John W. Allran Jonathan A. O’Donnell Michelle Bartsch William B. Richardson Teresa J. Newton 2003 Ammonia is a relatively toxic compound generated in water and sediments by heterotrophic bacteria and accumulates in sediments and pore water. Recent data suggest that unionid mussels are sensitive to un-ionized ammonia (NH3) relative to other organisms. Existing sediment exposure systems are not suitable for ammonia toxicity studies with juvenile unionids; thus, we modified a system to expose juveniles to ammonia that was continuously infused into sediments. This system maintained consistent concentrations of ammonia in pore water up to 10 d. Juvenile <i>Lampsilis cardium</i> mussels were exposed to NH<sub>3<sub> in pore water in replicate 96-h and 10-d sediment toxicity tests. The 96-h median lethal concentrations (LC50s) were 127 and 165 &mu;g NH₃-N/L, and the 10-d LC50s were 93 and 140 &mu;g NH₃-N/L. The median effective concentrations (EC50s) (based on the proportion affected, including dead and inactive mussels) were 73 and 119 &mu;g NH₃-N/L in the 96-h tests and 71 and 99 &mu;g NH₃-N/L in the 10-d tests. Growth rate was substantially reduced at concentrations between 31 and 76 &mu;g NH₃-N/L. The lethality results (when expressed as total ammonia) are about one-half the acute national water quality criteria for total ammonia, suggesting that existing criteria may not protect juvenile unionids. application/pdf 10.1897/02-342 en Wiley Effects of ammonia on juvenile unionid mussels (<i>Lampsilis cardium</i>) in laboratory sediment toxicity tests article