The exposure filtering rate is then compared to the control. The test 

 result is a sublethal response (percent reduction in filtering rate, 

 relative to the control) as opposed to an "all or none" (death or sur- 

 vival, toxic or nontoxic) type of response. The inhibition of the 

 filtering performance of the clams is proportional to the severity of 

 the stress (Sparks et al . 1992). For purposes of evaluating sediments 

 for toxicity, it is useful to be able to rank sites based on relative 

 toxicity. Only the 1990-1991 porewater samples were evaluated using 

 this assay because it was not fully developed until late 1990. 



The results of the various assays were standardized for easier 

 comparisons. The treatment results were divided by the control results 

 and then 1 was subtracted from the quotient. A negative value indicates 

 inhibition (toxicity), a positive value indicates stimulation, and 

 indicates no response (no difference with respect to the control). If 

 we use the fingernail clam filtering bioassay as an example: 



T = test response to sample of sediment porewater 



C = control response to uncontaminated dilution water 



T = 3.4 mg yeast/g clam/hour 



C = 6.5 mg yeast/g clam/hour 



T/C = 3.4/5.5 = .52 



.52 - 1.00 = -.48 A decline of 48% from the 



control value, a marked inhibition of the filtering 



ability of the clams. 

 Results of the C. dubi'a bioassay are expressed in toxicity units, 

 as well as 48-hour LC50s, where toxicity units = 100/(48-hour LC50). 

 The 48-hour LC50 is the percent dilution of porewater (or treated pore- 

 water) that kills 50% of the test organisms in 48 hours. For example, 



19 



