The reversibility of lake acidification: A diatom study from the Round Loch of Glenhead, Galloway, Scotlan

The lake-water pH of the Round Loch of Glenhead has risen by approximately 0.2 pH units over the last decade, probably as a result of a recent decline in acid deposition. This study is concerned with the response of the diatom flora to this short-term low-magnitude pH increase, and the ability of the diatom biostratigraphic record to reflect this response. Evidence of live diatom response to the lake-water pH increase is ambiguous. A comparison of epilithic diatom samples from 1983 and 1989/90 indicates significant differences between the two periods, but these are difficult to relate directly to the pH increase. A survey of the surface sediment diatom assemblages reveals the presence of non-contemporary sediments over extensive areas of the loch bed. These sediments contain diatom taxa which are no longer an important part of the diatom flora. Recent trends in diatom biostratigraphy are evaluated using two methods; core replication and the fine-slicing of cores. The results reveal consistent trends in biostratigraphy in the uppermost sediments indicative of a recent reversal of lake acidification. The biostratigraphy indicates that there was little delay in the response of the diatom flora to the increase in lake-water pH. The reversal can be replicated in cores sliced at 5 mm intervals with accumulation rates over 0.7 mm a-1, and it is clearly resolved in the finely-sliced cores. The ability to detect the reversal is dependent on the time-average of core slices, the reversal being apparent when this time-average is less than approximately six years. Data from both sediment cores and sediment traps indicate that sediment mixing has led to some attenuation of the biostratigraphic record. Attenuation is not caused by the re-working of non-contemporary taxa. The recent reversal is a real floristic signal, not the product of variability, and represents a reestablishment of past-communities. Lake sediment diatom records can be used with confidence to reconstruct and monitor diatom responses to low-magnitude changes in lake-water pH. The study also demonstrates the utility of sediment traps for monitoring low-magnitude changes in the diatom flora of a lake, such as those associated with reversibility.