Atmospheric acidification history of Loch Laidon: a comparison of pollution records from 1985 and 1995 sediment cores

Like many oligotrophic softwater lochs in Scotland, Loch Laidon has been acidified by atmospheric pollution (Flower et al. 1988). The process started in the 19th century when industrial emissions began to contribute acidity (as sulphur and nitrogen oxides), heavy metals, fly ash and other trace contaminants in significant quantities to the atmosphere. Acidification of many sensitive freshwater lochs typically proceeded throughout the 20th century ( e.g. Battarbee et al. 1988a). However, as a result of industrial decline and implementation of some source controls, UK total acid emissions declined by about a third between the mid- l 970s and the mid-1980s (Irwin et al. 1990). This change has been linked to small decreases in the measured acidities of some Scottish lochs during the late l 970s and 1980s (Harriman et al. 1996). Biological improvements over this period are less clear, although diatom communities in two upland lochs in Galloway (Battarbee et al. 1988b, Allott et al. 1992) do show minor floristic recoveries since ca. 1980. The time-scale and extent of acidification of Loch Laidon was revealed by microfossil and geochemical analysis of a radiometrically dated sediment core retrieved from the loch in 1985 (Flower et al. 1988). The most recent sediment in this core showed no evidence of any significant recovery trend in loch water acidity. Similarly, sediment contamination by spheroidal carbonaceous particles (SCPs), a fly ash component of high temperature fossil fuel burning, showed an increasing trend to the sediment surface (1985). To test if any biologically significant changes or any decline in atmospheric contamination (as defined by SCP analysis) of the loch had occurred in the 10 years subsequent to the 1985 palaeo-survey, re-sampling was carried out in 1995. This re-survey entailed collecting a sediment core from approximately the same location as the 1985 core and analysing it in similar ways. Changing pollutant deposition patterns and possible lag-effects in biological responses to the late 1970s acid emission decline, makes examination of the sediment record of the intervening 10 years of particular interest. This report presents results of radiometric dating, diatom analysis and SCP analysis of the 1995 sediment core. Using the time scale provided by sediment dating, diatom analysis is used to reconstruct recent trends in water pH and SCP analysis is used to identify recent trends in direct atmospheric contamination. By comparing the 1995 results with those obtained from the earlier core (Flower et al. 1988), not only is the post 1985 sediment record made available but also replicability of the methods for the pre- 1985 sediment sections is tested