Influence of pH and hardness on toxicity of ammonia to the amphipod Hyalella azteca

Abstract

The amphipod Hyalella azteca is frequently used for freshwater sediment tests throughout North America. A common potential toxicant in sediments is ammonia. Our objective was to characterize the influence of two key water quality variables, pH and hardness, on toxicity of ammonia to the amphipod. Ninety-six hour, water-only exposures of H. azteca to ammonia were conducted using three different water types with hardnesses of 42, 100, and 270 mg/L as CaCO₃ and three levels of pH (ca. 6.5, 7.5, and 8.5). In the soft water, toxicity of total ammonia was constant across test pH. As water hardness increased, toxicity of ammonia (on a total basis) to the amphipod decreased and became more pH dependent. Our data suggest that in softer water the amphipod was quite sensitive to the ionized (NH₄⁺) form of ammonia. This contrasts with most other species that have been tested, which typically are more sensitive to un-ionized (NH₃) ammonia than to NH₄⁺. These data provide baseline values for interpreting the possible contribution of ammonia to sediment toxicity in tests conducted with H. azteca and also indicate that in some situations NH₄⁺ may be important in determining ammonia toxicity.