Methylmercury production and accumulation in sediments and soils of an amazonian floodplain - Effect of seasonal inundation

This study investigated the spatial and seasonal variations of MeHg concentrations and burdens of different sediments and soils of the Tapajos river floodplain, one of the major clear-water tributaries of the Amazon. The small floodplain of the Tapajos is typical of Amazonian floodplain ecosystems. The studied lakes are bordered by inundated forest (igapo), while floating macrophyte mats (Paspalum sp.) develop at the margin of lakes during the flooded season. During the flood, we observed very low MeHg concentrations in the open water lake sediments (<0.5 ng g(-1) d.w or <0.5 mug m(-2) cm(-1) d.w.) as compared to the semi-aquatic sediments of the macrophyte zone (0.2-1.4 ng g(-1) d.w or 1-3 mug m(-2) cm(-1) d.w.) and the igapo semi-terrestrial soils (0.2-3 ng g(-1) d.w or 2-12 mug m(-2) cm(-1) d.w.). The litter horizon from the igapo soils showed the highest value of MeHg (4-8 ng g(-1) d.w.) representing 0.2-2 mug m(-2) cm(-1) d.w. at the sediment/water interface during the aquatic phase. The inundation had no effect on the concentrations and burdens of MeHg in the sediment of the central part of the lake. The inundation had a clear effect on the methylation of Hg at the surface of semi-aquatic shoreline sediments (macrophyte zone) and semi-terrestrial forest soils, where MeHg concentrations and burdens appeared to be 3 times greater following inundation. In all cores, total Hg concentrations follow those of Fe and Al oxy-hydroxides, whereas the MeHg concentrations are linked to organic matter quality and quantity. It is suggested that organic matter and inundation control MeHg production and accumulation at the benthic interface. These results confirm previous observations, in the same study area, of net Hg-203 methylation potentials. The fresh and labile organic matter in the litter of the igapo forest appears as the most important factor leading to significant enrichment of MeHg in these particular terrestrial/aquatic sedimentary environments.