- 52 ~ form and evidently select the insoluble material somewhat indiscriminately as indicated by the fact that most of the fallout radioisotopes found in sea water are also found in plankton. able, Later, when the insoluble material is not avail- only those isotopes for which there is an apparent physiological need are found in the plankton. concentrate radioisotopes from the sea, Plankton often by a factor of a thousand or more, and are good biological indicators of contamination. A sample of plankton would be the best source of information to find out quantitatively if radioisotopes are present in a particular area of the ocean. The fallout radiolsotopes in ocean water are often difficult to identify because of the great amount of salts present. Identification of the radioisotopes in a plankton sample is more precise than the analysis for water samples because the isotopes taken up by plankton are more abundant and the techniques of analysis are simpler. Where both plankton and water data are available for samples collected soon after fallout, similar. the lists of isotopes are generally The radioisotopes listed in Table III were deter- mined for plankton samples collected soon after fallout but also are representative of the radioisotopes to be found in water from which the plankton samples were collected. The