The fate of radiocesium in freshwater communities—Why is biomagnification variable both within and between species?

In the Ottawa River, levels of 137Cs exhibit classic biomagnification with about a 4-fold increase with each trophic level. For a given trophic level, organisms with a benthivorous diet have about 2-fold higher 137Cs levels than planktivorous organisms of the same trophic level. Lower Bass Lake exhibits a similar pattern of biomagnification, as does Great Slave Lake. 137Cs concentrations in 18 species of fish from the three systems average about 3-fold those of their food, and always exceed the concentration of 137Cs in their food. However, there are no significant differences in biomagnification within functional groups or between functional groups and systems. These results conclusively demonstrate food-chain biomagnification of 137Cs. Within species variation in biomagnification follows three patterns: linear increase with age (4 species); higher biomagnification in mature fish than immature fish, increasing in a step-like pattern (3 species); no increase with age (1 species). These patterns can be explained by the ratio of assimilated consumption to growth and elimination: a linear increase in biomagnification occurs when the ratio of assimilated consumption to growth and elimination increases with age; a step-wise increase in biomagnification at maturity occurs when the ratio of assimilated consumption to growth and elimination increases at maturity; no increase in biomagnification with age occurs when the ratio of assimilated consumption to growth and elimination remains constant throughout the fish lifecycle. This explains why no consensus has been reached regarding 137Cs biomagnification and fish age and points to the predominance of consumption rates in determining the biomagnification of 137Cs by fish. Thus, to accurately predict 137Cs concentrations in fish under steady-state or dynamic conditions, consumption rates need to be known.