Acute ammonia toxicity of Texas unionid mussels

Degradation of water quality, including wastewater discharges and non-point source nutrient pollution threatens freshwater mussels. Early life history stages of unionid mussels are highly sensitive to ammonia, and toxicity increases with rising water temperatures. Although ammonia toxicity is a known threat to several federally endangered mussel species endemic to the state of Texas, lethal concentrations of most species' early life stages in the state remain unknown. Hence, the objective of this study was to determine acute lethal concentrations (LC50s and LC05s) of ammonia for mussel glochidia (24-hr exposure) and newly metamorphosed juveniles (96-hr exposure). We tested three federally protected and five nonprotected species following ASTM International standards. For glochidia, the average LC50s for Cylonaias necki, Lampsilis bergmanni, L. hydiana, L. satura, L. teres, Fusconaia askewi, F. mitchelli, and Potamilus purpuratus ranged between 32.5 to 88.8 mg/L of total ammonia nitrogen (TAN) at pH 7. Juvenile LC50s for juvenile life stages of L. bergmanni, L. hydiana, L. satura, and P. purpuratus ranged from 29.5 to 43.1 mg/L of TAN (at pH 7). The average LC05s for glochidia ranged from 5.6 to 51.3 mg/L, and for juveniles, from 5.9 to 19.1 mg/L. The acute ammonia toxicity concentrations found in this study are comparable with existing literature, supporting the suitability of current U. S. Environmental Protection Agency aquatic life ambient water quality criteria for ammonia in freshwater to guide future regulatory protections and conservation efforts for unionid mussels in Texas.