First quantitative study of macrozoobenthos subjected mountain spring stream Travyanistyi (“Grassy”) on the Northern slope of the KHamar-Daban Ridge (Eastern Siberia). The stream's water is bicarbonate-calcium, with very low mineralization (0.04 g/l), which is significantly lower than the mineralization of Lake Baikal water (0.09–0.1 g/l), and with the nearly neutral pH. It was found that in different parts of the stream and even in one short section the communities can be radically different. The study identified communities with an extremely high abundance of planaria (up to 10118 specimens/m² and 29.06 g/ m²) or amphipods (up to 12176 specimens/m² and 117,00 g/ m²), as well as a community with the dominance of oligochaetes (up to 2648 specimens/m² and 2.06 g/ m²). Amphipods in the communities are represented by the single species Gammarus dabanus Tachteew et Mekhanikova, 2000, endemic for Hamar-Daban Ridge. Core of populations of this endemic amphipod serve shallow geocrenes with abundant detritus, over-grown with moss. In the middle reaches of the stream, the larvae of caddisflies Rhyacophila cedrennsis Schmid, 1993 were detected for the first time for Baikal Lake tributaries. It is discovered a representative of stoneflies that is close to the endemic for Lake Baikal genus Baikaloperla. To clarify the taxonomic composition and determine the quantitative characteristics of zoobenthos in small mountain streams, recommendations for mandatory sampling in the coastal zone, which is characterized by the greatest biotopic diversity, are justified. The originality of mountainous watercourses of the northern macroslope of the Khamar-Daban is manifested not only at the level of faunal composition, but also at the biocenotic level, which is expressed in the presence of uncommon for most mountain streams of Eastern Siberia benthic communities.

Eropova Irina Olegovna, Postgraduate, Baikal Museum ISC SB RAS, 1, Akademicheskaya st., Listvyanka settl., Irkutsk Region, 664520, Russian Federation; Engineer, Irkutsk State University, 1, K. Marx st., Irkutsk, 664003, Russian Federation, e-mail: eropova.irina@yandex.ru 

Takhteev Vadim Victorovich, Doctor of Sciences (Biology), Professor, Irkutsk State University, 1, K. Marx St., Irkutsk, 664003, Russian Federation; Leading Research, Scientist Baikal Museum ISC SB RAS, 1, Akademicheskaya st., Listvyanka settl., Irkutsk region, 664520, Russian Federation, e-mail: Amphipoda@yandex.ru 

Rozhkova Natalya Anatolyevna, Candidate of Science (Biology), Senior Research Scientist, Limnological Institute SB RAS, 3, Ulan-Batorskaya st., Irkutsk, 664033, Russian Federation, e-mail: rozhkova@lin.irk.ru 

Batranin Dmitry Aleksandrovich, Student, Irkutsk State University, 1, K. Marx st., Irkutsk, 664003, Russian Federation, e-mail: badmal3477@gmail.com 

KHadeeva Ekaterina Romanovna, Lead Engineer, V.B. Sochava Institute of Geography SB RAS, 1, Ulan Batorskaya st., Irkutsk, 664033, Russian Federation, e-mail: war_ker@mail.ru Lopatovskaya 

Lopatovskaya Olga Gennadyevna, Candidate of Sciences (Biology), Associate Professor, Irkutsk State University, 1, K. Marx st., Irkutsk, 664003, Russian Federation, e-mail: lopatovs@gmail.com

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