consistent in their responses to the sediment porewaters. Both organ- 

 isms exhibited no inhibitory response to porewaters from the lower 

 Illinois River or from the reference site in the Upper Mississippi River 

 (Figure 4.1). The stimulation of filtering performance in the finger- 

 nail clam, M. transversum, may have been caused by favorable ratios of 

 dissolved sodium, potassium, calcium, and magnesium salts in porewaters 

 from the lower river. Anderson, Sparks and Paparo (1978) demonstrated 

 the importance of these salts in regulating the beating of the cilia on 

 the gills of the clams. Salts that affect the cilia are likely to 

 affect filtering performance because the lateral cilia produce the water 

 currents that bring food into the clam and the latero-frontal cilia act 

 as filters. Also, the presence of organic matter in the sediment pore- 

 waters may have stimulated a feeding response in the clams, which are 

 deposit feeders, as well as water column filterers. The clam and the 

 water flea likewise are consistent in indicating toxicity in the upper 

 waterway. Filtering performance in the clam was inhibited starting with 

 sediment porewaters from IR248.2 near Marseilles and water flea mortali- 

 ty started at DP277.0, just above the mouth of the Du Page River near 

 the Interstate 55 bridge. Sediment porewaters from 7 of the 13 upstream 

 sites were toxic to C. dubia, and 12 of 13 inhibited the fingernail clam 

 (Figure 4.1). 



Since the fingernail clam is the organism of main interest in this 

 study, the response of the clams is extracted from Figure 4.1 and 

 presented separately in Figure 4.2. With the exception of a stimulatory 

 response to porewater from one station in the Sanitary and Ship Canal, 

 SS315.3, all the upper waterway stations exhibited some degree of 



27 



