Methylmercury production and distribution in aquatic systems.

Mercury methylation and partitioning between sediment, water and fish were studied in river sediment spiked with mercuric chloride at levels of 1, 5 and 10 ppm as Hg. Maximum methylmercury production in the sediment occurred during the first week of incubation, with concentrations reaching 45.5 ng/g (dry wt.). Equilibrium conditions were established 3-4 weeks after the contamination of the sediment with mercury. Methylmercury partitioning between sediment and water yielded methylmercury levels of 25-154 ng/l in the fish aquariums and 0.26 ng/l in the fish-free control after 1 week of incubation. Equilibrium levels were 1.5-5.5 ng/l and 0.53 ng/l, respectively, during the 7th week. Over 50% of the methylmercury in whole water were in soluble form or associated with colloidal particulate < 1 micron. Mercury concentration in fish increased almost exponentially from 30 ppb to an average of 345 ppb within 3 weeks. Mercury uptake rates by fish were in the range of 10-18 ng/g per day during the 2nd and 3rd weeks, high rates occurring in water with methylmercury to total mercury ratios > 0.45. The partition coefficients for total mercury and methylmercury between fish and water (Kf-w) were 5000-7000 and 10,000-22,000, respectively. Large differences were observed in methylmercury production in sediment-water incubation with fish and the fish-free control. Equilibrium methylmercury concentrations in sediment were in the range 15-32 ng/g in the aquariums containing fish and 3-4.5 ng/g in the fish-free control. The significance of fish in mercury methylation in the aquariums still remains to be clarified as fish itself cannot methylate mercury in vivo.