“20+ involve either adsorption of the particle onto the organism or ingestion of particulate material. The latter would include both food perticles and non-living particulate detritus. Several other factors, however, also control the uptake of specific radioelements. In the case of strontium and cesium chemical competition plays a major role. occurring potassium, the competitor to cesium, sea water at a high level of about 380 ppm. Naturally is present in In comparison to this level of occurrence, Cs137 would be present in very sm&ll amounts, even in an area of heavy fallout. Because of the limited ability of animals to differentiate between potassium and cesium, uptake of the latter would be low. Natural calcium occurs at a level of about 400 ppm in sea water and exhibits chemical competition to strontium with regard to uptake by marine organisms. elements, however, the uptake In the case of these is not directly proportional to occurrence in the water; rather strontium is discriminated against with reference to and in the presence of naturally oc- curring calcium by factors of approximately 3 in calcareous a&lgae and foraminiferans, 2 in arthropods, 2 to 7 in molluscs, 2.5 in bryozoans (25) and 3 to 10 in marine fishes (19). DOE ARCHIVES In laboratory experiments utilizing radioactive strontium Chipman (2) observed that Artemia larvae reached a steady state in the level of radiostrontium about 0.7 that of sea water and Burroughs, Townsley, and Hiatt (1) reported a value 25