Mercury methylation along a lake-forest transect in the Tapajos river floodplain, Brazilian Amazon: seasonal and vertical variations

The seasonal and spatial variations of net methylmercury production in sedi ments, soils and other sites were evaluated by assays with Hg-203 at differ ent depths and locations along a lake-forest transect at lake Enseada Grand e, Tapajos river. Soil and sediment samples were taken at the surface and a t different depths up to 9 cm. Fresh samples and acidified controls (1-3 g dry wt.) were slurried with local water and incubated in the dark at 25-28 degrees C for 3 days with 0.5-1.6 mu g Hg g(-1) (dry wt.) added as (HgCl2)- Hg-203. CH3 Hg-203 was extracted and measured in scintillation cocktail aft er acid leaching. Methylmercury production varied by orders of magnitude am ong sites and among sediment or soil layers. Seasonal variations were small er than those with sample depth and location. In both seasons, MeHg formati on in sediment and soil or flooded soil decreased with depth and was, in th e top layers, one order of magnitude higher in the C-rich littoral macrophy te zone (2.3-8.9%) and flooded forest (3.2-4.5%) than in the center of the lake (0.2-0.56%). Similar MeHg production was found in slurried dry soils ( dry season) and in soils already flooded for months. In the macrophyte zone soil (dry season), methylation was mainly associated with the thin Paspalu m sp. rootlet layer. In the forest site, vertical variation in methylation was less pronounced in flooded than in dry soils and during the inundation the higher methylation rate was found in the flocculent sediment settled ov er the litter layer. The roots of floating Paspalum sp. were an important H g methylation site, particularly those heavily colonized with periphyton (3 .4-5.4%). Methylation in surface or near-bottom water was undetectable (< 3 x 10(-2)%) at all sites. Flooded forests and macrophyte mats are specific features of the Amazon and are important links between Hg inputs from natur al and manmade sources and MeHg exposure of local populations through fish intake. (C) 2000 Elsevier Science B.V. All rights reserved.