THE COUPLING OF MERCURY AND ORGANIC-MATTER IN THE BIOGEOCHEMICAL CYCLE - TOWARDS A MECHANISTIC MODEL FOR THE BOREAL FOREST ZONE

In boreal forest lakes, high Hg concentrations in fish are common, even in remote areas. In this paper, the effects of atmospheric Hg pollution in Sweden are synthesized and related to a concept based on the strong interaction of Hg with biogenic matter (Hg/B). Based on this concept, a compartment model is developed to predict concentrations, pool sizes, flux rates and turnover times of Hg along the biogeochemical cycle, including atmosphere, forest soils, surface runoff, lake waters, and aquatic biota. The aim is to provide a conceptual framework, both for a comprehensive mechanistic model, and for predictions from readily available information, such as regional data on acid deposition, air temperature and surface runoff, and local data on the trophic status of lakes with respect to humus and nutrient concentrations. The model is in good agreement with observations from recent Swedish field studies in all compartments. It suggests a strong influence of climate on the susceptibility of soil and lake ecosystems in the boreal region to Hg contamination. The high Hg concentrations in fish from forest lakes can be largely attributed to the low productivity of both terrestrial and aquatic biota. The impact of historical point sources of Hg is considered, as well as the slow turnover of Hg in forest soils, both resulting in elevated fish Hg levels in humic lakes for centuries following atmospheric deposition.