Abstract: In terms of principle components analysis (PCA), detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA), a diatom-pH transfer function could be reconstructed by using the modern diatom data and pH from 34 lakes in Wales. In light of root mean square error (RMSE) and residuals results of the experiments, x-validated weighted averaging with classical deshrinking (WA_Cla_X) and x-validated weighted averaging partial least squares component 2 (WAPLS_C2_X) were selected for pH reconstruction with WA with classical deshrinking (WA_Cla) as comparison. According to these diatom-pH transfer functions, the fossil diatom in the sediment were analysed to reconstruct the pH value in Llyn Hir (lake) from 1956 to 1995. It showed that improvement of pH in 1985 was the result of liming by the history record and there was an obvious effect in the following six years but the effect weakened gradually until vanished. Although liming can increase pH in a relatively short period, it cannot eradicate lake acidification.

Key words: diatom assemblage, build the transfer function, reconstruct the past pH change, Llyn Hir