energetic gamma- emitter with a half- life of 

 30 years. Cesium is a rare element in the 

 biosphere; it is not even required for the 

 metabolism of biological systems. Small 

 amounts of cesium, however, are widespread 

 in nature, usually associated with the much 

 more abundant alkali metals- - especially with 

 potassium. Cesium is concentrated by animals 

 probably because of its chennical similarity to 

 potassium, which is the principal cation of 

 cytoplasm and which undergoes active ion 

 transport through cellular membranes of most 

 organisms. The intensive nuclear testing sev- 

 eral years back resulted in a large stratos- 

 pheric reservoir of long-lived radioisotopes, 

 and the levels of cesium 137 at the earth's 

 surface will probably increase until about 1970, 

 when the rate of physical decay will exceed the 

 rate of input from additional fallout. This 

 cesium 137 from fallout has proved a useful 

 tracer in studies on the biogeochemical cycling 

 of cesium in the estuarine environment. 



We have measured cesium 137 from fallout 

 for nearly 2 years in Rangia from six widely 

 separated stations on the Trent and Neuse 

 Rivers in eastern North Carolina. Rangia 

 thrives over a broad area, in salinities of less 

 than 0.1 p.p.t, to over 15 p.p.t. Clams were 

 collected periodically by raking at each sta- 

 tion; the soft tissues were dried, then ashed at 

 450° C; and the ash was analyzed by gamma 

 spectroscopy. Naturally occurring potassium 

 40 and cesium 1 37 from fallout were determined 

 by spectrum stripping of their respective 

 photopeaks at 1.46 and 0.662 Mev. The average 

 contents of cesium 137 in Rangia at each sta- 

 tion are shown in table 1. The clams at Wilson 

 Creek contained, on the average, more than 

 twice as much cesium 137 as clams from 40 

 knn, farther downstream. The average concen- 

 tration decreased consistently from station to 

 station in a downstream order. The grand 

 mean is 3.2 // /i c./ 100 g. wet weight. The large 



Table 1. --Concentration of cesium 137 in 

 Rangia euneata 



standard deviations shown in table 1 arise 

 partly because the cesium 137 content shows 

 a cyclic seasonal variation in Rangia . 



At the four upstream stations, cesium 137 

 showed a pronounced seasonal variation; the 

 maximum was in late spring and the minimum 

 in December. The concentration ranged from 

 1.1 to about 10//// c./ 100 g. wet weight. The 

 cycle was not particularly noticeable at the two 

 downstream stations. This seasonality cor- 

 responded to the seasonal variation in the dep- 

 osition of worldwide fallout but also was 

 related inversely to a seasonal change in 

 salinity at stations. The salinity varied 

 cyclically over a wider range than did the 

 content of cesium 137 in Rangia , and the cyclic 

 variation of salinity was completely out of 

 phase with the cycle of cesium 137 in Rangia , 

 that is--the maximum cesium 137 concentra- 

 tion in Rangia during the period of lowest 

 salinity. The concentration of cesium 137 in 

 Rangia probably is related to the rate of po- 

 tassium turnover which must in turn be affected 

 by changing availability of potassium in the 

 environment. The concentration of potassium 

 40 in Rangia was nearly constant, despite a 

 150-fold variation in external potassium con- 

 centration--that is, from a salinity of 0.1 to 

 15 p.p.t. Rangia was hypertonic to its environ- 

 ment with respect to both potassium and cesium 

 over this salinity range, and the relative 

 constancy of internal potassium concentration 

 indicated the ionic regulation of potassium to 

 circumvent the loss of potassium by diffusion. 



To determine concentration factors for cesi- 

 um and potassium, we collected water samples 

 concurrently with Rangia at three of the sta- 

 tions during the fall of 1967. We evaporated 

 40 1 . of water per sample and analyzed the 

 residual salts for gamma radioactivity of 

 cesium 137 and potassium 40. Comparison of 

 these levels to the contents of the respective 

 isotopes in Rangia gave us the concentration 

 factors (fig. 4). The concentration factors for 

 cesium and potassium are similar to each 

 other and are related inversely to the salinity 

 of the water in a log-log manner. This same 

 relation has been demonstrated experimentally 

 with isotopic tracers for brackish-water iso- 

 pods and snails at the Plymouth Laboratory in 

 England. The biological turnover of cesium 

 was demonstrated experimentally to be much 

 slower than that of potassium. Rangia is almost 

 isotonic with the environment at about 20 p.p.t. 

 salinity, whereas at lower salinities both po- 

 tassium and cesium must be pumped against 

 a large concentration gradient (fig. 4). 



The actual cesium concentrations in Rangia 

 can be described fairly accurately as a function 

 of salinity by a log-log regression (fig. 5), but 

 the cesium 137 concentration in Rangia was 

 not affected nearly so greatly by salinity 

 changes as we would predict from the concen- 

 tration factors in figure 4. In fact, above sa- 

 linities of 4 or 5 p.p.t., the cesium 137 content 



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