The late-glacial (late-Devensian) and post-glacial (Flandrian) diatom assemblages from lochs in northern Scotland.

A core from sediments of Loch Sionascaig, north-west Scotland was originally analysed to determine the changes in the late-glacial and post-glacial diatom assemblages. These indicate a trend from diatoms of alkaline assemblages in the late- and early post-glacial to those of acid and unproductive waters. This is apparently a response to changes in soils and vegetation upon the catchment area of the lake. Three diatom assemblage zones have been demonstrated within the late-glacial and zone 2 contains an unusually high proportion of planktonic Cyclotella spp. Three other late-glacial profiles were analysed, from L. Cam and L. Borralan in the same neighbourhood, and L. Tarff in the L. Ness area, to discover 1) whether this zonation applied generally, 2) whether taxa were characteristic of certain parts of the profiles, and 3) if other profiles include a similar planktonic phase. The sites varied in size and morphometry but the three zones could always be distinguished although assemblages differed slightly; none included a high percentage of Cyclotella in zone 2. Closer interval analysis has resulted in recognition of several subzones including one that can be correlated with the Bolling-Allerod stade. Esnvironmental interpretation is based on comparison with modem diatom assemblages and most taxa present in the late-glacial now occur in alkaline, nutrient-rich lakes. The lack of diatom plankton in the late-glacial suggests a similarity with some alpine lakes where diatoms are predominantly benthic. This similarity may have been initially due to severe climate and low nutrient supply. Continuation of this type of assemblages throughout the interstadial and into the early post-glacial suggests that the environment depends on the type of soils, vegetation and the amount of inwash of material into the lakes, rather than on climate.