Mercury transfer from watersheds to aquatic environments following the erosion of agrarian soils: A molecular biomarker approach

Lake St. Pierre, an important freshwater location for sports and commercial fisheries in Canada, is composed of a 120 km2 stretch of the St. Lawrence River, located at the center of the St. Lawrence Lowlands. Receiving its waters from the St. François, 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. François 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