TRANSURANIC RADIONUCLIDES IN MARINE ENVIRONMENT 53 1 



plutonium, i.e., high concentrations in the shell and lesser amounts in flesh. The 

 exoskeleton of the shrimp accumulates the major portion of neptunium, and loss rates are 

 strongly influenced by molting. When the shrimp were placed in unlabeled seawater, the 

 loss was biphasic; the largest part of the ^'^'Np initially present was lost with a TbVj of 4 

 days, and about 3% of the original activity exhibited a Tbi^ of 252 days. Mussels held in 

 the laboratory had Tbij values of between 180 and 226 days; faster turnover was observed 

 in animals held at temperatures of 25°C than in those held at 13°C. Mussels placed in the 

 sea showed whole-body Tt^ values of some 81 days; the faster turnover rates were 

 attributed to active growth of the organism. 



As yet there is no information regarding the assimilation of ^^"^Np by marine 

 organisms fed labeled food and subsequent turnover rates of assimilated material. 



Extrapolation of Laboratory-Derived Information to Natural Conditions 



The entire subject of biokinetics of radioactive and stable isotopes of elements and the 

 extrapolation of these laboratory-derived data to the real world has been a point of 

 discussion among aquatic scientists for a number of years. It is extremely difficult to 

 design laboratory experiments that are short term and simplistic relative to oceanic 

 processes which will provide predictive information on the accumulation and redistribu- 

 tion of radionuclides by marine organisms. In addition, field verification of laboratory- 

 derived information often is not possible. Inherent problems in experimental designs for 

 studying the transfer of radionuclides in marine food chains are discussed in a recent 

 review by Cross, Renfro, and Gilat (1975). 



Laboratory experiments must be designed which will present the radionuclide to the 

 test organism in a manner similar to that which occurs in nature; i.e., it must be allowed 

 to distribute between particulate and dissolved fractions realistically and must be of a 

 similar specific activity. In addition, feeding rates of the test organisms must reflect 

 natural conditions, as should population densities in the experimental aquaria. These 

 conditions require a combination of laboratory and field observations and basic 

 information on the general ecology of the test organism, which often is not available. 

 Realistic estimates of uptake rates, assimilation efficiencies, and turnover rates can only 

 be obtained by carefully designed experiments (Cross, WiUis, and Baptist, 1971; Cross 

 et al., 1975; Cross, Renfro, and Gilat, 1975; Willis and Jones, 1977). Perhaps one of the 

 major problems in radiotracer experiments is that of incomplete labeling (Willis and 

 Jones, 1977). 



Another important aspect of biokinetics of radionuclides which warrants discussion 

 here is the use (or misuse) of concentration factors. At this time we do not know what 

 fraction of an element in nature is bioavailable relative to concentration factors, and we 

 obviously are not in agreement (Lowman, Rice, and Richards, 1971; Cross, Renfro, and 

 Gilat, 1975; Fowler and Beasley, 1977). Examples can be found in the literature which 

 base concentration factors on (1) dissolved concentrations in the water, (2) total 

 concentrations in the water, (3) concentrations in food organisms, and (4) concentrations 

 in sediment. The use of any of these four fractions will depend on the author's prejudice 

 relative to the source of the element to the organisms under study. Presently, we do not 

 know what physicochemical forms of elements are most bioavailable to marine organisms. 

 In fact, we have not even developed adequate experimental designs to determine the 

 relative importance of food and water in conveying elements to aquatic animals (Cross 

 and Sunda, 1979). In reality, bioavailable fractions of elements probably consist of a 



