Chapter 9 



Uptake by Fresh-water Organisms 



93 



fectively, if not to a greater extent, than some 

 crustaceans. This may be especially true for 

 those long-hved isotopes which are incorporated 

 into the shell. 



The length of exposure to water that con- 

 tains radioisotopes will also greatly affect the 

 concentration in different organisms. The con- 

 centration of isotopes in phytoplankton and 

 other micro-organisms will reach equilibrium 

 with the water in a relatively short period of 

 time. For radiophosphorus, this is estimated at 

 about 15 hours (Whittaker, 1953). The larger 

 animals, such as fish, will approach equilibrium 

 much more slowly, however. Coffin, et al. 

 (1949) found that radiophosphorus introduced 

 into an acid bog lake did not appear in the fish 

 until two days later. Several weeks of chronic 

 exposure to an environment containing long- 

 lived, bone-seeking isotopes is undoubtedly 

 necessary before maximum concentrations will 

 result in large fish. 



Variations with season, age, and growth 



So far as is known, all cold-blooded fresh- 

 water organisms exhibit seasonal changes in the 

 assimilation of radiomaterials through metabolic 

 processes. There is a direct correlation between 

 an increase in temperature and an increase in 

 the accumulation of radiomaterials through 

 metabolic processes in the invertebrates and 

 fishes of the Columbia River (Foster and Davis, 

 1955) and the fishes of White Oak Lake 

 (Krumholz, 1954, 1956). However, in White 

 Oak Lake it was found that the amounts of ra- 

 diomaterials in all fish tissues decreased mark- 

 edly after August 1, even though the tempera- 

 tures at that time were similar to those during 

 the early summer when there was a rapid in- 

 crease in the accumulation of radioactive ma- 

 terials. This may well be a suggestion that some 

 warm-water fishes enter a period of estivation 

 or summer dormancy. A decline in radioactivity 

 of Columbia River organisms during the winter 

 months correlates with cessation of feeding. 



No seasonal pattern of change in the ac- 

 cumulation of radiomaterials has been demon- 

 strated for any of the warm-blooded aquatic 

 vertebrates, but this may well occur. It is 

 known, for example, that the V-^'^ content of 

 rabbit thyroid glands changes markedly with 

 the season (Hanson and Kornberg, 1955). 



Among the fishes, it has been established by 



Olson and Foster (1952) that the younger, 

 more rapidly growing individuals accumulate 

 relatively greater amounts of radioactivity than 

 the older, more slowly growing ones. This 

 phenomenon is probably a reflection of the more 

 rapid anabolism that accompanies the growth 

 of younger fish. It is not known whether any 

 of the other fresh-water vertebrates or inverte- 

 brates exhibit this same phenomenon. 



Any accumulation of radioactive materials in 

 an organism is subject to biological dilution. 

 Such dilution results from cell division and 

 growth. It is especially manifest in rapidly 

 growing organisms and is particularly notice- 

 able following an acute short-term exposure to 

 the radiomaterials. 



Retention and elimination 



Radioisotopes will be deposited and retained 

 in the organisms according to the physiological 

 behavior of the particular element involved. 

 Highly mobile isotopes, such as tritium, may be 

 eliminated in a matter of minutes or hours 

 (Foster, 1955), but certain bone-seekers, such 

 as strontium or phosphorus, may be so tightly 

 fixed that little loss occurs, except by radioac- 

 tive decay, during the life of the organism. The 

 metabolism of the radiophosphorus in trout has 

 been studied by Hayes and Jodrey (1952) and 

 by Watson (Hanford Laboratories, unpub- 

 lished). Little information is available on the 

 metabolism of other isotopes in other aquatic 

 animals, however. 



The recognized methods of elimination of 

 radiomaterials are: (1) through surface ex- 

 change, (2) excretion through the natural 

 physiological channels, (3) through moulting 

 where this occurs, and (4) through death. In 

 any of these processes of elimination, the radio- 

 materials are released into the environment and 

 can be immediately taken up by other organ- 

 isms. 



Discussio)2 



Based on our present knowledge, there can 

 be no broad statement to the effect that "aquatic 

 organisms will concentrate radioactivity in their 

 tissues." Rather, each individual situation must 

 be appraised separately in the light of the fol- 

 lowing basic considerations which are concerned 

 with the accumulation of radiomaterials by 

 fresh-water organisms: (1) the particular ele- 



