METHYLMERCURY PRODUCTION AND ACCUMULATION IN SEDIMENTS AND SOILS OF AN AMAZONIAN FLOODPLAIN – EFFECT OF SEASONAL INUNDATION ⋆ M. ROULET 1∗ , J.-R. D. GUIMARÃES 2 and M. LUCOTTE 1 1 Chaire de Recherche en Environnement H-Q/CRSNG/UQAM, University du Québec à Montréal, Montréal, Québec, Canada; 2 Laboratório de Radioisótopos, Inst. de Biofísica da UFRJ, Rio de Janeiro, Brazil ( ∗ author for correspondence, e-mail: r34664@er.uqam.ca; fax: +1 514 9873635) (Received 6 January 1999; accepted 24 May 2000) Abstract. This study investigated the spatial and seasonal variations of MeHg concentrations and burdens of different sediments and soils of the Tapajós river floodplain, one of the major clear-water tributaries of the Amazon. The small floodplain of the Tapajós is typical of Amazonian floodplain ecosystems. The studied lakes are bordered by inundated forest (igapó), 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 µgm −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 µgm −2 cm −1 d.w.) and the igapó semi-terrestrial soils (0.2–3 ng g −1 d.w or 2–12 µgm −2 cm −1 d.w.). The litter horizon from the igapó soils showed the highest value of MeHg (4–8 ng g −1 d.w.) representing 0.2–2 µgm −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 203 Hg methylation potentials. The fresh and labile organic matter in the litter of the igapó forest appears as the most important factor leading to significant enrichment of MeHg in these particular terrestrial/aquatic sedimentary environments. Keywords: Amazonia, floodplain, igapó, methylmercury, seasonal inundation 1. Introduction Recent advances have been made in the understanding of the origins and dispersion of mercury (Hg) in the Amazon (Forsberg et al., 1994; Lechler et al., 1997; Roulet et al., 1996, 1998a, 1999a; Silva-Forsberg et al., 1998). In the lower Tapajós valley ⋆ The present investigation is part of an ongoing study, the CARUSO project (CRDI-UFPa- UQAM), initiated to determine the sources, fate and health effects of the presence of MeHg in the area of the Lower Tapaj´ os. Water, Air, and Soil Pollution 128: 41–60, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.