UPTAKE BY AQUATIC ORGANISMS 615 



TABLE 4 Primary Food Sources for Various Great Lakes Species 



Species Primary food sources 



Zooplankton (general) Phytoplankton 



Mysis, Pontoporeia (crustaceans) Periphyton near surficial sediment layer 



Sculpin Pontoporeia 



Chub, alewife, smelt, and whitefish Zooplankton or mixture of zooplankton 



and benthic invertebrates 



Perch Mixture of invertebrates and fish 



Coho salmon, chinook salmon, and lake trout Smaller fish 



As summarized by Marshall, Waller, and Yaguchi (1974), "The results clearly indicate 

 that although the concentration of plutonium in phytoplankton is several thousand times 

 that in the water, it decreases by an order of magnitude in each successive link in the food 

 chain leading to man." 



Wahlgren and Marshall (1974) studied the distribution of residual fallout plutonium 

 in Lake Michigan between the water and the various trophic levels of the food chain. 

 They found tliat the CR value for plutonium in phytoplankton compared with that in 

 water was about 5000. A reduction in concentration by a ratio of about 10 was 

 observed at each trophic level consisting of zooplankton, planktivorous fish, and 

 piscivorous fish. The top predators had concentrations only slightly greater than the lake 

 water. However, the concentration in benthic (bottom-feeding) fish was considerably 

 higher than tliat in the planktivorous fish. It should be noted that the fish were not 

 dissected but were analyzed in their entirety (including GI tract). From Fig. 1 we 

 estimate the planktivorous fish to have a CR of about 20 over water and the piscivorous 

 fish a CR of about 2, whereas benthic fish exhibit a CR of approximately 300. 



The major role of phytoplankton in plutonium kinetics in aquatic systems has been 

 postulated to be one of the removal of a significant fraction of plutonium from tlie water 

 column (Walilgren et al., 1976; Hetherington, 1976). However, collection techniques are 

 such tliat phytoplankton cannot readily be separated from inorganic suspended 

 parficulate matter. The CR values for algae reported in Table 3, in fact, may be high 

 owing to the inclusion of suspended inorganic particulate matter, which would have a CR 

 value of approximately 10^ (see below). The correlafion between percent silicon content 

 and plutonium concentrations in phytoplankton samples (Yaguchi, Nelson, and Marshall, 

 1974) was attributed to the predominance of diatom frustules in the samples analyzed. 

 Wahlgren et al. (1976) reported a correlation between percent ash weight and plutonium 

 concentrafions. They also concluded that the plutonium was associated with diatom 

 frustules in the plankton samples. However, in neitlier case was the contribution of 

 associated suspended inorganic particulate matter quantified. Wahlgren et al. (1976) 

 reported a distribuUon coefficient 



„ _ Concentrafion on solid phase (g/g) 

 f^d 



Concentration in liquid phase (g/ml) 



of plutonium for suspended sediment materials of about 3 X 10^. The inclusion of a 

 small amount of those materials in the ash residue of phytoplankton samples is a plausible 

 alternative interpretation of the observed correlafions. 



