Zoobenthos of the freezing water bodies of the “Medvezh`i ostrova” Nature Reserve, East Siberian Sea

While monitoring the Polar bear in the “Medvezh`i ostrova” (= Bears’ Islands) Nature Reserve in October 2023, a number of mainland water bodies were surveyed: the lakes “Nesolyonoe” and “Unnamed”, the Krestovaya River, the “Unnamed” watercourse on Chetiryokhstolbovyi Island, as well as the sea bay of Kolyma Bay, all in the East Siberian Sea. Representatives of zoobenthos were found in each of the water bodies studied, but the fauna was not very diverse. A total of only 7 species of benthic invertebrates were revealed in the surveyed water bodies of the Bears’ Islands Nature Reserve, of which 3 species were found in sea water: Pacifoculodes pallidus (G. O. Sars 1892) (Amphipoda), Yoldiella nana (M. Sars 1865) (Bivalvia), and Enchytraeus albidus Henle 1838 (Oligochaeta). In the water bodies feeding the Galgavaam River, 2 species from the family Chironomidae and 3 species of eurytopic Oligochaeta were recorded. Their abundance ranges from 0.20 to 1.70 thousand ind./m2 and 0.10 to 3.70 g/m2 biomass in freshwater bodies, vs not exceeding 0.60 thousand ind./m2 and 1.90 g/m2 biomass in the sea. Seasonal cohorts are formed there, 37% of which are merohydrobionts (Сhironomidae) and amphibiotic Оligochaeta, and 13% each are neritic amphipods and bivalves. The formation of seasonal cohorts is determined by a short growing season and an extended ice formation on island and mainland watercourses and reservoirs.