Abstract
The mechanism of methylmercury accumulation in fish and aquatic organism was studied in aquatic food chains reproduced in the model streams. The results were compared with those of the direct absorption from surrounding water in the tank experiments.
Benthic algae obtained from the Tama River were planted in the model streams. After culturing them for 1-2 weeks, methylmercury was added 5, 10 or 20ppb every 24 hours. When the mercury level in the algae had become constant, the addition of methylmercury was terminated and the medium was completely replaced by a fresh one. Mayfly larvae (Eperorus sp.) and Tribolodon sp. (Ugui) were kept for 10-17 days in the model streams.
More than 95% of methylmercury was absorbed rapidly within 24 hours by the algae and as a result, methylmercury disappeared from the water. The level of methylmercury in the algae reached 69ppm on dry weight basis within 24 hours after the first addition of methylmercury at a 5ppb level. However, after the termination of methylmercury addition the mercury level in the algae decreased quite slowly not because of elution from the algae into the water, but because of its biological dilution.
The methylmercury absorbed in the algae was rapidly decomposed. Especially, species of algae that were resistant to organic pollution and generally appeared in relatively polluted water showed a higher ability to decompose methylmercury than one grown in oligotrophic water.
The higher the level of methylmercury was in the algae, the greater the quantity of methylmercury, contained in algae taken by fish or water-insects as their feed, was transferred into organisms at upper trophic levels. When 5ppb of methylmercury was added into the model stream, where the algae has a lesser ability to decompose methylmercury, more methylmercury was transferred to the organism at upper trophic levels than when 20ppb of methylmercury was added into the model stream, where the algae has a higher ability of decomposing methylmercury.
Of the methylmercury accumulated by fish or insect larvae through food chains and/or direct absorption, accumulation through food chains by Tricoptera, Ephemeroptera and Tribolodon was 83, 58-83 and 63%, respectively, at 5ppb of methylmercury in the water. On the other hand, accumulation by Tribolodon was reduced to 15% at 10ppb and 9% at 20ppb, respectively. Thus, it could not be unequibocally concluded whether or not the food chain plays an important role in the transfer of methylmercury in the aquatic ecosystem. But it is thought that the algae is an important organism for the transfer of methylmercury in the food chains.
