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A key concern in the assessment of the recovery of acidified lakes from the effect of acid deposition is the reference pH of acidified lakes and how reference values can be used to establish targets for restoration. In this paper we evaluated the accuracy of three different, although overlapping, diatom-pH transfer functions using UK Acid Waters Monitoring Network (AWMN) data to compare inferred pH from diatoms collected in annually exposed sediment traps with measured water column pH for 11 acid-sensitive lakes. We then used these transfer functions to infer lake-water pH from approximately AD 1850 (taken as the reference date) to AD 1990 (the core-top date) for radiometrically dated sediment cores from each site. When applied to the sedimenttrap samples all three models tended to underpredict measured pH in the less acidic sites, with the SWAP model being the most biased. All three models were in good agreement in reconstructing pH for the AD 1850 reference period. Reference pH varied between sites from approximately 4.9 to 6.4, and was correlated with modern basecation status. In the acidification phases of the cores discrepancies in pH inference between models appear to be linked to differences in pH optima for a minority of taxa. Current (2006) diatom-inferred pH values from the most recent sediment traps, when compared with sediment core tops, provide evidence of recovery at most sites but its extent, so far, is slight when compared with the reference period. The substantial difference between sites in their reference conditions indicates clearly that recovery success can not be measured against a single target pH or, by inference, ANC value and must be assessed on a site-specific basis. [ABSTRACT FROM AUTHOR]

Abstract: We assess the recovery of UK lakes from acidification using the combined data from sediment cores and sediment traps to track changes in diatom assemblages in 11 UK upland lakes from pre-acidification times (prior to ca. 1850 AD) to the present (2008 AD). We projected the data into a Principal Component Analysis (PCA) of diatom assemblage data from 121 low-alkalinity lakes in the UK to show how the floristic composition of the core and trap diatom assemblages for each site has changed through time. The results show that the degree of recovery from acidification varies amongst sites but in all cases its extent is limited when compared with the pre-acidification reference. In most cases the recovery, although usually slight, is characterised by a decline in acid tolerant taxa and a return towards taxa that occurred previously at each respective site. In a few cases, however, the floristic composition of recent samples is different from those that occurred during and before the acidification phase. The reasons for this are not yet clear but it is possible that nutrient enrichment from atmospheric N deposition and/or climate change is beginning to play a role in driving water quality as acidity decreases. More generally the results show that annually recovered samples from sediment traps can be successfully combined with sediment core data to provide a continuous record of environmental change in lake systems, and that diatoms collected in sediment traps can be used to provide a very powerful lake monitoring tool. [Copyright &y& Elsevier]