Dissolved organic carbon (DOC) is known to affect the Hg cycle in aquatic environments due to its overriding influence on complexation, photochemical, and microbial processes, but its role as a mediating factor in the bioaccumulation of Hg in aquatic biota has remained enigmatic. Here, we examined 26 tundra lakes in Canada's western Arctic that span a large gradient of DOC concentrations to show that total Hg (HgT) and methyl mercury (MeHg) accumulation by aquatic invertebrates is defined by a threshold response to Hg-DOC binding. Our results showed that DOC promotes HgT and MeHg bioaccumulation in tundra lakes having low DOC (<8.6 - 8.8 mg C L(-1); DOC threshold concentration, TC) whereas DOC inhibits HgT and MeHg bioaccumulation in lakes having high DOC (>DOC TC), consistent with bioaccumulation results in a companion paper (this issue) using a microbial bioreporter. Chemical equilibrium modeling showed that Hg bioaccumulation factors were elevated when Hg was associated mainly to fulvic acids, but became dramatically reduced when DOC was >8.5 mg C L(-1), at which point Hg was associated primarily with strong binding sites on larger, less bioaccessible humic acids. This study demonstrates that the biological uptake of Hg in lakes is determined by binding thresholds on DOC, a water quality variable predicted to change markedly with future environmental change.

Gold mines in the Yellowknife, NT, region—in particular, the Giant Mine—operated from 1949–99, releasing 237,000 tonnes of waste arsenic trioxide (As2O3) dust, among other compounds, from gold ore extraction and roasting processes. For the first time, we show the geospatial distribution of roaster-derived emissions of several chemical species beyond the mine property on otherwise undisturbed taiga shield lakes within a 25 km radius of the mine, 11 years after its closing. Additionally, we demonstrate that underlying bedrock is not a significant source for the elevated concentrations in overlying surface waters. Aquatic arsenic (As) concentrations are well above guidelines for drinking water (10 μg/L) and protection for aquatic life (5 μg/L), ranging up to 136 μg/L in lakes within 4 km from the mine, to 2.0 μg/L in lakes 24 km away. High conversion ratios of methyl mercury were shown in lakes near the roaster stack as well, with MeHg concentrations reaching 44% of total mercury. The risk of elevated exposures by these metals is significant, as many lakes used for recreation and fishing near the City of Yellowknife are within this radius of elevated As and methyl Hg concentrations.

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This open file reports on recent geoscience data collected and monitoring sites installed by the Geological Survey of Canada (GSC) in collaboration with Aboriginal Affairs and Northern Development Canada (AANDC), Northwest Territories Geoscience Office (NTGO), Government of the Northwest Territories (GNWT) Environment and Natural Resources (ENR) and the Department of Transport (DOT), BGC Engineering Inc., Carleton University, and the University of Ottawa. The report represents the first of several co-published GSC / NTGO Open File reports under the Climate Change Geoscience Program. A 2010-11 field program under the Transportation Risk in the Arctic to Climatic Sensitivity (TRACS) activity in the Climate Change Geoscience Program included extensive fieldwork in the Yellowknife area between June and September, 2010. Field data collection by the Geological Survey of Canada and Carleton University included 14 CRREL-cored boreholes and ecological descriptions at 48 sites. An additional 20 ecological site descriptions were made by University of Ottawa students. Collaborative work with BGC Engineering Inc. included field observations and soil analysis (grain size and Atterberg limits) at test pits along Highway 3, and 15 water-jet drilled holes for subsequent temperature cable installations. Numerous temperature measurement sites were also established with AANDC along a 170 km transect between Behchoko and Tibbitt Lake including six air temperature sites, six multi-channel near-surface temperature sites and seven water temperature sites. In March 2011, additional field data were collected including snow depths at 45 sites, densities at 18 sites, and ice thicknesses at eight pond sites. Snow depth transect surveys were also conducted at 11 sites along Highway 3, across the highway embankments and right-of-ways. These data were collected in order to provide baseline information regarding the nature and properties of permafrost in the Yellowknife area.