Short- and long-term patterns of 137Cs in fish and other aquatic organisms of small forest lakes in southern Finland since the Chernobyl accident

We summarize the patterns of 137Cs activity concentrations and transfer into fish and other biota in four small forest lakes in southern Finland during a twenty-year period following the Chernobyl accident in April 1986. The results from summer 1986 showed fastest accumulation of 137Cs into planktivorous fishes, i.e. along the shortest food chains. Since 1987, the highest annual mean values of 137Cs have been recorded in fish occupying the highest trophic levels, for perch ( Perca fluviatilis) 13,600 Bq/kg (ww) and for pike ( Esox lucius) 20,700 Bq/kg (ww). At the same time, activity concentrations of 137Cs in crustacean zooplankton and Asellus aquaticus have ranged between 1000 and 19,500 Bq/kg (dw). In 2006, 5–28% of the 1987 137Cs activity concentration levels were still present in perch and pike. Since 1989 their 137Cs activity concentrations in oligohumic seepage lakes have remained significantly higher than in polyhumic drainage lakes due to the increased transfer of 137Cs into fish in the seepage lakes with lower electrolyte concentrations, longer water retention times and lower sedimentation rate. ► In summer 1986 highest 137Cs levels in planktivore fishes of short food chains. ► Since 1987 highest 137Cs were recorded in predatory fish of highest trophic levels. ► High variation found also in 137Cs of crustacean zooplankton and Asellus aquaticus. ► In long-term fish 137Cs higher in clear seepage lakes than in humic drainage lakes. ► Increased transfer of 137Cs into fish in seepage lakes was the reason suggested.