Potential effects of Asian clam (Corbicula fluminea) die-offs on native freshwater mussels (Unionidae) I:: water-column ammonia levels and ammonia toxicity

The Asian clam (Corbicula fluminea) co-occurs with unionid mussels in many riverine ecosystems. Clam populations can reach high densities and may undergo rapid die-offs, particularly during the low flow and warm temperatures of summer drought. Our study objective was to determine whether ammonia produced by decaying clam tissues during die-offs could affect unionid mussels. We induced C. fluminea die-offs in artificial streams to simulate die-off effects in natural habitats, and measured total and unionized ammonia (NH3-N) and dissolved oxygen (DO) in the overlying water. When water flow was stopped, reductions in DO preceded the onset of mortality in streams having the highest density of clams (similar to 10,000 clams/m(2)). NH3-N concentrations in the overlying water increased in association with clam mortality and reached concentrations of up to 5.04 mg/L at 26 +/- 2 degrees C. Temperature significantly influenced the rate of DO reduction and NH3-N production in the systems, while resumption of water flow led to rapid dissipation of NH3-N from the water column. We also conducted laboratory experiments to determine median lethal concentrations (LC(50)s) of total ammonia and NH3-N for glochidia and juveniles of the unionid mussel Villosa iris, adults of the unionid Pyganodon grandis, and juveniles and adults of C. fluminea. The 96-h LC(50)s (24-h for V iris glochidia) for NH3-N ranged from 0.11 mg/L for V iris glochidia to 0.8 mg/L for adult C. fluminea, indicating that NH3-N levels produced by Asian clam die-offs have the potential to exceed acute effects levels for at least some species of unionid mussels.