TEMPORAL CHANGES IN CONCENTRATION OF RADIOCESIUM IN LAKE SEDIMENT ANDFISH OF SOUTHERN FINLAND AS RELATED TO ENVIRONMENTAL-FACTORS

In 1992 the concentration of radiocaesium in the surface layer of bott om sediment of 52 lakes of southern Finland was found to have declined on average to 27.4% of 1987-88 levels and to 25.7, 28.0 and 38.6% of 1988 levels in the tissues of perch, pike and roach, respectively. The sediment radiocaesium concentration was higher in larger lakes and in lakes having a large catchment, but lower in oligotrophic than eutrop hic lakes, when the total amount of electrolytes and organic matter wa s high. The concentration of radiocaesium in perch and pike was lower in eutrophic than oligotrophic lakes and lower in all three fish speci es when the concentration of electrolytes was high. Lower radiocaesium values in both sediments and fish of eutrophic when compared with oli gotrophic lakes may be due to a greater dilution of this contaminant i nto a higher biomass. Water colour and pH were negatively correlated w ith radiocaesium in perch only. A lower radiocaesium level in perch in humic water lakes may be due to an adsorption of radiocaesium onto hu mus particles which reduces its bioavailability and transfer along the food chain. Radiocaesium in fish and in sediment were positively corr elated with each other for perch and roach whose diets comprise mainly the detritivore or sediment feeding bottom animals. In perch the radi ocaesium concentration was high when the water retention time of the l ake was long. A tropic level bioconcentration was evident in the three fold higher radiocaesium level in predatory pike compared with roach. The decline in radiocaesium was most pronounced in sediment when the c atchment area was large, when the water pH was high and when the sedim ent surface contained a low proportion of organic matter. In fish the largest decline in radiocaesium was recorded in pike from eutrophic or shallow lakes and when the lake water retention time was short. In pe rch and roach the reduced radiocaesium levels were not due to effects of water quality or lake morphometrical properties. Two different sedi ment core samplers, a Kajak corer and a Limnos corer, gave comparable results.