New species of Eunotia (Bacillariophyta) from Lake Baikal with comments on morphology and biogeography of the genus

In this article, we describe four species of Eunotia new to science from Lake Baikal based on light and scanning electron microscopic examination of valve morphology. Unlike most other species of the genus, all of the newly described species possess widely spaced transapical striae, a feature unique for Eunotia. Amongst the characters upon which we distinguish the new species from Lake Baikal are size ranges, raphe structure and position, stria density and the presence/absence of rimoportulae. Only a few established taxa, such as E. minor, E. submonodon and E. incisa, show weak morphological similarities to the newly described species. Hence, phylogenetic relationships are difficult to assess. All new species are described from recent samples; these are E. dorsicrenata sp. nov., E. leptocrenata sp. nov., E. pseudincisa sp. nov. and E. immanis sp. nov. One of the new taxa (E. dorsicrenata) occurs in the geological record dated to c. 7.7 million years ago. For the other species, information of their distribution in the geological record is missing. However, none of the species described in this article were observed outside of Lake Baikal, even in lakes occurring in the Baikal rift zone. These species are found in a deeper part of the lake, adding support to the notion of the immiscibility of Baikal's flora and fauna. We also discuss the idea of species flocks in diatoms and whether this group of species of Eunotia may represent a species flock in Lake Baikal.