MATCHING DIATOM ASSEMBLAGES IN TAKE SEDIMENT CORES AND MODERN SURFACESEDIMENT SAMPLES - THE IMPLICATIONS FOR LAKE CONSERVATION AND RESTORATION WITH SPECIAL REFERENCE TO ACIDIFIED SYSTEMS

Restoration goals for damaged freshwater habitats can be defined accor ding to ecological as well as to chemical criteria. For disturbed lake s, the sediment microfossil record can be used to select potential mod ern analogue sites as possible restoration target ecosystems. Fossil d iatom assemblages in two acidified lakes (Round Loch of Glenhead and L och Dee) in Galloway, Scotland, were compared floristically with moder n surface sediment samples from ca. 200 lakes in Britain, Ireland, Swe den and Norway using numerical techniques. Mean squared Chi-squared di ssimilarity (SCD) values based on between sample Chi-square distance m easures were used to compare samples. 'Space-for-time substitution' us ing diatom assemblage matching techniques identified several modern an alogue sites with Hebridean Loch Teanga and Irish Lough Claggan posses sing modern diatom floras most similar to those which existed at the R ound Loch of Glenhead and Loch Dee before acidification. From the poin t of view of atmospheric pollution, the most closely matching modern a nalogue sites were not necessarily in the most pristine regions. Some analogues occurred in UK regions of moderate or low acid deposition an d modern diatom assemblages in atmospherically cleaner mid Norway were generally less similar floristically. It is argued that identificatio n of modern analogue sites raises the possibility of using time-space substitution of closely matched modern and fossil samples to infer who le lake ecosystems. Diatoms are however poor indicators of some water chemistry variables and the two closest matched modern analogue sites have too high calcium concentrations making faunistic comparisons ques tionable. Identification of good modern analogue lakes can be improved by using selection criteria, other than diatoms, to pre-select sites. Screening inappropriate sites according to water chemistry and basin features combined with a larger biological database of modern and foss il samples offers a promising way of refining the selection processes. Despite necessary refinements, modern analogue matching can potential ly identify whole lake ecosystems that can serve as biological target communities for currently disturbed sites. Being based on biological r ather than chemical criteria, this approach does not rely on species-w ater chemistry transfer functions. It is therefore directly relevant t o lake conservation and restoration objectives and offers an alternati ve method for reconstructing lake palaeo-environments.