Сharacteristics of aquatic invertebrates in the upper basin of Bakhta River (Krasnoyarsk Krai, Tungus Plateau) in the context of the distribution of relict and Baikal species

Pioneering studies were conducted on zooplankton, meiozoobenthos and macrozoobenthos assemblages in mountain-type taiga rivers: the Bakhta, Surindakan and Maigungda, as well as in the Yenisei River at the confluence with the Bakhta River. The studied watercourses are oligotrophic, and their fauna is characterized by high taxonomic diversity. Zooplankton organisms were extremely scarce in the studied watercourses. The meiobenthos assemblages consist mainly of ostracods (class Ostracoda) and copepods from the order Cyclopoida. The macrozoobenthic assemblages consist primarily of insects: mayflies (order Ephemeroptera), caddisflies (order Trichoptera) and midges (order Diptera, family Chironomidae), as well as gastropods (class Gastropoda). Most of the benthic fauna consists of widely distributed Palearctic species. However, five species crustacean and polychaeta from Baikal Lake origin have been identified, which are potentially allochthonous. Of these, Manayunkia baicalensis (Nusbaum, 1901) and Harpacticella inopinata (Sars, 1908) were recorded in the upper reaches of the Bakhta River, while Eulimnogammarus lividus (Dybowsky, 1874), Pallasea cancelloides (Gerstfeldt, 1858) and Gmelinoides fasciatus (Stebbing, 1899) were found only in the Yenisei River. In addition, two southern species from Oriental-Far Eastern origin – Ectocyclops polyspinosus Harada, 1931 and Nigrobaetis acinaciger (Kluge, 1983) – have been identified in the Bakhta River. These species are likely relics of earlier warm climatic periods and survived the last glaciation in the mountainous refugia of Middle Siberia.