Natural and anthropogenically-induced hydrological connectivity produces methylmercury hotspots in the Hudson Bay Lowlands, Canada

English Abstract:

The Hudson Bay Lowlands is the second largest contiguous peatland complex in the world, and is a critical source of water and solutes to the James and Hudson Bays. Water quality in this extensive, low-gradient catchment is strongly influenced by the saturated organic matter that comprises ~ the top 2 m extensive surficial deposits. The fish supported by this freshwater system are critical to human residents who rely on it in whole or in part for their subsistence, however it has been known since the 1970s that fish of edible size frequently exceed consumption guidelines for tissue methylmercury (MeHg) concentrations. In the absence of point sources in this remote landscape, the sources of Hg are entirely atmospheric.

We hypothesized that the source of MeHg was in situ methylation of atmospherically-deposited inorganic Hg in the extensive peatlands; a well-documented occurrence in more southern latitudes. Extensive sampling within both bog and fen peatland types in the Attawapiskat River catchment in the central James Bay Lowlands revealed that MeHg concentrations were vanishingly low in the peatlands, despite at least superficial similarity to strong methylating environments in other geographic regions. Instead, extremely localized zones of elevated MeHg concentrations were found in near-stream pore waters and surface pools associated with surface water-ground water convergence. These hotspots appear to play an important role in the flux of MeHg to streams, and food web exposure. Similar biogeochemical hotspots for Hg methylation were produced in both an operational and an experimentally-manipulated wastewater treatment wetland, with elevated sulphate and labile carbon generated localized MeHg concentrations 3 orders of magnitude higher than background. Hydrological connectivity supporting methylation is much more localized in this vast landscape than previously thought. Knowledge of relatively fine-scale hydrological processes is required to predict biogeochemical hot spots and understand the implications of future changes in hydrology due to land-use or climate change.