Caron, S., Lucotte, M. and Teisserenc, R. 2008. Mercury transfer from watersheds to aquatic environments following the erosion of agrarian soils: A molecular biomarker approach. Can. J. Soil Sci. 88: 801Á811. Lake St. Pierre, an important freshwater location for sports and commercial fisheries in Canada, is composed of a 120 km 2 stretch of the St. Lawrence River, located at the center of the St. Lawrence Lowlands. Receiving its waters from the St. Franc¸ois, Yamaska, Ottawa, and St. Lawrence Rivers, it is subjected to important inputs of mercury (Hg) and suspended particles eroded from its watershed. This study aims at tracing back the origin of terrigenous Hg loadings to Lake St. Pierre. The specific phenol signatures yielded by a mild oxidation of the terrestrial organic matter (TOM) carried in the water column was used as a tracer to identify the different sources of terrigenous Hg to the lake. Our results demonstrate that most of the Hg bound to suspended particulate matter (SPM-bound Hg) found in Lake St. Pierre is associated with TOM. We were also able to distinguish the relative influence that forested soils, mainly drained by the Ottawa River, and agrarian soils, located on nearby watersheds, exert on the lake's Hg burden. Our data strongly suggest that the erosion of vast areas of agrarian soils, drained by the Yamaska and St. Franc¸ois rivers to Lake St. Pierre, greatly facilitates the transfer of Hg from the watersheds to the lake. This study stresses the need to improve the management of agrarian soils and protect them from extensive erosion in order to preserve the integrity of the fish resources harvested in Lake St. Pierre.Key words: Mercury, soil erosion, Lake St. Pierre, lignin biomarkers Caron, S., Lucotte, M. et Teisserenc, R. 2008. Transfert du mercure des bassins versants vers les milieux aquatiques suite al 'e´rosion de sols agricoles: une approche par biomarqueurs mole´culaires. Can. J. Soil Sci. 88: 801Á811. Le lac Saint-Pierre, un site important pour la peˆche commerciale et sportive au Canada, est un e´largissement du fleuve Saint-Laurent de 120 kilome`tres carre´s au coeur des basses-terres du Saint-Laurent. Outre le fleuve, le lac Saint-Pierre est alimente´par les rivie`res Saint-Franc¸ois, Yamaska et des Outaouais. Il rec¸oit une quantite´importante de mercure (Hg) et de particules en suspension provenant de l'e´rosion de son bassin versant. L'objectif de cette e´tude est de retracer l'origine des apports de Hg terrige`ne dans le lac. Les signatures phe´noliques spe´cifiques de la matie`re organique terrige`ne (MOT), obtenues apre`s une oxydation alcaline par l'oxyde de cuivre, ont e´te´utilise´es comme traceurs des diffe´rentes sources de Hg terrige`ne retrouve´es dans la colonne d'eau. Nos re´sultats de´montrent que la plus grande partie du Hg lie´e a`la matie`re particulaire en suspension (MPS) retrouve´e dans le lac Saint-Pierre est associe´e a`la MOT. Nous avons e´galement mis en e´vidence l'influence relative que les sols forestiers, essentiellement draine´s par la rivie`re de...

Lake St. Pierre (LSP), constituted of a 120 km 2 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 resuspension 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.