below which mussels lose weight and eventually die. We need to determine such threshold 

 filtering rates for critical mussel species. 



Although porewater has been recognized as a suitable '■surrogate" for assessing the toxicity of 

 whole sediments, researchers have not found a suitable extraction method that does not alter the 

 physical and chemical properties of the porewaters in the "natural state" (Carignan el al. 1985; 

 Ankley et al. 1994). Also, the problems associated with the collection of sediment porewaters 

 (e.g., time and amount of sediment required to provide sufficient volumes of porewater for 

 testing) warrants the use of whole sediment studies. Recently, the U.S. Environmental 

 Protection Agency (1994) approved a sediment testing intermittent-renewal (STIR) system for 

 invertebrate sediment toxicity testing. The STIR system enables the maintenance of acceptable 

 water quality parameters (e.g., dissolved oxygen and temperature) by automatically renewing 

 overlying water in sediment tests at rates ranging from 1 to 21 volume renewals per day. The 

 STIR system reduces the labor associated with the renewal of overlying water by hand and 

 affords a gentle exchange of water that results in virtually no sediment suspension. This method 

 is simple and inexpensive to assemble and operate. 



We recommend testing sediments of Reach 15 and sediments of remaining high quality 

 mussel refuges in the Upper Mississippi and Illinois rivers, using the species and test procedures 

 previously mentioned. Tests conducted in this study indicate that chronic toxicity is present in 

 the sediments of Reach 15. By comparing sediments from other mussel refuges to Reach 15 

 sediments, we can determine if sediment toxicity is caused by local non-point pollution sources 

 along Reach 15 or by upstream sources. 



Macroinvertebrates are continuous indicators of environmental quality. The composition of 



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