Interaction of Mercury and Organic Matter in the Yukon River Basin

The Yukon River Basin, the fourth largest drainage basin in North America, provides a unique opportunity to study the interaction of mercury (Hg) and organic matter (OM) at a large scale in a relatively undisturbed system. The U.S. Geological Survey recently completed an extensive five-year water-quality study of the Yukon River from its headwaters in Canada to Pilot Station, Alaska near its mouth. One aspect of the project was to establish a water quality baseline to assess possible future changes from a warming climate. Two of the key water-quality components analyzed were Hg and OM in surface water. It is well known that Hg and OM have strong interactions in aquatic ecosystems. However, the vast majority of what is known is derived from relatively small streams, lakes and reservoirs at lower latitudes. The high latitude and vast scale (longitude and flow rate) of the Yukon River, its naturally high suspended sediment loads, and complex geologic terrain make it a unique setting to examine whether Hg-OM interactions hold in these large-scale northern ecosystems. For water years 2001 through 2004, dissolved organic carbon concentrations in the Yukon River mainstem and two major tributaries ranged from 1 to 15 mg/L and particulate carbon ranged from 1 to 34 mg/L. Likewise, dissolved total Hg ranged from 0.1 to 4.3 ng/L, and total particulate Hg ranged from 0.1 to 75 ng/L. These high Hg concentrations in such a remote location are suggestive of contributions from geologically enriched materials. For both Hg and OM, the highest concentrations were observed during periods of high flow, and both were dominantly in the particulate state. The data show a strong positive correlation among dissolved and particulate mercury concentrations and their respective dissolved and particulate carbon concentrations. This research suggests that Hg-OM interactions described for numerous smaller northern catchments can be scaled up to large undisturbed basins in northern temperate and subarctic climates. Verification of Hg-OM interactions in large-scale northern ecosystems may prove to be valuable given the vast reservoirs of OM that exist in arctic regions. Large expanses of permafrost are melting in the arctic and subarctic regions, the soil active layer is deepening, upland soils are drying, the growing season is lengthening, and, as a result, fire frequency is increasing. Along with these changes, as permafrost melts, the frozen soil is transformed into biogeochemically active zones. The changes in climate affecting the overall transport and reactivity of this carbon pool could have a large impact on the long term fate of the associated OM-bound Hg.