Long-term dynamics of Chernobyl 137Cs in freshwater fish:: quantifying the effect of body size and trophic level

1. Freshwater fish are a potentially important link in the transfer of radionuclides from polluted ecosystems to people. A pulsed contamination event such as the Chernobyl fallout in 1986 is a challenge to the prediction of radioactivity in biota, because activity concentrations of radionuclides can change dynamically among populations during an initial equilibration phase. This was demonstrated from time-series of (137)caesium (Cs) in fish from three Swedish lakes (1986-2000, eight species, > 7600 individuals). In addition, we used these data to test hypotheses about the influence of fish size and trophic level on the temporal patterns of Cs-137. 2. In order to facilitate comparisons, a pulse-response model was developed to extract key parameters from field data: the timing (t(max)) and level (Cs-max) of the peak concentrations, the near steady-state level (Cs-base) and the long-term decline rate (lambda). 3. Peak concentrations in different fish were attained 56-806 days after the fallout. This delay (t(max)) increased with body size and trophic level. Cs-max increased with fish size but was highest at intermediate trophic levels. Cs-base increased by a factor of 1.8 per trophic level, but was not influenced by size across species. The Cs-137-size relationship within species, however, shifted from negative to positive during the first 2 years. 4. The apparent ecological half-life (T-1/2) for Cs-137 increased after t(max) and initially differed among fish and lakes. About 10 years after the fallout T-1/2 was no longer significantly different among fish or lakes, suggesting that steady-state among Cs-137 levels in fish had been reached. The mean T-1/2 during 1996-2000 was 46 years, implying that the future recovery from present Cs-137 levels (more than 10-fold higher than pre-Chernobyl levels) will be dominated by the physical decay of Cs-137 (30 years). 5. The influence of fish size and trophic level on Cs-137 dynamics has been consistent among the three lakes. The duration of the Cs-137 pulse in fish appeared to be regulated by fish ecophysiology, whereas the amplitude also appeared to be regulated by lake characteristics. 6. Synthesis and applications. These data have implications for the predictive modelling of pulsed contaminants in lake ecosystems and show that (i) complex temporal concentration trends in fish can be adequately modelled by a combination of simple first-order equations; (ii) a one-component decline function is insufficient to describe the observed increase in ecological half-life over time; (iii) fish weight and trophic level can and should be treated as independent variables because their influence differs and changes over time. For management purposes, a pulse-response model for fish can be scaled to different systems by using established relationships to account for differences in fallout and catchment biogeochemistry.