The History of Two Linked but Contrasting Lakes in North Wales from a Study of Pollen, Diatoms and Chemistry in Sediment Cores

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Abstract
The history and evolution of 2 linked lakes, Lyn Padarn and Lyn Peris, are reconstructed from a study of the pollen, diatom and chemical stratigraphy in long sediment cores. A study of methods of matching replicate cores collected from a single sampling site was carried out to date the long cores used in these investigations. Magnetic susceptibility and 210Pb-dating were used to enable correlations to be made between the cores used in these biological studies and replicates previously dated by 14C-techniques. The possibility of using sediment-pigment content as a means of matching cores was investigated. Analyses of diatoms, pollen and chemistry were carried out on sediment cores from the 2 lakes: a core of 4.75 m length representing 6000 yr was obtained from Llyn Padarn and a core of 2.1 m representing 900 yr from Lyn Peris. A more detailed picture of the history of the lakes was obtained using multivariate analytic techniques as a means of integrating biological and chemical sequences. Ordinations generated by reciprocal averaging or principal components analysis were constructed for each set of data, and these ordinations were used to differentiate the diatom, pollen and chemical sediment stratigraphy into zones which could be correlated and compared. The principal changes in the diatom communities in Llyn Padarn probably occurred as a result of vegetational changes in the catchment area. In contrast to this, it is suggested that the diatom communities of Llyn Peris were influenced to a large extent by mining and quarrying carried out within the catchment area of the lake.