Ž . The Science of the Total Environment 261 2000 4359 Methylmercury in water, seston, and epiphyton of an Amazonian river and its floodplain, Tapajos River, ´ Brazil  M. Roulet a,  , M. Lucotte a , J.R.D. Guimaraes b , I. Rheault a ˜ a Chaire de Recherche en En ironnement H-Q  CRSNG UQAM, Uni  . du Quebec a Montreal, C.P. 8888, Succ. Centre ´ ` Ville, H3C3P8 Montreal, Quebec, Canada ´ ´ b Laboratorio de Radioisotopos, Inst. de Biofisica da UFRJ, Bloco G-CCS, Rio de Janeiro, CEP 21949-900, Brazil ´ ´ Received 27 September 1999; accepted 17 April 2000 Abstract Levels and accumulation of MeHg were characterized in filtered water, suspended organo-mineral matter, phytoplankton, zooplankton and epiphyton during the dry season and the wet season. In open water of the lentic and Ž . Ž . lotic ecosystems, the MeHg in filtered water 0.2 m was near or below the detection limit 0.02 0.03 ngl. Ž . These concentrations represent 5% of the Hg content in filtered water. Inundated forests Igapo and ´ tot. Ž . macrophyte floating mats were the only sites where MeHg was significantly detected 0.07 0.24 ngl , representing Ž 3  22% of the Hg in filtered water. MeHg concentrations in organo-mineral suspended matter 2  26 ngg dry wt. tot. . representing 0.6 7.3% of Hg were correlated with the N content but not with the C content. Data suggest that tot. MeHg enrichment of suspended matter is strongly influenced by the presence of degraded planktonic remains Ž . relatively rich in N and MeHg. In zooplankton, MeHg concentrations 20  140 ngg dry wt. increased from the dry season to the end of the wet season. This increase was followed by higher proportions of MeHg during the wet Ž . season in comparison to the dry season 15  40 to 50  70% of the Hg . The epiphytic material collected from the tot. roots of macrophyte floating mats contained 2  8.5 ngg dry wt. of MeHg. The proportion of MeHg to Hg in tot. Ž . epiphyton 1.5 8.3% correlated with its C and N contents. The data suggest a greater bioavailability of MeHg in the Tapajos River ecosystems due to the seasonal increase in water level and the consequent inundation of the ´ floodplain. Inundation favours the development of large macrophyte floating mats which increases the bioavailability of epiphyton to herbivorousdetritivorous fish. The root zone of floating macrophytes and the flooded organic horizon of the Igapo forest are the only sites along the Tapajos River where significant MeHg can be detected in the ´ ´  Ž . The present investigation is part of an ongoing study, the CARUSO project IDRC-UFPa-UQAM , initiated to determine the sources, fate, and health effects of MeHg in the lower Tapajos area. ´  Corresponding author. Fax: 1-514-987-3635. 0048-969700$ - see front matter  2000 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 4 8 - 9 6 9 7 00 00594-5