Publicações 2018

Mercury in Black-Waters of the Amazon


Chapter 3: Mercury in Black-Waters of the Amazon

Daniele Kasper, Bruce Rider Forsberg, Helena do Amaral Kehrig, João Henrique Fernandes Amaral, Wanderley Rodrigues Bastos, and Olaf Malm

Abstract: Negro river, the largest black-water tributary of the Amazon, has soils naturally rich in mercury (Hg) that is leached to the aquatic ecosystem by pedologic processes. The flooded areas of its basin are conducive to mercury methylation. Therefore, highest mercury concentrations have been observed in its black-water and biota even in regions without anthropogenic impacts. Here we show an integrated analysis of total mercury (THg) and methylmercury (MeHg) data of environmental samples collected along the Negro basin. THg concentrations in whole water of Negro river and its tributaries did not vary between two hydrological seasons, but in suspended matter were highest during high-water season showing that mercury was more associated to particulate form during this time. During high-water season, waters of Negro river and its tributaries showed highest MeHg concentrations, were more acid, and less oxygenated. The %MeHg in relation to THg of plankton from floodplain lakes of Negro basin was in the same range or higher than the values observed in plankton from natural lakes or reservoirs. Therefore, some processes can occur specially in the environmental conditions of Negro basin such as highest and fastest MeHg absorption by plankton and greater availability of MeHg to bioaccumulation. We conclude that probably MeHg is formed mainly in the flooded area, where dissolved oxygen in depleted and the microbial activity of anoxic organisms is increased. The Amazonian lakes are hot spots for MeHg absorption by planktonic communities and can have an important contribution to Hg concentrations of the basin by exporting large MeHg levels associated to these organisms. Along the food chain of the Negro basin, it was possible to observe the biomagnification of mercury from phytoplankton to dolphin, with increase of mercury concentrations around one order of magnitude from one trophic level to other. Amazon basins, including Negro, have been frightened by diverse land use that can change the mercury cycle and increase concentrations in aquatic systems.

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