the food chain to the carnivores the lower. concentrations become lower and Within each organism there is a differential concentration from tissue to tissue, the digestive organs having a higher concentration than the other tissues, where a more selective deposition as to specific isotopes has taken place. More specifically, plankton, the oceanic plants and animals that drift about passively with little or no resistance to water movements, may influence greatly the distribution of radioactive materials in the sea. These forms include many groups of organisms from the simple one-celled plants to the larval forms of vertebrates. Plankton acquire radioisotopes by absorption, adsorption, or both. Plankton, especially phytoplankton, present a greater absorptive surface to the environment than any other group of marine organisms. Thus, the major initial concentration of radioactive isotopes probably occurs in the phytoplankton -- the same organisms which comprise the foundation of the food chain in the sea. The isotopes especially concen- trated by these forms are, for the most part, representatives of those elements which tend to form strong complexes with organic material. They include nearly all of the anionic radioisotopes, with the exception of iodine, and the cationic radioisotopes produced by neutron induction, including radioactive zinc, cobalt, iron and manganese. of the cationic radioisotopes logically concentrated important elements comprising the All in the plankton are bioessential parts of enzyme