Ammonia Toxicity to Fishes. Effect of pH on the Toxicity of the Un-ionized Ammonia Species

摘要

United States and European water quality criteria to protect aquatic life assume that the toxicity of aqueous ammonia is attributable to the un-ionized (NH3) chemical species, and that the ionized (NH4+) species is nontoxic or significantly less toxic than NH3. Data are presented here on the toxicity of ammonia to rainbow trout (Salmo gairdneri) and to fathead minnows (Pimephales promelas) from two series of 96-h flow-through toxicity tests in which pH was controlled within the range 6.5-9.0. Results show that the toxicity of ammonia in terms of NH3 increased at lower pH values. It is concluded that NH4+ exerts some measure of toxicity and/or that increased H+ concentration increases the toxicity of NH3. It is recommended that water quality criteria be revised to consider the pH dependence of the toxicity of ammonia.