Dinâmica sazonal do metilmercúrio em ecossistemas fluviais amazônicos.

Abstract

Amazonian rivers are characterized by high natural diversity of water quality among subbasins. Extensive areas of floodplain associated with these rivers are inundated seasonally leading to thermal stratification, which combined with large inputs of allocthonous organic matter results in anoxic conditions especially conducive to mercury (Hg) methylation. The large seasonal and spatial variation of limnological conditions and the relatively undisturbed nature of most drainage basins make Amazonian rivers interesting sites for studying the natural dynamics of methylmercury (MeHg). The huge discharge of Amazonian rivers also makes them important sites for hydroelectric impoundments. In Brazil, a large percentage of electric energy comes from hydroelectric dams, with many of them planned or already constructed on Amazonian rivers. Damming a river can have a major impact on MeHg dynamics in a river system, both above and below the dam. Studies have shown that rivers downstream from a dam can be more contaminated by Hg than in the reservoir itself. However, it is not clear how far downstream this contamination occurs and how it varies seasonally. This study aimed to investigate the influence of the seasonal flood pulse and associated variations in limnological parameters on the dynamics of MeHg in the principal Brazilian tributaries of the Amazon river. It also aimed to investigate the effects of impoundment on MeHg dynamics and bioaccumulation both above and below a dam. Water samples were collected in the Amazon main stem and its main Brazilian tributaries (n = 38) during two distinct phases of the flood-pulse cycle (high and low-water seasons). Water, plankton and fish were collected for one year in Balbina reservoir and in different sites between 0.5 and 250 km downstream from the dam. Limnological conditions were measured at the same time of water collection at each site. The MeHg concentrations in river waters varied between 0.02 and 0.76 ng.L-1. These concentrations increased with water level (r2 = 0.528%; p < 0.0001) and decreased with dissolved oxygen concentrations (multiple regression: F = 11.5; r2 = 0.443; p < 0.001). Due to higher MeHg concentrations and water discharge, MeHg transport was much higher in all rivers at high water, demonstrating a dominant influence of the flood pulse on MeHg dynamics in river systems. The MeHg concentrations in water from Balbina reservoir were substantially higher in the anoxic hypolimnion when compared with the epilimnion. The high MeHg concentrations in hypolimnetic water exported from the reservoir to the river declined gradually to background levels 200 km downstream from the dam. Uniform depth distributions of MeHg and oxygen were encountered in the reservoir only during the rainy season, when vertical mixing was greatest and coincided with uniform MeHg concentrations along the river downstream from the dam. MeHg in plankton and total Hg concentrations in fish from downstream sites were higher than reservoir values, suggesting that MeHg exported from the dam was accumulated by biota. MeHg concentrations in both natural and impounded river systems were influenced by seasonality, with dissolved oxygen being the main predictor of variation. In tributaries of the Amazon river, changes in water level influence in the floodplain and in the limnological parameters, specially dissolved oxygen, and MeHg dynamics. In the Balbina reservoir, where the water level is manually controlled, the rainy and dry seasons control the limnology of reservoir, and, consequently, MeHg dynamics. The influence of the dam on MeHg concentrations decreases downstream, while the influence of the natural flood cycle increases.