Achnanthidium zhakovschikovii sp. nov. (Bacillariophyta) and related species from rivers of Northwestern Russia

Achnanthidium zhakovschikovii is described as a new epiphytic species from spring-fed rivers near St. Petersburg, Russia. This species belongs to the group of Achnanthidium taxa with terminal raphe fissures curved to one side of the valve. The new species differs from other taxa in this group by the broadly linear to elliptical shape of the valve and the wide sternum of the rapheless valve. Achnanthidium zhakovschikovii and two other Achnanthidium species with curved raphe fissures, A. pyrenaicum (Hustedt) Kobayasi and A. thienemannii (Hustedt) Lange-Bertalot, are found in the St. Petersburg region, mostly in waters of relatively high mineral content.