HG METHYLATION IN SEDIMENTS AND FLOATING MEADOWS OF A TROPICAL LAKE IN THE PANTANAL FLOODPLAIN, BRAZIL

Potential net Hg-203 methylation was assayed in different substrates a nd conditions in Fazenda Ipiranga Lake, 30 km downstream from gold min ing fields near Pocone, Pantanal, Brazil, during the dry season. Sampl es and acidified controls of surface sediments and roots of dominant f loating macrophytes (Eichhornia azurea Salvinia sp.) were incubated in situ for 3 days with approx 43 ng Hg.g(-1) (dry weight), added as (Hg Cl2)-Hg-203. Methylmercury ((MeHg)-Hg-203) was extracted in toluene an d measured by beta counting. Net methylation was about 1% in sediments under floating macrophytes, both at an open lake site and at a littor al site. Parallel incubations of sulphate or molybdate amended samples suggest that sulphate reducing bacteria may be important Hg methylato rs at both sites, and that their activity is sulphate-limited in parti cular at the littoral site. In laboratory experiments, net methylation in the same sediments was highest at temperatures in the 33-45 degree s C range but was completely inhibited at 55 degrees C. NaCl addition had a strong inhibiting effect on net methylation. In an intact open-l ake sediment core, spiked with Hg-203 in the overlying water and incub ated for 3 days, total Hg-203 was detectable down to a depth of 14-16 cm, coinciding with the depth reached by the galleries of chironomid l arvae present in the core. Swimming insects caused Hg-203 penetration down to 4 cm. (MeHg)-Hg-203 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 m acrophyte stands, a distinctive feature of tropical rivers and lakes, as potentially important sites for the production of highly available MeHg. (C) 1998 Elsevier Science B.V.