COX: UPTAKE, ASSIMILATION. AND LOSS OF DDT RESIDUES 



In another experiment, 12 animals were placed 

 in a vessel and fed "C-DDT labelled nauplii for 

 1 hr. Six animals were taken and processed for 

 "C activity, and the remaining six were allowed 

 to feed on unlabelled nauplii, for 2 days before 

 they were processed. The assimilation efficien- 

 cies were computed as a ratio of '•'C activity in 

 the animals processed after 2 days to the "'C 

 activity in the animals processed immediately 

 after the 1-hr feeding period. This method 

 yielded an assimilation efficiency of 76 '"r. 



For calculations, I took a mean of the first 

 four animals' assimilation efficiencies. It is un- 

 certain whether this figure (62*;; ) adequately 

 reflects the influence of moulting on DDT assim- 

 ilation efficiency. Moulting probably plays an 



Figure 4. — Loss curve constructed from data such as 

 that presented in Figure 3. See text for details. 



Figure 5. — DDT residue concentrations in different sizes 

 of Eiiphausia pacifica. The numbers next to the data 

 points indicate the numbers of animals in the pooled 

 sample analyzed; horizontal brackets indicate the range 

 of weights of individual animals within the groups. 



important role in DDT loss from the organism; 

 DDT incorporated into the moult is lost when 

 the moult is shed. 



NATURAL LEVELS OF DDT 



Figure 5 shows the results of gas chromato- 

 graphic analyses of E. pacifica samples collected 

 in August 1970, the same time that most of the 

 experimental animals were collected. On the 

 basis of DDT acquisition from food, a rising 

 trend in the DDT residue concentrations would 

 be expected as animals grew and aged. In order 

 to examine the discrepancy between the observed 

 DDT values and that which might be expected 

 from cumulative assimilation of DDT residues 

 from food, a model was constructed. 



Woodwell, Wurster, and Isaacson (1967) 

 found 0.04 ppm in plankton hauls from a polluted 

 estuary; I have found 0.25 ppm in large, pooled 

 samples of copepods from Monterey Bay. The 

 mean weight of these copepods, 0.95 mg, was 

 only slightly higher than for those eaten by E. 

 pacifica. E. pacifica also feeds on phytoplankton. 

 The concentrations of phytoplankton, when the 

 density of the standing crop of phj-loplankton 

 is high enough to stimulate feeding, are probably 

 below 0.1 iipm, wet weight (Cox, 1970). An 

 intermediate figure can be taken as representa- 

 tive of the DDT concentration of the food of E. 

 pacifica. I chose 0.1 ppm as the mean concen- 

 tration of DDT i-esidues in food. 



Employing the carbon budget parameters 

 published by Lasker (1966) and the estimate of 

 DDT residue concentration in food organisms, 

 I calculated the cumulative DDT content of the 

 ingested food of animals of three diflferent dry 

 weights (Table 3). The computed values are 

 compared with values interpolated from the ar- 

 biti'arily drawn dotted line in Figure 5. 



Two conclusions may be drawn from a com- 

 parison of columns 7 and 8 in Table 3. First, 

 the estimated values are close enough to the ob- 

 served values to indicate that ingestion is a suf- 

 ficient source of DDT residues in E. pacifica. 

 Second, the concentration vs. size function of 

 the observed values is quite different from that 

 of the calculated values, indicating that processes 

 other than simple accumulation of a fraction of 



631 



