Interaction of Mercury and Organic Matter in the Yukon River Basin
By Paul F Schuster,
George R Aiken, Michael M Reddy, David P Krabbenhoft, Robert G Striegl, John F DeWild, Mark L Olson, and Shane D Olund
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 that
project was to establish a water quality baseline to assess possible future
changes from a warming climate. Two of
the key water-quality components we 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 (80% for Hg, and 50-80% for OM). The data show a strong positive correlation among
dissolved (R2=0.56, p=0.001) and particulate
mercury concentrations (R2=0.57, p=0.01) 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.
This
abstract will be presented at the Mercury Conference, Madison, WI,
August 6-11, 2006.