Mercury in Natural Lakes and Unperturbed Terrestrial Ecosystems of Northern Québec

Abstract

Mercury (Hg) in natural lakes and unperturbed terrestrial systems is ubiquitous throughout northern Quebec, at sites situated 200 to 1 400 km away from the closest industrial centers. The diagenetic stability of Hg in recent lacustrine sediments makes it possible to give historic interpretations of the deposition of this heavy metal in remote regions. Accelerated anthropogenic activities over the last century are responsible for the 2- to 3-fold increase in the atmospheric Hg deposition rate in all lakes studied between latitudes 45°N and 56°N in northern Quebec. Hg concentrations in lake sediments are proportional to the amounts of terrestrial organic carbon brought from the catchment area. Once incorporated into the organic horizons of the soils of the boreal forest, the airborne Hg efficiently binds itself to humic matter. Natural Hg has been accumulating in these horizons since the beginning of soil formation at the end of the ice age. The recent additional inputs of anthropogenic Hg to these soils mixes with the existing natural Hg pool and represents only a small fraction of the total Hg burden. The cumulative burden of Hg in the humic horizon of podzolic soils averages 2 mg m-2, with less than 25% of it being attributable to anthropogenic Hg deposition, while peatlands accumulate 4 to 5 times more Hg in their thick peat layer. In boreal forests a substantial fraction of newly deposited atmospheric Hg is readily transported to lacustrine systems by snow melt in the spring or by surficial (i.e., not fully penetrating the humic horizons) runoff during summer storms. This explains the paradox between the 2- to 3-fold increase in Hg deposition rates in recent sediments, with the Hg concentration present in lake sediments being of terrestrial origin in spite of the small fraction of anthropogenic Hg in soils. Using a combined set of environmental indicators, a fairly uniform pattern of Hg atmospheric deposition, with no detectable gradient decreasing away from the Great Lakes industrial belt, can be drawn for the entire domain of the Quebec boreal forest south of latitude 54°N. These include Hg concentrations in snow, in yearly shoots of spruce or in epiphytic lichens, Hg burdens normalized to carbon in humic layers of podzols, and normalized sedimentary Hg enrichment since the onset of the industrial era. Particulate matter at the bottom of a lake or in suspension in the water column plays a key role in the transfer of Hg to the base of the aquatic food chain. It has not yet been quantified to which extent the substantial increase of anthropogenic Hg in lake sediments relates to an equivalent increase in Hg bioaccumulation in aquatic organisms.