The seasonal dynamics of the taxonomic composition, average abundance, average biomass of mollusks and the size structure of some individual species in the waters of the Biyuk-Karasu River (Karasevka), which is the largest tributary of the Salgir River, from January to December 2024 were shown. The Mollusca fauna was also compared in different sampling sites, but mostly in 3 stations. In this river 10 taxa of mollusks, belonging to classes Gastropoda (5 species) and Bivalvia (5 species) in total were found. Only, three taxons were found at all the studied stations, with the exception of one-time ones near the Ak-Kai (Belaya) rock area. The maximum number of taxa of mollusks during the year was noted at station M – 8, including 1 specimen of the bivalve mollusk Unio sp. which was taken in July at this station in one qualitative sample. The average abundance at different stations ranged from 109 (station U 0) to 359 (station M) ind./m2. Depending on the stations, gastropods Limnaea stagnalis (Linnaeus, 1758) (in stations U 0 and U 1), Physa sp. (in station M) and Planorbis sp. (in station U 2) in average were prevailed. Such differences may be related to the characteristics of biotopes in these areas. Average abundance also varied significantly depending on the seasons, from 0 ind./m2 in March to 2138 ind./m2 in November, due to the high abundance of Physa sp. in this month at the station M. The average biomass ranged at the stations: from 0.75 g/m2 (station U 0) to 6.433 g/m2 (station M). The dominance of species at different stations is almost the same as in abundance, except for station U 2, where L. stagnalis prevailed instead of Planorbis sp. By seasons average biomass changing from 0 g/m2 in March to 29.363 g/m2 in December, due to the high biomass (as well as abundance) of Physa sp. in this month at the station M. The size structure of mass and numerous species (L. stagnalis and Physa sp.) was analyzed. In the area of the river mouth near village Uvarovka, from January to May and from August to October, small–sized L. stagnalis specimens (less than 10 mm) dominated, but in June, mollusks of this species with a shell height of more than 40 mm prevailed, in July –10-20 mm. Conditionally in the middle course of the river near village Melniki from October to December, very small specimens of L. stagnalis mollusks (no more than 5 mm) dominated. Also, in this station very small (no more than 3 mm) individuals of Physa sp. predominated in this area in October-November and slightly larger (3.1–6 mm) in December.
taxon, abundance, biomass, size structure, distribution
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