Hg methylation in sediments and floating meadows of a tropical lake in the Pantanal floodplain, Brazil

Potential net 203Hg methylation was assayed in different substrates and conditions in Fazenda Ipiranga Lake, 30 km downstream from gold mining fields near Poconé, Pantanal, Brazil, during the dry season. Samples and acidified controls of surface sediments and roots of dominant floating macrophytes (Eichhornia azurea, Salvinia sp.) were incubated in situ for 3 days with approx. 43 ng Hg.g−1 (dry weight), added as 203HgCl2. Methylmercury (Me203Hg) was extracted in toluene and measured by beta counting. Net methylation was about 1% in sediments under floating macrophytes, both at an open lake site and at a littoral site. Parallel incubations of sulphate or molybdate amended samples suggest that sulphate reducing bacteria may be important Hg methylators at both sites, and that their activity is sulphate-limited in particular at the littoral site. In laboratory experiments, net methylation in the same sediments was highest at temperatures in the 33–45°C range but was completely inhibited at 55°C. NaCl addition had a strong inhibiting effect on net methylation. In an intact open-lake sediment core, spiked with 203Hg in the overlying water and incubated for 3 days, total 203Hg was detectable down to a depth of 14–16 cm, coinciding with the depth reached by the galleries of chironomid larvae present in the core. Swimming insects caused 203Hg penetration down to 4 cm. Me203Hg was detected only in the upper layers (0–2 cm) of the sediment, with concentrations reaching 0.47–0.75% of total Hg. This suggests an important role for bioturbation in the exchange of Hg and MeHg between sediment and water. Methylation was up to nine times more intense in floating macrophyte roots than in the underlying surface sediments: an average of 10.4% of added Hg was methylated in samples of Salvinia sp. roots during the 3-day incubation, and 6.5% in E. azurea roots. This adds to previous findings on the role of such macrophyte stands, a distinctive feature of tropical rivers and lakes, as potentially important sites for the production of highly available MeHg.