Concentrations and Yields of Total Hg and MeHg in Large Boreal Rivers Linked to Water and Wetland Coverage in the Watersheds

Large rivers are major contributors of mercury (Hg) exports to the ocean, as they integrate processes of loading and loss occurring at the watershed level. Within a watershed, stream-scale studies have revealed that specific landscape features, such as wetlands or lakes, are hotspots for Hg and methylmercury (MeHg) loading, sinks and transformation, but we still do not know how these landscape features operate at the whole network scale and over large geographic gradients. In this study, we evaluate how landscape metrics (vegetation types, wetland and lake cover, climate, hydrology) are related to riverine concentrations and watershed yields of Hg and MeHg in 18 large boreal rivers draining watersheds that range from 44 km2 to 209 453 km2, distributed along a 650 km latitudinal transect in the James Bay region of Québec. Our results reinforce the role of wetlands as sources of MeHg, but further show that surface coverage of water in the watershed is the major driver of both Hg and MeHg concentrations and exports to the coast at the whole network scale. Our findings also demonstrate that seasonality modulates the relationship between landscape features and the various Hg forms. Based on hydrometric data, we additionally estimate annual exports for the whole Eastern James Bay to 441 kg Hg and 15 kg MeHg, for an average landscape yield of 1.24 g Hg km-2 y-1 and 0.041 g MeHg km-2 y-1. Our study provides tools to broadly predict riverine Hg concentrations and exports with only a few easily accessible landscape metrics.