COX: DDT RESIDUES IN CALIFORNIA CURRENT SYSTEM 



< 



Q 

 O 

 I 

 O 



750figmC/l 



600 700 



18- 

 16- 

 14- 

 12- 

 10- 

 8- 

 6- 

 It- 

 2 



100 200 300 400 500 600 700 



VOLUME ALGAL SUSPENSION ADDED (ml) 



Figure 4. — Percentage of total "C-DDT in sample 

 aliquots recovered on GFC filters, plotted as a function 

 of total volume of Dunaliella salhia culture added to a 

 constant volume system. See text for detailed discussion. 



DDT which was available. Both curves show 

 inflection points after the density of cells in- 

 creases beyond 750 ^ug C/1. The fact that the up- 

 take per cell was constant over the linear range 

 indicates that each cell has a saturation value 

 for uptake of DDT, which is independent of the 

 ambient concentration of DDT available. The 

 curves presumably begin to level off when the 

 "C-DDT which was available for uptake is most- 

 ly associated with the algal mass already added. 

 If algal cells exhibit a saturation value for 

 uptake of DDT, then adsorption of DDT to the 

 cell surface is a more likely explanation for DDT 

 uptake than phase partitioning of DDT between 

 seawater and the lipid component of the algal 

 cell, as has been previously hypothesized (Cox, 



1970b). Each Dunaliella salhia cell probably 

 had a total cell surface area of 240 fjr (Mullin, 

 Sloan, and Eppley, 1966). The cells in Experi- 

 ments 1 and 2 took up a mean of 5 X 10~^ pico- 

 grams '^C-DDT//i-. This value may be near the 

 asymptotic saturation value for Dunaliella salina 

 for the experimental conditions described above. 

 The validity of a saturation value of this kind 

 needs to be tested with other phytoplankton 

 species over a wide range of ambient DDT con- 

 centrations. 



A quantitative solution to simultaneous 

 Freundlich adsorption equations for the algal 

 cells and the <l-2 ix particles could explain the 

 uptake curves if the adsorption energy coeffi- 

 cient were known in each case. Studies such 

 as those of Weber and Gould (1966) should 

 therefore be applied to uptake of DDT residues 

 by phytoplankton and smaller particles to elu- 

 cidate the relationships discussed here. 



No measurements were made of the concen- 

 tration of the <l-2 fi particles in the untreated 

 seawater of Experiment 1 or the treated sea- 

 water of Experiment 2. Thus the differences 

 can only be explained qualitatively. The higher 

 concentration of '^C-DDT in Experiment 2 

 (30 ppt) was appai'ently reflected in the uptake 

 of '^C-DDT per unit of cell surface area ; Ex- 

 periment 2 yielded a value of about 6 x 10~^ 

 picograms "C-DDT'/i-, which is higher than the 

 mean for both experiments quoted above. The 

 uptake of "C-DDT per unit of cell surface area 

 in the case of Experiment 2 is probably closer 

 to the asymptotic saturation value because of 

 the higher concentration of '^C-DDT in the me- 

 dium. The main difference between the curves 

 for Experiments 1 and 2 is the position of the 

 inflection point. Experiment 2 shows an ap- 

 parent inflection point which is lower than the 

 apparent inflection point of Experiment 1, 

 indicating a lowering of the available percent- 

 age of total "C-DDT in the system. The total 

 small particle concentration of the system was 

 not measured, so this apparent change must be 

 regarded as a presumptive effect of the Nuchar- 

 attaclay addition. 



If a large percentage of the DDT added to 

 aqueous systems is fixed to a particle fraction 



449 



