Guimarães J R, Roulet M, Lucotte M, Mergler D
Lab. de Radioisótopos, Inst. de Biofísica, Universidade Federal do Rio de Janeiro (UFRJ), Bloco G CCS, Rio de Janeiro, Brazil.
Sci Total Environ. 2000 Oct 16;261(1-3):91-8. doi: 10.1016/s0048-9697(00)00627-6.
The seasonal and spatial variations of net methylmercury production in sediments, soils and other sites were evaluated by assays with 203Hg at different depths and locations along a lake-forest transect at lake Enseada Grande, Tapajós river. Soil and sediment samples were taken at the surface and at 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 microg Hg g(-1) (dry wt.) added as 203HgCl2. CH3 203Hg was extracted and measured in scintillation cocktail after acid leaching. Methylmercury production varied by orders of magnitude among sites and among sediment or soil layers. Seasonal variations were smaller than those with sample depth and location. In both seasons, MeHg formation in sediment and soil or flooded soil decreased with depth and was, in the top layers, one order of magnitude higher in the C-rich littoral macrophyte 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 Paspalum 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 over the litter layer. The roots of floating Paspalum sp. were an important Hg 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 natural and manmade sources and MeHg exposure of local populations through fish intake.
通过在塔帕若斯河大恩塞阿达湖沿湖 - 森林样带不同深度和位置进行的²⁰³Hg分析，评估了沉积物、土壤及其他地点甲基汞净生成的季节和空间变化。在地表以及直至9厘米的不同深度采集土壤和沉积物样本。将新鲜样本和酸化对照（1 - 3克干重）与当地水制成泥浆，添加0.5 - 1.6微克汞/克（干重）的²⁰³HgCl₂后，在25 - 28摄氏度黑暗条件下孵育3天。酸浸后，在闪烁鸡尾酒中提取并测量CH₃²⁰³Hg。甲基汞生成在不同地点以及沉积物或土壤层之间相差几个数量级。季节变化小于样本深度和位置的变化。在两个季节中，沉积物、土壤或淹水土壤中的甲基汞形成均随深度降低，并且在顶层，富碳的沿岸大型植物区（2.3 - 8.9%）和淹水森林（3.2 - 4.5%）中的甲基汞形成比湖心（0.2 - 0.56%）高一个数量级。在泥浆状干土（旱季）和已淹水数月的土壤中发现了类似的甲基汞生成情况。在大型植物区土壤（旱季）中，甲基化主要与薄叶雀稗的细根层有关。在森林地点，淹水土壤中甲基化的垂直变化不如干土明显，在淹水期间，在落叶层上沉降的絮凝沉积物中发现了较高的甲基化率。漂浮的薄叶雀稗的根是汞甲基化的重要位点，尤其是那些被周丛生物大量定殖的根（3.4 - 5.4%）。在所有地点的表层或近底层水中均未检测到甲基化（< 3×10⁻²%）。淹水森林和大型植物垫是亚马逊地区的特有特征，是自然和人为来源的汞输入与当地居民通过鱼类摄入甲基汞暴露之间的重要联系。
