FISHERY BULLETIN: VOL. 70, NO. 3 



relative concentration factors in zoo- and pliyto- 

 plankton and a mean carbon assimilation of 50^/r . 

 On this basis, assimilations for 24 elements 

 ranged from 1 to 85 ''r — indicating nonparallel- 

 ism with carbon assimilation — but Lowman went 

 on to say: "The major weakness in this method 

 of calculating conversion factors for a variety 

 of elements is the uncertainty of the accuracy 

 of the concentration factors for phytoplankton 

 and zooplankton." Similar uncertainty exists 

 for published concentration factors for various 

 elements in estuarine organisms, especially since 

 environmental conditions (and therefore ele- 

 mental concentrations in estuarine water) are 

 subject to such wide variation. A further com- 

 plication is the uncertainty concerning the pro- 

 portion of total elemental intake represented by 

 food sources. According to Polikarpov (1966), 

 marine animals satisfy their requirements for 

 most elements by direct absorption from the sur- 

 rounding water. Considerable experimental evi- 

 dence, however, supports the importance of food 

 as a source of elements for many organisms 

 (Rice, 1963; Hoss, 1964; Baptist and Lewis, 

 1969) . Atoms adsorbed directly from the water 

 onto body surfaces, whether internal or external, 

 do contribute to the concentration factor but 

 have no relevance to assimilation of food. Sur- 

 face adsorption frequently results in assimila- 

 tion, however, as in phytoplankton (Goldberg, 

 1952) and the mantle epithelium of Pelecypoda 

 (Nakahara and Bevelander, 1967). For organ- 

 isms with well-defined and easily analyzed in- 

 ternal tissues, e.g., crustacean and fish muscle, 

 internal concentrations of elements probably rep- 

 resent the assimilated fraction, but for many 

 smaller or less differentiated organisms, internal 

 tissues cannot readily be separated from adsorp- 

 tive surfaces. Assimilation efficiency may de- 

 pend also on biochemical composition of the food 

 — at least at certain trophic levels. For example, 

 the assimilation of ^^Zn by human subjects was 

 35 "^r from a diet of whitefish (Honstead and 

 Brady, 1967) and 13. 5''/^ from oysters (Honstead 

 and Hildebrandt, 1967) , showing a high (though 

 perhaps coincidental) positive correlation with 

 the protein content of the foodstuff (Wolfe and 

 Rice, 1968). 



The concentration of an element in represent- 



atives of a population of organisms is a function 

 of the turnover time for the element and the av- 

 erage life span of the organism. Long-lived or- 

 ganisms probably achieve a steady state for the 

 turnover of most elements after the cessation of 

 growth — and if the availability of the element 

 from the organism's environment is stable. In 

 organisms with rapid growth and high popula- 

 tion turnover, net accumulation probably pro- 

 ceeds for most metallic elements throughout the 

 entire short life span of the organism, and steady 

 state is not reached before the organism is con- 

 sumed by the next trophic level. The environ- 

 mental and physiological factors determining the 

 steady state conditions are not known, however. 

 Many organisms may accumulate metallic ele- 

 ments far in excess of their biological require- 

 ments (Wolfe, 1970b; Pequegnat, Fowler, and 

 Small, 1969), and accumulation of metals may 

 continue independent of the biological necessity 

 in some cases until available reaction sites (e.g., 

 between metal and proteins or tissue surfaces) 

 are saturated. This process is suggested also by 

 the increasing concentration of mercury with age 

 (or size) in various fish (Westo, 1969; Bache, 

 Gutenmann, and Lisk, 1971). 



In a dynamic estuarine system, where envi- 

 ronmental levels of metallic elements are subject 

 to rapid fluctuations, the organismic response to 

 environmental change must be identified. Infor- 

 mation of this nature is sorely lacking in the lit- 

 erature. Pringle et al. (1968) tested the re- 

 sponse of oysters to various increased experi- 

 mental levels of lead, and after 49 days exposure, 

 accumulation had proceeded in direct relation to 

 availability of lead in the environment. Other 

 data (Chipman, Rice, and Price, 1958; Preston, 

 1967; Wolfe, 1970a) suggest that concentration 

 factors for Zn in oysters are inversely related 

 to zinc content of water, implying that net ac- 

 cumulation would diminish or cease at some high 

 environmental concentration (low concentration 

 factor in oyster) and steady state would be esta- 

 blished. In these cases, however, the variability 

 of instantaneous uptake of the element can only 

 be inferred from the amounts contained after a 

 long period of accumulation. In natural ecosys- 

 tems, fallout radioisotopes appear in estuarine 

 organisms very quickly after initial entry of the 



968 



