inhibited the cilia on the gills of the clams, and the growth of the 

 clams in the laboratory was reduced at concentrations between 0.20 and 

 0.34 mg/1 NH3-N. The C. dubia acute LC50 for ammonia is 1.04 mg/1 NH3-N 

 (Ankley et al . 1990). Arthur et al . (1987) reported un-ionized ammonia 

 toxicity to 5 invertebrates ranged from 1.95 to 18.3 mg/1 NH3-N and 

 mollusks (snails) were most sensitive. 



Concentrations of this magnitude (1.0-8.0 mg/1 NH3-N) are commonly found 

 in the sediments in the Upper Illinois Waterway, based on total ammonia 

 concentrations (23.8-59.8 mg/1) and naturally occurring pHs. Ammonia 

 places organisms in double jeopardy because it exerts an oxygen demand 

 in the process of nitrification (conversion to nitrites and then ni- 

 trates) and low oxygen levels place organisms under additional stress 

 (USEPA 1985). Ammonification may be occurring in the deep, anaerobic 

 zones of the sediments and nitrification in the shallower, aerobic 



47 



