THE EFFECT OF SELENIUM AND ORGANIC MATERIAL IN LAKE-SEDIMENTS ON THE BIOACCUMULATION OF METHYLMERCURY BY LUMBRICULUS-VARIEGATUS (OLIGOCHAETA)

The accumulation of methylmercury (MeHg) to an oligochaete worm Lumbri culus variegatus (Muller) was measured in two different lake sediments in the laboratory. C-14- labelled MeHg was added to sediments at the nominal concentration of 95 ng/g dw sediment. Groups of six oligochaet e worms were exposed in glass beakers to 35 g of spiked sediment for 1 4 days. The two sediments had organic carbon concentrations of 3.4% an d 9.9% and natural selenium concentrations of 1.45 and 0.28 mg/kg (dw) , respectively. After two weeks exposure, both the accumulation rate o f MeHg and the body residue in the worms were much lower in the sedime nt having a high organic carbon content. The effect of selenium concen tration in the sediment on bioaccumulation of MeHg in Lumbriculus vari egatus was measured in one sediment (organic carbon 3.4% and Se 1.45 m g/kg) by adding sodiumselenite (Na2SeO3) at different concentrations. The added amounts of selenium were 0, 0.1, 0.5, 2.5, 15.0, and 50.0 mg Se/kg dry sediment. In this exposure the nominal concentration of MeH g was 102 ng/g dw sediment. The two lowest selenium concentrations did not affect the bioaccumulation of MeHg. But, the dose of 2.5 mg Se/kg resulted in a 25% reduction in the body residue after two weeks expos ure. When 15 and 50 mg Se/kg were added to the sediment the accumulati on of MeHg in the organisms was decreased by 75% and 86%, respectively , as compared to the reference.