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Depositional records of methylmercury (MeHg) are rare, especially for remote and mountainous areas. Our data from Lochnagar, a mountain lake in Scotland, covering a 7-year period from 2001-2008, show an unusual seasonal pattern in that elevated MeHg concentrations occur each winter while concentrations each summer fall below the limit of detection. To our knowledge this is the first time this seasonal pattern has been reported. Peak concentrations at the site in October 2006 (1.2 ng L(-1)) are amongst the highest reported depositional values in the literature. As the soils and lakes in the region are frozen or snow covered for much of each winter, we consider possible sources of this winter deposited MeHg to be either aqueous phase methylation in the atmosphere or marine evasion. However, the factor driving this seasonal pattern is likely to be scavenging by snow, as elevated concentrations in deposition coincide with periods of snowfall at the site. If this mechanism is correct, then predicted impacts of climate change, which will reduce annual snowfall at the site by between 50% and 100% by 2080, will effectively eliminate this input source. However, other climate-influenced inputs of mercury, such as remobilisation from catchment soils, are likely to increase and negate any benefit., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

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.

Anthropogenic trace metals enter the entire ecosystem of Lochnagar solely through atmospheric deposition. Trace metals, including Hg, have been monitored in atmospheric deposition and lake water, and measured in catchment vegetation, aquatic plants and zooplankton, revealing contamination levels in the ecosystem. Furthermore, 17 sediment cores were taken from different areas of the lake. Hg, Pb, Cd, Zn and Cu were analysed in all the cores, which show that the sediments have been heavily contaminated by these trace metals since the 1860s. The distribution of trace metals in the lake sediments was found to be heterogeneous, with concentrations in the surface sediments varying significantly: 110-250 ng/g, 100-360 microg/g, 39-180 microg/g, 0.3-1.9 microg/g and 8-25 microg/g for Hg, Pb, Zn, Cd and Cu, respectively. Trends in the concentration profiles for different trace metals in the same core are different, as are the trends of the profiles for the same metal in different cores. Hence, a single sediment core cannot represent the pollution history of the whole lake. As the soils and sediments contain a high proportion of plant debris and the debris has a high affinity for Hg, resulting in Hg enrichment. Hg was measured in plant debris (> 63 microm) separated from catchment soils and lake sediments. Plant debris may play an important role in storing and transferring Hg in this ecosystem.