Heribaudiella fluviatilis (Aresch.) Svedel. presents the most frequently reported freshwater species of the brown algae, being known mostly from central and northern Europe and western North America with only a few locations in Asia, including those in Japan and China. The discovery of the new H. fluviatilis locations from small Olkha River draining the Olkha plateau, Irkutsk Region, Southern Siberia, significantly extends the species distribution. The species was first collected in summer 2012 and then recollected in the same location only in August 2015, and in the second year one more location of H. fluviatilis was discovered a kilometer upstream. The alga inhabits cobbles and broken bricks on the shallows (5–30 cm depth) partially drying during short late summer low-water period. In the both new locations, H. fluviatilis forms dark-brown patches up to 2 cm in diam. with typical system of creeping and erect filaments documented with light micrographs. Macroscopic algae co-occurring with H. fluviatilis are presented by Cladophora glomerata (L.) Kütz., C. floccosa C. Meyer, Ulothrix zonata (Weber et Mohr) Kütz. and Spirogyra sp. ster. Associated diatoms including native Didymosphenia geminata indicate fairly eutrophentic conditions. Heribaudiella fluviatilis is proposed to be included in the Red Data Book of Irkutsk Region due to its uniqueness for Siberia, as well as high anthropogenic impact on Olkha River increasing the risk of H. fluviatilis extinction.
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