Palaeolimnological Evidence for the Acidification and Contamination of Lakes by Atmospheric Pollution in Western Ireland

1 Diatom analysis of Pb-210-dated sediment cores from four lakes in the west of Ireland has demonstrated that only the two upland sites are recently acidified.2 Trace metal and fossil fuel-derived carbonaceous particle analyses of each sediment core show that atmospheric contamination of the sites began in the mid-19th century.3 Atmospheric pollution indicators in the sediment record are compatible with the hypothesis that acidification of the upland sites is caused by acid deposition, Although all four lakes lie in acid sensitive granitic drainage basins, local alkalinity sources from geologic faulting and basic intrusions are, in part, thought responsible for mitigating acidification of the two lowland lakes.4 Lake acidification began as early as the 1860s but, compared with strongly acidified UK lakes, even in the most acidified Irish lake (Lough Muck) acidity increased by less than 50% of that recorded for the most heavily impacted site in SW Scotland.5 The timing of lake contamination by atmospherically derived trace metals and carbonaceous particles is generally later than for UK lakes. The extent of contamination is intermediate between strongly contaminated regions (SW Scotland) and relatively unpolluted areas (NW Scotland).6 Application of unmodified steady state prediction models of acidification to the four lakes gave inconsistent results and all were incompatible with the diatom evidence of acidification. Marked sea-salt and brown water effects at the acidified sites probably interfere with ionic equilibria and influenced water acidity in ways which invalidate some assumptions implicit in the models.7 Regionally computed and measured nonmarine sulphur deposition rates for the lake regions are conflicting and underestimate deposition at the upland sites where enhanced acid deposition must occur.8 Acidification models perform poorly when applied to remote unmonitored sites because of poor quality S deposition estimates and inadequate water chemistry data. At these sites only the palaeolimnological records can supply unequivocal evidence of atmospheric acidification and contamination as well as providing a recent history of ecological change.