The Turnover of Metals in a Eutrophic and an Oligotrophic Lake in Sweden

Trace metal dynamics were studied in lakes to illustrate the direct and indirect effects of eutrophication on the turnover of metals. The decreasing atmospheric load of Pb was well reflected by the Pb-profile in dated sediment cores. The trend was, however, biased by resuspension which at times dominated the flux of particles to the sediments. Since sulfides played an important role for the sequestering of metals in the sediments, resuspension, and water circulation, would facilitate the dispersal of pollutants and the solubilization of metal sulfides by sulfide oxidation. Significant effects of “seston dilution” was demonstrated as decreased Pb and Hg levels in the eutrophic lake sediments. However, the bioavailability in fish for those metals did not decrease, as it did for Cd and Tl. In the eutrophic lake, higher concentrations of Hg in fish muscle were accompanied by increasing net production of methyl-Hg in hypolimnetic waters. Higher fluxes of metals in the eutrophic lake may partly be explained by a larger biomass and partly by different geology and pH. Eutrophication decreased the residence time of Cu, Ni, and Tl, probably partly due to sulfide fixation, and decreased the concentration of Cd and Mo in water, due to a large biomass and an efficient scavenging of metals.