added for maximum sediment concentration did not exceed 1 

 percent of bottle volume. An appropriate volume of radio- 

 active M. lutheri cells was added by micropipet, as 

 before, to each bottle to provide a cell concentration of 

 50,000 cells ml" . 



The natural sediments and mineral solids used were so fine-grained 

 that no settling of particulate material occurred during the short 

 time of the experiments. Therefore, bottles containing the zooplank- 

 ton were not shaken. 



Preliminary work showed that the natural sediments and mineral 

 solids used did not sorb carbon-14 counts that were significantly 

 higher than background. 



Results are reported as the difference between the uptake in con- 

 trol and sediment treatments and are expressed as a percent of the 

 control value. The differences between carbon uptake in controls and 

 sediment treatments were tested for significance by Student's "t" 

 test (Snedecor and Cochran, 1967). Values for individual zooplankters 

 were coded and logarithmically transformed before testing was done. 



f . Determination of the Effects of Suspended Patuxent River Sed- 

 iment (500 milligrams l~^) on Zooplankton Feeding Activity with In- 

 creasing Time of Exposure . The procedure for this determination was 

 identical with that used for the determination of time to maximum 

 ingestion with these exceptions: 



(1) Replicate pairs of bottles were not used because of the ex- 

 tensive time series (5, 10, 15, 30, 45, and 60 minutes; 1.25, 

 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3, 3.25, 3.5, 3.75, and 4 

 hours). Instead, two sets of bottles (one control and one 

 sediment treatment) , each containing 10 adult zooplankters 

 were used at each interval in the series. 



(2) Sufficient natural sediment slurry was added to each sediment 

 treatment bottle to make the concentration 500 milligrams i~ . 

 Radioactive M. Zutheri cells were pipetted simultaneously 

 into a control bottle and its corresponding sediment treatment 

 bottle for a specific feeding time interval. This procedure 

 was continued throughout the time series of control and treat- 

 ment bottle pairs. Bottle pairs for the long-time intervals 

 were inverted every 15 minutes to maintain the particles in 

 suspension. 



Results are reported as cells ingested per organism per day (cells 

 org'^day"-^). This daily rate (f) was calculated from the formula: 



f = C(R2 X 24) /(R^ X t), 



where 



22 



