The Recent Acidification of a Large Scottish Loch Located Partly within a National Nature Reserve and Site of Special Scientific Interest

(1) The recent acidification history of Loch Laidon is reconstructed from diatom analysis of a210Pb-dated sediment core and the effect of atmospheric pollution is assessed from carbonaceous particle ('soot') and trace metal concentrations in the sediment. (2) Diatom analysis revealed marked changes in species composition, with the planktonic diatom Cyclotella kuetzingiana common until the mid-nineteenth century and Brachysira vitrea declining in the 1940s and being replaced by acidophilous species, notably Tabellaria flocculosa and Eunotia veneris. (3) Lead, zinc and 'soot' concentrations were highest in sediment deposited during this century, with the trace metals increasing from c. 1850 AD and 'soot' from c. 1930. (4) The results show that Loch Laidon has become more acid and has been affected by lead and zinc contamination since the mid-nineteenth century. Soot contamination of the sediment probably reflects the national increase in oil combustion since the 1930s.