perris. There was a progressive loss in enzyme activity below and above these pH ranges. The results suggest that the decline in aquatic amidohydrolase activities in relation to the pH‐profile near their pHopt was caused by a reversible reaction involving the ionization or deionization of functional groups of the active center of the protein, but under highly acidic or alkaline conditions (pH < 4 to > 9) the reduced activity appears to be due to irreversible inactivation of the enzyme.