On the morphological variability of Aulacoseira baicalensis and Aulacoseira islandica (Bacillariophyta) of Lake Baikal, Russia

Reliable identification of the diatoms is of crucial importance to understand the changing environmental signals stored in the sediments of Lake Baikal. Light and scanning electron microscopy combined with mathematical methods, including principal components analysis, were used to estimate morphological diversity in the structure of mantles of the two dominant species of Lake Baikal, Aulacoseira baicalensis (K. Meyer) Simonsen and Aulacoseira islandica (O. Muller) Simonsen. Species-specific features, such as areolar density, ultrastructure and form of linking spines, were found to be widely variable within cells of the same filaments of A. baicalensis. This finding suggests that the diagnosis of this species will have to be broadened. Besides the thin-walled frustules and the ability to form spores, the population of A. islandica in Lake Baikal is characterized by a higher density of areolae on the mantle compared to the published diagnoses, including the diagnosis of the subspecies helvetica. Morphological features of A. baicalensis and A. islandica are very similar in early spring when the cells produce auxospores, but diverge in summer. The most important distinguishing feature is the density of areolae-the number of areolar rows per 10 mu m and the number of areolae per 10 mu m of a row.