Experimental Study of Inorganic and Methylmercury Bioaccumulation by Four Species of Freshwater Rooted Macrophytes from Water and Sediment Contamination Sources

An experimental study based on four species of rooted macrophytes (Elodea densa, Ludwigia natans, Lysimachia nummularia, Hygrophila onogaria) was carried out to quantify and compare their mercury bioaccumulation capacity from the water column and the sediment compartments as initial sources of contamination. The first stage consisted of analyzing plant growth (weight and total length) in relation to six different characteristics of the biotopes, resulting from the combination of three types of sediment (natural sediment, sediment + sand, clay) and two types of aquatic medium (synthetic river water, dechlorinated tap water). This preliminary experiment permitted selection of the basic structure of the experimental units-a mixed biotope consisting of dechlorinated tap water and a mixture of natural sediment + sand-for the comparative analysis of mercury bioaccumulation capacities. In the second stage, two chemical forms of mercury were considered (HgCl2 and CH3HgCl), the metal being introduced into the sediment at the beginning of the experiment ("sediment" source) or into the water column by twice-daily additions ("water" source). Results showed very great accumulation differences after 18 or 21 days exposure: (a) Hg concentrations in the plants (stems + leaves) were always greater when the metal was introduced in organic form, with even greater differences when initial contamination was via the sediment; (b) according to the selected experimental conditions, mercury bioaccumulation by the macrophytes from the water source was about 10 times greater than that observed when the metal was introduced into the sediment, and thus, despite the strong differences between the contamination levels of these two sources, in favor of the sediment (factor close to 103 for the two compounds); and (c) major interspecies differences emerged in Hg burdens accumulated by the plants, with differences being very small when results were expressed as concentrations, thus taking account of the different biomasses of the species.