The fingernail clam filtering assay used in this study is based on 

 observations by Aldridge et al . (1987), Sparks and Sandusky (1983), 

 Sparks et al . (1981), and Anderson et al . (1978) that stresses, includ- 

 ing toxicants, impair the ability of bivalves to filter particles from 

 water (including the food particles on which the clams feed). The assay 

 is outlined below and a detailed description is given in Sparks et al . 

 (1992). Filtering rates are determined by measuring the fingernail 

 clams' -ability to filter yeast from a suspension of known concentration. 

 Fingernail clams are first exposed to the porewater sample for one hour. 

 They are then placed in a yeast suspension and allowed to filter for one 

 hour. Two controls are used: the first consists of the yeast suspen- 

 sion alone and is used to determine the change in concentration due to 

 settling of the yeast. The second control determines the baseline 

 filtering rate of clams exposed for 1 hour in clean, uncontaminated 

 water. The yeast concentrations are measured at the beginning and end 

 of the filtering period. The filtering rates of the exposure and 

 control tests are then determined by taking the initial yeast 

 concentration minus the final concentration minus the amount settled 

 divided by the weight of the test organisms. Filtering rates are 

 expressed as the concentration of yeast filtered per unit weight of 

 organism per unit time. 



C^ = initial concentration of yeast 



Cf = final concentration of yeast 



W = wet weight of clams, in g (grams) 



C3 = change in yeast concentration due to settling 



- Cr - C- 



filtering rate in mg (milligrams) yeast/g clam/hour 



18 



