Empirical data on the behavior of radionuclides in the biosphere are needed for validating radioecological models. In this study, data collected from two lakes in Northern Finland were used to investigate the transfer of 137 Cs from lake water into fish during a 20-year period after the Chernobyl nuclear power plant accident. The results indicated that transfer of 137 Cs in the food chains investigated is nonlinear: the water-to-fish concentration ratios (CRs) decreased with increasing 137 Cs concentration in water. Major deviation from linearity (constant CR) commonly assumed in radioecological modeling was observed only at low 137 Cs concentrations in water. Other findings of potential importance for radioecological models were that 137 Cs concentrations were threefold higher in piscivores than in nonpiscivores and that no differences in 137 Cs uptake were found between the two piscivorous species (pike and perch) studied or between three non-piscivorous species (vendace, white fish, and roach) representing different feeding habits (planktivore, benthivore, and mixed diet).

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Abstract. Soil-grass-milk is one of the main food chains leading radioactivity in man in Finland. From 1963 onwards dairy and farm milk and from 1972 deposition have been regularly collected at several sites in Northern Finland; some grass and AIV-silage samples have been taken simultaneously.137 Cs and 90 Sr have been analysed by the Radiation and Nuclear Safety Authority (STUK) in the Regional Laboratory in Northern Finland. In this study the activity concentrations and decrease rates of 90 Sr and 137 Cs were studied in the years following the atmospheric nuclear weapon testings and after the Chernobyl accident in 1986 until 2008. Also the transfer factors were calculated and the ecological half-lives were estimated in the study. The concentrations of 90 Sr and 137 Cs were highest in the beginning of 1960 due to the nuclear weapon tests. The radioactive fallout in Finnish Lapland from the Chernobyl accident in April 1986 was low. The increase of the 137 Cs concentrations in milk was found out in July 1986 and it decreased rapidly after summer 1987. There was no increase of 90 Sr concentrations in milk and in deposition after Chernobyl fallout.