-25- 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 particulate matter in the water would be available to the invertebrate filter feeders. Thus the second concentration stage for non-fission radloisotopes 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 zooplankton. These factors are not known. DOE ARCHIVES 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°7 and C060 found in plankton samples from the different stations. the heavier elements at least, For organisms have little or no ability to differentiate between isotopes of a given element. Thus, the variation in ratio of Coo! to C69 probably reflects the variation in level of occurrence of these two isotopes in the fallout material in different geographical locations. =o