zones, or in the boundary water at the sediment surface, so benthic 

 invertebrates are exposed to the worst of both worlds. They are exposed 

 to ammonia and to low oxygen at the same time. 



The highest ammonia concentrations in sediments are associated 

 with nitrogen-enriched sediments or high organic loading, as from sewage 

 treatment plants (Brezonik 1973; Ankley et al . 1990; and Schubauer- 

 Berigan and Ankley 1991). Although most sewage treatment plants remove 

 a substantial portion of carbon in municipal waste, most do not remove 

 nitrogen, but convert it from ammonia into nitrate. It is possible that 

 nitrate is carried down into the sediments where it is converted back 

 into ammonia in the anaerobic zones. If this is the case, ammonia 

 toxicity in the sediments might be reduced by reducing the nitrogen 

 loading of the river. 



During the course of this study, several species of fingernail 

 clams, including M. transversum, reappeared in the Chicago area water- 

 ways and in the Illinois River at Peoria and Havana. There are at least 

 four possible explanations for this surprising reappearance of clams in 

 the same general areas where the porewaters tested toxic. First, we 

 found that clams recolonizing the upper Illinois were more resistant to 

 ammonia than the clams from the lower Illinois, where the organisms were 

 obtained for all of the early bioassays. Second, our previous research 

 demonstrated that the surface layers of sediment in some areas were less 

 toxic than layers a few centimeters deeper (Sparks, Sandusky and Paparo 

 1981; Blodgett et al . 1984). Toxicity may have been overestimated in 

 tests where surface and deep layers of sediment were mixed prior to 

 testing. Third, toxic episodes may be brief and infrequent, allowing 

 organisms to colonize in between episodes. Fourth, the distribution of 



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