The hydroelectric reservoir of Petit Saut, French Guiana, was created in 1994-1995 by flooding 350 km(2) of tropical forest. When sampled in 1999, the lake exhibited a permanent stratification separating the 3-5 m thick, oxygenated epilimnion from the anoxic hypolimnion. The rate of anaerobic organic carbon mineralization below the oxycline was on the order of 1 mumol C m(-2) s(-1) and did not show a pronounced difference between wet and dry seasons. Methanogenesis accounted for 76-83% of anaerobic carbon mineralization, with lesser contributions of sulfate reduction and dissimilatory iron reduction. Upward mixing of reduced inorganic solutes explained 90% of the water column O-2 demand during the dry season, while most O-2 consumption during the wet season was coupled to aerobic respiration of organic matter synthesized in the surface waters. Inorganic mercury species represented 10-40% of total dissolved mercury in the epilimnion, but were of relatively minor importance (less than or equal to10%) in the anoxic portion of the water column. Net production of soluble organic mercury compounds in the flooded soils and anoxic water column did not vary significantly between wet and dry seasons. Methylmercury accounted for about 15% of total dissolved mercury below the oxycline. Its estimated net production rate, 0.04 mg m(-2) yr(-1), is of the same order of magnitude as values reported for contaminated lakes and flooded terrestrial ecosystems.