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Slash-and-burn agriculture practiced by several thousand small-scale farmers in the Tapajós region of the Brazilian Amazon has contributed to accelerated deforestation over the past decades. The present study aims to quantify and qualify changes in the transfer of terrestrial organic matter (TOM) to aquatic environments following deforestation. Lignin biomarkers have been analyzed from sediment cores collected in three floodplain lakes and in suspended particulate matter sampled during both wet and dry seasons. These analyses are interpreted with regard to lignin biomarker signatures of surface and deeper horizons of common soils, and of dominant plant species from forested and deforested environments. Dating the sediment cores with 210 Pb allows reconstructing the successive deforestation cycles since the onset of European colonization two centuries ago. Further, satellite images coupled to a GIS approach is used to correlate the evolution of sedimentary TOM and anthropogenic land-use from 1986 to 2009. Over this period, sedimentation rates have sharply increased, and the nature of the sedimentary TOM has been shifted from being linked to primeval forest soils to degraded soils following deforestation for subsistence cropping and/or pasture lands. The intensity of changes in the nature of sedimentary TOM appears inversely related to the connectivity of flood lakes to the river. In the least connected flood lake, weathering of pasture soils in the watershed dominates TOM inputs particularly during the dry season. Massive deforestation in the Amazon thus triggers major changes in the nature of TOM transferred and sedimented in aquatic systems. [ABSTRACT FROM AUTHOR]

Lake St. Pierre (LSP), constituted of a 120 km2 stretching of the St. Lawrence River (Southern Québec), hosts the largest freshwater fishery industry in Canada. The lake drains, through its main tributaries, an important area of agrarian land and was subjected to intense industrial activities in the past century. In this paper, we present (1) an estimation of the seasonal aquatic mercury (Hg) inputs to LSP from the St-Lawrence River and two major tributaries; (2) a reconstruction, by the analysis of sediment cores, of the historic inputs of Hg into LSP and in a large riparian wetland, Bay St. François. Our results indicate that the aquatic Hg inputs to LSP (290 kg Hg between April 2003 and April 2004) are moderately elevated with most of the inputs occurring from the St. Lawrence River, either in spring or early winter, during high flow episodes. The sediment profiles suggest a recent decrease in Hg inputs, likely attributable to improvements of industrial practices. The observed perturbation of the surface sediments give evidence of an active hydrodynamic regime, suggesting that LSP could only act as a transitory system for suspended sediment and Hg, with seasonal accumulation and recurrent re-suspension resulting from changes in the hydrodynamic regime. Finally, we observed positive MeHg fluxes from the sediment to the water at different seasons in Bay St. François. However, Hg levels in fish like walleyes of LSP are reported to be low, which could be explained by faster fish growth rates following in part intense fishing pressure in LSP. [ABSTRACT FROM AUTHOR]

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Relationships between organic matter (OM) compounds and methyl mercury concentrations ([MeHg]) have been examined in littoral and offshore sediments of Lake Lusignan (Québec). The highest [MeHg] were generally found at the sediment–water interface with exceptionally high concentrations at littoral sites (5.8 ± 1.3 ng·g dry weight–1), which were four times more elevated than at offshore sites (1.6 ± 0.77 ng·g dry weight–1). Source-specific fatty acid (FA) biomarkers identified that littoral sediments contained more than twice as much terrestrial and bacterial OM compounds than offshore sediments, whereas the amount of labile algal OM was three times higher at littoral sites. Results indicate that [MeHg] were higher in the presence of labile OM substrates, and the amount of terrestrial OM compounds could not predict [MeHg]. Correlations between [MeHg] and FA of Desulfovibrio desulfuricans (a sulfate-reducing bacterium producing MeHg) could significantly account for 36% of [MeHg] at offshore sites; however, no significant relationships were found at littoral sites. This study illustrates that the microbial dynamics involved in producing and degrading MeHg in lacustrine sediments are complex and cannot be predicted solely by the quantification of FA biomarkers in D. desulfuricans or by biomarkers in the OM itself. [ABSTRACT FROM AUTHOR]