CYCLING OF METHYL MERCURY AND MERCURY - RESPONSES IN THE FOREST ROOF CATCHMENT TO 3 YEARS OF DECREASED ATMOSPHERIC DEPOSITION

Studies of the biogeochemistry of total mercury (Hg) and methyl mercur y (MeHg) in the Lake Gardsjon watershed have shown that the atmosphere is the most important source of Hg and MeHg in the ecosystem. Soils a re accumulating most of the deposited Hg and MeHg, but transport of Hg and MeHg from the forested catchments into the lake ecosystems is eno ugh to explain elevated concentrations of MeHg in fish in more than 10 000 Swedish lakes. An experimental roof was constructed to study effe cts of decreased atmospheric input on an entire forested catchment. Th e experiment started in April 1991, and decreases in the output of bot h MeHg and Hg occurred during 1991, 1992 and 1993. Runoff fluxes from the control catchment during the pre-treatment period were related to the experimental catchment using regression analyses. Since April 1991 , after three year experiment, predicted compared to measured fluxes s howed that Hg output decreased by 32% and MeHg by 28%. The decrease in Hg was most obvious during high water flows in winter/spring while Me Hg decreased during all seasons of the year. The decreased input of Hg and MeHg to the Forest Roof Catchment is the most probable explanatio n to the rapid decrease in output of Hg and MeHg by runoff from the ca tchment basin.