THE REPRESENTATION OF DIATOM COMMUNITIES BY FOSSIL ASSEMBLAGES IN A SMALL ACID LAKE

The representative quality of fossil diatom assemblages in the recent sediment of a lake is compared with its contemporary diatom flora. In April 1986 experimental liming of the catchment of a small acidified l ake, Loch Fleet (Galloway, U.K.), produced immediate changes in water quality. Lakewater pH rose from a mean of approximately 4.5 to 6.5, an d in the two year period following liming a consistently higher pH was maintained. The marked response of diatom species to changing water q uality provided a means of tracing events from living communities to t he fossil assemblages. Diatom periphyton and plankton were sampled dur ing a 20 month period and archived material was used to characterise e arlier diatom communities. A comparison is made between living diatom communities and diatom assemblages collected by sediment traps and fro m sediment cores taken during the same period. Following liming, the d iatom communities were found to respond within days or weeks to the ch anges in water quality. There is an initial change from acidobiontic c ommunities, dominated by Tabellaria quadriseptata, to dominance by the acidophilous species Eunotia incisa and Peronia fibula. However, in t he epipsammic community the acidobiontic species Tabellaria binalis fo , elliptica remains abundant after liming. Approximately one year afte r liming the abundances of species such as Achnanthes minutissima and Brachysira vitrea increase in the epilithon, epiphyton and epibryon, w hilst in the epipsammon T. binalis fo. elliptica is replaced by small Eunotia spp. and Achnanthes altaica. During the latter part of 1987 an d in 1988, despite a stable pH, fluctuating patterns of species abunda nces are seen in the epilithon, epiphyton and epibryon whilst the spec ies composition of the epipsammon remains relatively stable. Spring bl ooms of the planktonic species Synedra acus and Asterionella formosa o ccur during 1988 and 1989 respectively. Sediment trapping, which began in April 1987, records shifts in species composition corresponding wi th those seen in the epilithon, epiphyton and epibryon and with the bl ooms of planktonic species. The signal from the smaller, and probably less easily transportable, epipsammic community is not so clearly disc ernible. Although the fundamental record of the sediment traps is one from living diatom communities, the appearance of taxa 'extinct' durin g the post-liming period reflects a low, but significant level of sedi ment resuspension. In contrast to the rapid response of living communi ties and their record in sediment traps, sediment cores do not begin t o reflect changes in diatom composition until about 14 months after th e initial liming. The first appearance of circumneutral taxa in signif icant abundance occurs only approximately 17 months after liming. The delayed reaction of sediment assemblages cannot be attributed principa lly to a slow rate of transport from the littoral to the profundal zon e. Time-averaging processes within the sediment appear to be the main cause of the lag in core response. In contrast, blooms of planktonic s pecies are quickly reflected in the stratigraphy of cores, but indicat e that a considerable degree of downward mixing occurs. Comparison of the time trajectories of whole species assemblages in living communiti es, sediment traps and core surface sediments shows that the direction of change is similar in all three, but that the magnitude of change i s attenuated in sediment assemblages.