Mercury and methylmercury in individual zooplankton: Implications for bioaccumulation
Using trace-metal-clean sampling and handling techniques along with ultrasensitive analytical procedures, it is possible to measure both total Hg and monomethylmercury (methyl-Hg) in natural planktonic communities with the same level of taxonomic, ontogenic, and trophic resolution that is currently possible in fish communities. In an experimentally manipulated lake, both acidification and trophic position enhanced the bioaccumulation of methyl-Hg in the plankton. A consistant pattern of methyl-Hg enrichment (2-4 ×) in water, bulk phytoplankton, and individual zooplankton was associated with a 1.5 unit pH decrease in Little Rock Lake. Regardless of pH, bioconcentration factors [Bf = log(Cb/Cw), where Cb and Cw are Hg concentrations in biota and water] were substantially higher for methyl-Hg than those for total Hg or nonmethyl-Hg at three pelagic trophic levels ( 10-100×). Between each trophic level, the Bf(methyl-Hg) increased by 0.5 log units, clearly indicating biomagnification. Although somewhat higher in the acidified basin, Bf(methyl-Hg) was more strongly influenced by trophic position than by pH. This suggests that methyl-Hg was bioaccumulated largely in proportion to supply and that acidification may have directly increased supply to the base of the food chain.