First Record of Stephanodiscus hantzschii (Bacillariophyceae) from Lake Khalaktyrskoye (Kamchatka)

A new find of the diatom Stephanodiscus hantzschii Grunow, 1880 is described from Lake Khalaktyrskoye (Kamchatka) during the period of its mass growth in April 2023. The density of this species was 4.95 million cells/L, or 92.6% of the total density. A list of 45 species of associated diatoms is provided. According to electron microscopy data, it has been established that the S. hantzschii population was represented by two morphotypes, hantzschii and tenuis, which differ in quantitative (number of areolae and rows of areolae in 10 μm) and qualitative (shape of spines and arrangement and shape of areolae on the valve face) features. Features of the shape of areolae and their arrangement have also been determined for different frustules of the hantzschii morphotype. The mass growth of this species and revealed morphological features of the structure of its frustules might be due to the high level of eutrophication of Lake Khalaktyrskoye.