2a02 
involve either adsorption of the particle onto the organism 
or ingestion of particulate material. The latter would 
include both food particles 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. Naturally 
occurring potassium, the competitor to cesium, is present in 
sea water at a high level of about 380 ppm. In comparison 
to this level of occurrence, Cs137 would be present in very 
small amounts, even in an area of heavy fallout. Because of 
the limited ability of animals to differentiate between po- 
tassium 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. In the case of these 
elements, however, the uptake 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 
algae and foraminiferans, 2 in arthropods, 2 to 7 in molluscs, 
2.5 in bryozoans (25) and 3 to 10 in marine fishes (19). 
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 
