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Palaeolimnological data from six mesotrophic, eutrophic and hypertrophic lakes in the Irish Ecoregion, in the form of microfossil (cladocera, diatoms and pollen) and sediment chemistry data from radiometrically dated sediment cores, were used to reconstruct past variations in lake water quality and catchment conditions. Basal sediments from sediment cores from the six sites ranged in age from the late 18th century to the early 20th century. A weighted averaging partial least squares regression model was developed to reconstruct past epilimnetic total phosphorus concentrations. The results indicate that all but one of the study sites currently are in a far more productive state compared with the beginning of the sediment core record and that those same five lakes have experienced accelerated enrichment post c. 1980. Two of the sites demonstrated long-term enrichment, in one case beginning in the late 19th century, while both eutrophication and oligotrophication have occurred at three sites. The results highlight the difficulties in applying a general temporal end-point for reference conditions and demonstrate that productive lakes in the Irish Ecoregion have complex, locally specific and often long histories of enrichment. These may not be responsive to reduced external loadings of phosphorus and, as a result, restoration could prove particularly challenging. The results also provide evidence of the ways in which palaeolimnological techniques can assist implementation of the EU Water Framework Directive.

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