Direct and trophic contamination of the herbivorous carp Ctenopharyngodon idella by inorganic mercury and methylmercury.

A comparative analysis of inorganic mercury (Hg(II)) and methylmercury (MMHg) bioaccumulation by the herbivorous fish Ctenopharyngodon idella, via the direct and "direct+trophic" routes, was investigated experimentally. The direct contamination was based on fairly low exposure conditions (Hg(II), 300 ng small middle dotL(-1); MMHg, 30 ng small middle dotL(-1)), for 15 and 30 days. The trophic uptake was based on ingestion of macrophyte leaves (Elodea densa), previously exposed for 87 days to similar Hg concentrations. Results revealed high capacities of bioaccumulation in fish via the direct route, with accumulation factors of 1000 for Hg(II) and 13,000 for MMHg. Hg concentrations measured after direct+trophic contamination were very much greater than those measured after direct exposure, owing to considerable metal burdens accumulated in plants and to large biomasses ingested. Nevertheless, estimated trophic transfer rates were fairly low: 2% for Hg(II) and 13% for MMHg. MMHg determinations in elodea leaves and fish muscle samples reveal significant methylation and demethylation reactions.