Diatom analysis and the acidification of lakes

Diatom microfossils preserved in lake sediments can be used to provide evidence for lake acidification. Unlike documentary methods of historical reconstruction, the sediment record is potentially useful at all sites, it is usually continuous and it can be extended backwards in time as desired. Cores can be taken with little disturbance, by freezing in situ if necessary, and sediments deposited since about 1800 A.D. can be dated using $^{210}$ Pb analysis. The strong relationship between diatom occurrence and water pH allows pH to be reconstructed with a standard error of between ca. 0.25 and 0.5 pH unit using either an index system or multiple regression analysis. Although there is evidence that long term acidification is a natural process for lakes in areas of resistant base-poor bedrock, diatom analyses from NW European and North American sites shows that rapid acidification has taken place within the last 150 years. The first major sign of acidification is the loss of planktonic diatoms at pH values between about 5.5 and 5.8. Acidification below pH 5.5 leads to the decline of species typical of circumneutral water, such as Achnanthes microcephala Kutz. Cymbella gracilis (Rabh). Cleve and Anomoeoneis vitrea (Grun). Ross, and the expansion of acidophilous taxa such as Tabellaria flocculosa (Roth.) Kutz. and Eunotia veneris (Kutz.) O. Muller. Acidobiontic species become common at pH values less than 5.5 and as the pH drops to 4.5 these taxa begin to replace acidophilous taxa in the assemblage. Tabellaria binalis (Ehr.) Grun. is probably the most faithful member of this group. Diatom analysis for four acid lakes in Galloway, SW Scotland show that the beginning of acidification has varied from 1840 (Loch Enoch) to 1925 (Loch Grannoch) and that pH has declined by between ca. 0.5 units (Loch Dee) and ca. 1.2 pH units (Loch Grannoch) in these lakes. Since lakes without afforested catchments have been acidified and lakes with afforested catchments were acidified before afforestation it can be concluded that afforestation is not responsible for acidification in this region.

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