Mercury and lead budgets for Lochnagar, a Scottish mountain lake and its catchment

Lochnagar is a mountain lake located to the southeast of the Cairngorm Mountains in Scotland. The inputs and outputs of Hg and Pb and their distribution within the various ecosystem compartments were measured. Further, 17 sediment cores and 10 catchment peat cores were taken and dated using spheroidal carbonaceous particle (SCP) and 210Pb techniques. Total and anthropogenic Hg and Pb inventories since the 1860s for the lake basin and the catchment peats were calculated using this multiple core strategy. Hg sediment flux profiles based on the whole lake basin show that the flux to the sediments increased from the 1880s until the 1970s. This was followed by a relatively stable period (1970s to the present), during which the flux was approximately twice that of the 1880s. Similarly, the Pb flux increased from the 1860s until the 1940s and was also followed by a relatively stable period through to the present. Hg and Pb budgets for the whole catchment for 1998 indicated that 78% of the Hg and 91% of the Pb input to the lake were transported from the catchment. Hence, the expected decline resulting from the decrease in the atmospheric deposition of Pb was obscured in the sediment record. It is estimated that 77% of the total Hg and 90% of the total Pb deposited since the 1860s, and stored in the upper layers of the catchment peat soils, are from anthropogenic sources. The increased storage of Hg and Pb in the catchment implies that this will be a major source of these metals for the lake for many years. This will delay the restoration of the lake system, despite reductions in emissions to the atmosphere and subsequent deposition.