The diatom record preserved in a sediment core from Llangorse Lake in South Wales, a site of national and international conservation importance, was analysed to evaluate the post-1850 nutrient history of the lake. Fossil diatom assemblages were analysed in 13 samples of a dated sediment core from the easterly lake basin.
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A diatom-based transfer function was applied to the fossil diatom data to generate a quantitative reconstruction of in-lake total phosphorus (TP) concentrations, following taxonomic harmonization between the training set and core species data. The TP reconstruction was calculated using a north-west European calibration set of 152 lakes.
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The diatom assemblages were dominated by non-planktonic taxa, in particular Fragilaria spp. until ca 1950, since which time there has been a switch to domination by planktonic diatoms. This switch is most likely a response to reduced light conditions and the decline in submerged macrophytes in the lake, brought about by nutrient enrichment perhaps coupled with enhanced inwash of soils, and thus a loss of habitats for epiphytic and benthic taxa. The exact causes of enrichment remain unclear and are currently under investigation, although increased use of fertilizers, agricultural intensification, afforestation and development of tourism and recreation have all been suggested as possible factors. The recent (post-1985) diatom assemblages are associated with mesotrophic conditions and indicate some degree of lake recovery.
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Application of the transfer function showed that Llangorse Lake has experienced a decline in TP concentrations since the mid-1980s, following at least a century of high and stable TP levels of ca 150 mg TP L -1 . The timing of this decrease appears to be related to the sewage diversion in 1981 and a further smaller diversion in 1992. Future monitoring of the lake is recommended in order to observe whether the improvement in water quality is maintained or indeed enhanced.