Methylmercury in water, seston, and epiphyton of an Amazonian river and its floodplain, Tapajos River, Brazil

Levels and accumulation of MeHg were characterized in filtered water, suspe nded organo-mineral matter, phytoplankton, zooplankton and epiphyton during the dry season and the wet season. In open water of the lentic and lotic e cosystems, the MeHg in filtered water (< 0.2 mu m) was near or below the de tection limit (< 0.02-0.03 ng/l). These concentrations represent <5% of the Hg-tot. content in filtered water. Inundated forests (Igapo) and macrophyt e floating mats were the only sites where MeHg was significantly detected ( 0.07-0.24 ng/l), representing 3-22% of the Hg-tot. in filtered water. MeHg concentrations in organo-mineral suspended matter (2-26 ng/g dry wt. repres enting 0.6-7.3% of Hg-tot.) were correlated with the N content but not with the C content. Data suggest that MeHg enrichment of suspended matter is st rongly influenced by the presence of degraded planktonic remains relatively rich in N and MeHg. In zooplankton, MeHg concentrations (20-140 ng/g dry w t.) increased from the dry season to the end of the wet season. This increa se was followed by higher proportions of MeHg during the wet season in comp arison to the dry season (15-40 to 50-70% of the Hg-tot.). The epiphytic ma terial collected from the roots of macrophyte floating mats contained 2-8.5 ng/g dry wt. of MeHg. The proportion of MeHg to Hg-tot. in epiphyton (1.5- 8.3%) correlated with its C and N contents. The data suggest a greater bioa vailability of MeHg in the Tapajos River ecosystems due to the seasonal inc rease in water level and the consequent inundation of the floodplain. Inund ation favours the development of large macrophyte floating mats which incre ases the bioavailability of epiphyton to herbivorous/detritivorous fish. Th e root zone of floating macrophytes and the flooded organic horizon of the Igapo forest are the only sites along the Tapajos River where significant M eHg can be detected in the water column and sediment. This new study suppor ts the hypothesis that MeHg production and transfer to the first link of th e food chain in Amazonian river systems is closely related to organic matte r biogeochemistry in the floodplain environment. (C) 2000 Elsevier Science B.V. All rights reserved.