Experimental ecosystems: Comparative study of two methods of contamination of the water column by mercury compounds in relation to bioaccumulation of the metal by rooted macrophytes(Ludwigia notons).

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Abstract
In the context of work on bioaccumulation and transfer of mercury compounds within a multicompartment model (water, natural sediment, rooted macrophytes) a study was set up to select from several methods of water column contamination the one which best matched our experimental objectives. After some theoretical considerations based on a simulation using simple mathematical models, an experiment was carried out to compare two methods of water column contamination, with specific reference to the bioaccumulation of total mercury by one species of rooted macrophyte (Ludwigia natans): these were: 1) continuous additions of Hg in order to maintain a constant concentration in the water, and 2) twice daily additions of a constant quantity of Hg into all experimental units. For a better appreciation of the effect of each method, the experimental protocol considered several factors: photoperiod, temperature, chemical form of mercury and length of observation period. In view of the overall objectives of our research, which require several dozen experimental units to be set up and observed over periods of about a month, the second method of mercury addition was selected. In some cases, concentrations and burdens of Hg measured in the plants must be divided by a criterion “concentration days equivalent”;, calculated from Hg concentrations measured at intervals in the water.