The Anglesey lakes, Wales, UK — Changes in trophic status of three standing waters as inferred from diatom transfer functions and their implications for conservation

1. A diatom-phosphorus (P) transfer function was applied to fossil diatom assemblages in sediment cores from three Anglesey lakes of conservation importance, in order to reconstruct quantitatively the recent history of total P (TP) concentrations for each lake. The results indicate that all three lakes have been eutrophic throughout the period represented by the cores.  2. The model suggests that Llyn Coron and Llyn Dinam experienced gradual increases in TP concentration (core base to 20 cm), followed by marked, rapid increases from the time represented by a sediment depth of 20 cm (early 1970s for Llyn Coron), most probably associated with agricultural intensification.  3. A recent decline in TP concentration at Llyn Coron and Llyn Dinam, associated with reductions in external P loading, is indicated by the model, although current levels are still considerably higher than those estimated for the base of the cores (Dinam: current=112 μg L−1; base=65 μg L−1, and Coron: current=156 μg L−1; base=70 μg L−1).  4. No clear changes in diatom-inferred TP concentration were observed for Llyn Penrhyn over the 70 year period represented by the core. However, the reconstruction for this lake is inaccurate because: (a) the sediment record appears to be disturbed, and (b) unlike Llyn Coron and Llyn Dinam, diatom preservation and modern analogues are poor, and the present TP concentration (>1 mg L−1) is beyond the TP range of the model.  5. Continued control of the external TP loads from agricultural sources is essential to prevent further increases in lake TP concentrations at Llyn Coron and Llyn Dinam. Given the problems with the Llyn Penrhyn data, it is not possible to draw any firm conclusions regarding future site management. However, the impact of the recent installation of a phosphate stripping plant at the local sewage works should be monitored, to identify whether a reduction in external P loading results in a decrease in epilimnetic TP concentrations, and in turn whether this causes changes to the biological community structure of the lake.

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