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The probability that the non-fission radioisotopes would 
be present in particulate form in the sea was discussed 
earlier. Particles of non-fission-product material either 
adsorbed onto or assimilated into the phytoplankton or ex- 
isting as free perticulate matter in the water would be avail- 
able to the invertebrate filter feeders. Thus the second 
concentration stage for non-fission radioisotopes would occur 
in the zooplankton through the filter feeders. Of course 
factors such as (a) average life spans of the phytoplankton 
and zooplankton organisms (b) average biological half lives 
of the different radioactive elements within the zooplankton, 
and (c) species differences between filter feeders regarding 
choice of food, the assimilation of the different radioele- 
ments and retention of the radioelements would influence the 
levels of the non-fission radioactive elements in the zoo- 
plankton. These factors are not known. 
There is little probability, however, that the above 
mentioned factors or any other biologicel effects are re- 
sponsible for the variations in ratio between Co>’ and C060 
found in plankton samples from the different stations. For 
the heavier elements at least, organisms have little or no 
ability to differentiate between isotopes of a given element. 
Thus, the variation in ratio of Co! to 6000 probably reflects 
the variation in level of occurrence of these two isotopes in 
the fallout material in different geographical locations. 
