Sensitivity of juvenile freshwater mussels (Lampsilis fasciola, Villosa iris) to total and un‐ionized ammonia

Abstract This study evaluated the sensitivity of juveniles of two freshwater unionid mussel species ( Villosa iris [Lea] and Lampsilis fasciola [Rafinesque]) to un‐ionized and total ammonia. Five concentrations of ammonium chloride were tested using 96‐h staticrenewal toxicity tests at 12 and 20°C. Based on their respective mean 96‐h lethal concentration to 50% (LC50s), V. iris (0.11 mg/ L NH3‐N) was more sensitive than L. fasciola (0.26 mg/L NH3‐N). At 96 h, significant differences in sensitivity to un‐ionized ammonia between the two temperatures were not observed for either species. Comparison of LC50s reported for other aquatic organisms to the 96‐h LC50s calculated for juvenile L. fasciola and V. iris shows these two mussel species to be among the most sensitive to un‐ionized ammonia. Based on reported levels of un‐ionized ammonia in the aquatic environment from anthropogenic sources, un‐ionized ammonia may be an important limiting toxicological factor to freshwater mussel populations.