the change in yeast concentration due to the settling of the yeast during the 4-h filtering period of 

 the study. Yeast concentrations in all vessels (including the yeast control) were measured 

 indirectly by measuring absorbance of a 3-ml sample at 580 nm (Aldridge et al. 1987) using a 

 spectrophotometer (Milton Roy Spectronic 301, Milton Roy Analytical Products Division, 

 Rochester, New York 14625) at the beginning (e.g., initial yeast concentration) and end of the 4- 

 h filtering period (final yeast concentration). Filtering rates of the treatment were then 

 determined by the following equation (Sparks and Dillon 1993): 



Fr= (Y -Y 4 )-Y c - Wf'-h-' eqn. 1 



where: Y = Initial yeast concentration (mg yeast ■ L water" 1 ) 



Y 4 = Yeast concentration after 4 h filtering period ( mg yeast • L 



water' 1 ) 



i c = Difference in yeast concentration in yeast control (mg 



yeast- L water" 1 ). 

 Wt = Dry weight of mussel (g) 



h = Time (4 h) 



Fr = Mussel filtering rate (mg yeast • g mussel' 1, h" 1 ) 



Initial trials of the mussel filtering assay resulted in negative filtering rates for individual 

 mussels exposed to Sylvan Slough porewaters. All filtering rates were also below 1 mg yeast • 

 g dry wt" 1, mussel 1 ■ h' 1 . Since filtering rates are calculated as the difference between initial and 

 final yeast concentrations (eqn. 1 ). a negative filtering rate would indicate an increase in solids 

 (e.g.. yeast or some extraneous organic material) in the test beaker. This is possible since some 

 mussels were seen to expel fecal material while in the yeast solution. 



Negative filtering rates could have also been caused by differing settling rates of yeast in the 

 vessels. Yeast in vessels containing mussels will not settle at the same rate as yeast in the yeast 

 control vessels because of the filtering activity of the mussels. As mussels siphon the yeast 



1-; 



