Evolution of mercury distribution within an experimental system “water‐sediment‐macrophytes(Elodea densa)”

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Abstract
The aim of this study is to quantify mercury distribution within an experimental system, “water, natural sediment and rooted macrophytes (Elodea densa)”; after contamination of the water column by methylmercury. For the four contamination levels selected, the metal was introduced into the water column by two daily additions (0.55, 1.1, 2.75 and 5.5 μgHg/experimental unit/day). Eight observation periods were selected (3, 6, 9, 12, 15, 18, 21 and 24 days) to quantify total mercury distribution in the water column, sedimentary layers and macrophytes (leaves, stems, whole plants) in relation to pondéral growth. Results show a good degree of proportional relationships between mercury accumulated by the Elodeae and the two factors “length of exposure”; and “amount of Hg introduced”;. In relation with macrophyte growth, the evolution of Hg concentrations reached a plateau after about fifteen days. Analysis of the vertical stratification of Hg in the sediments clearly shows the predominant role of the “water‐sediment”; interface in metal accumulation, with no diffusion towards the lower layers observed. The level of contamination in the upper stratum is closely dependent on the length of exposure (non‐linear trend) and the contamination pressure; in some cases this may represent very high concentrations. From measurements of the mercury content of each of the three compartments studied, it was estimated that losses were about 40% of the total Hg introduced throughout the experiment. This percentage is independent of length of exposure and quantities of contaminant introduced into the water column.