Bioaccumulation and trophic transfer of ¹³⁷Cs in marine and freshwater plankton

An experimental study was conducted to investigate the trophic transfer of ¹³⁷Cs in marine and freshwater food chains, focusing on phytoplankton, zooplankton, and planktivorous fish. Algal concentration factors were 278 in freshwater and 69 in seawater. The weight-normalized uptake rate constants of ¹³⁷Cs were similar for both freshwater daphnids and marine copepods. Most of the ¹³⁷Cs in marine copepods was in the exoskeleton followed by polar and non-polar components. In freshwater daphnids, ¹³⁷Cs was highest in the polar fractions followed by exoskeleton and low amounts in the non-polar components. Fish that fed on contaminated marine copepods assimilated ¹³⁷Cs with an efficiency of 88%, while 91% was assimilated from freshwater daphnids. A bioaccumulation model demonstrated that diet accounted for ≤3% of the total body burden of ¹³⁷Cs in marine zooplankton and ≤12% in freshwater zooplankton, but ≥99% of the total body burden in fish. Rate constants of ¹³⁷Cs loss from fish following aqueous exposure were 0.2 d^-1 and 0.4 d^-1 in marine and freshwater conditions, respectively, but only 0.06 d^-1 and 0.3 d^-1, respectively, following dietary exposure. This model also indicates that trophic transfer factors from zooplankton to fish are up to 2.2 for marine conditions and up to 0.6 for freshwater. ¹³⁷Cs is unusual among metals in that it enters marine food chains primarily from the aqueous phase into zooplankton, from which it is highly assimilated by fish, resulting in detectable ¹³⁷Cs in fish tissues.