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.