R2 = average activity (counts per minute) for zooplankters 

 exposed to the sediment concentration for a given time, 



R_ = average activity (counts per minute) of food cells (M. 

 lutheri) in 1 milliliter of suspension, 



t = feeding time in hours, 



c = average concentration of food cells per milliliter over 

 the feeding periods. 



These values were plotted against feeding time. The differences 

 between sediment treatment and control values at each feeding time 

 were tested for significance with Student's "t" test after logarith- 

 mic transformation. Sample counts per minute were corrected by back- 

 ground subtraction and quench corrected from a curve constructed as 

 before. This curve relates machine counting efficiency of standard 

 toluene carbon-14 to counts of an external standard during addition 

 of increasing amounts of a severe quenching agent (chloroform) to 

 acidified Instagel-Soluene-100 liquid scintillation counting fluid. 



3. Results and Interpretation . 



a. Time to Maximum Ingestion . The shortest time in which a food 

 particle could pass through the gut of E. affinis was demonstrated by 

 a decrease in uptake of radioactivity for feeding periods in excess 

 of 10 minutes (Fig. 9). These results are in good agreement with 

 those of Heinle (1973, unpublished) who has noted formation of fecal 

 pellets at 10 minutes after start of feeding for this species. Fil- 

 tering rates for this species are lower than those reported for 

 similar organisms by Anraku (1964) . This may have been caused by a 

 decrease in the size of new generations of adults as the age of the 

 cultures increased. 



The shortest time in which a food particle could pass through the 

 gut of adult A. tonsa (Fig. 10) was 5 minutes. 



There are two inflection points in the feeding rate curve for the 

 first hour of feeding (Fig 9) . The first inflection point occurred 

 between and 5 minutes (Fig. 10) . These inflections are similar to 

 those reported by Bourne (1959) for Daphnia magna feeding on 60,000 to 

 250,000 cells ml"'^ of Chlorella vulgaris for 10 to 20 minutes and 

 again for 40 to 55 minutes (Figs. 9 and 10), depending upon the cell 

 concentration. Bourne's data demonstrated that the inflection of the 

 carbon uptake curve at 40 to 55 minutes was associated with the onset 

 of defecation following initial filling of the gut. 



McMahon and Rigler (1963) found that the initial peak represented 

 filling of the oral groove of this species. This collecting rate 

 was higher by about 5 percent than the actual ingestion rate. However, 

 the total time of these types of experiments, i.e., the time between 



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