The specificity of the composition, abundance and trophic structure of the summer macrozoobenthos of the Rybinsk Reservoir in the modern period

The main structural characteristics of the summer macrozoobenthos of the Rybinsk Reservoir have been studied according to the data of the expanded grid of stations in 2019 and 2021. 80 species and forms of benthic invertebrates were identified, most of which were chironomids, oligochaetes and mollusks. In all zones and parts of the reservoir, the basis of benthos species richness is represented by chironomids. The dominant complex on all soils included Limnodrilus hoffmeisteri Claparede, 1862, in most cases – Chironomus f. l. plumosus, with the exception of silted sand, where Cladotanytarsus gr. mancus and Tubifex newaensis (Michaelsen, 1902) were added to them. On the silted-up shell, the dominants included Potamothrix moldaviensis Vejdovsky et Mrazek, 1902. The basis of the quantity (93%) and biomass (86%) of animals were chironomids and oligochaetes. The greatest abundance of benthic invertebrates was recorded on gray silts and in the deep-water zone of the reservoir. The Sheksninsky part was characterized by the largest benthic biomass, and the Volzhsky part was characterized by the largest quantity. Among the trophic groups in the quantity of macrozoobenthos on all main soils, in all parts, in the deep-water zone and estuarine areas of the tributaries of the reservoir, the basis was formed by detritophages–swallowers. In the zone of open shallow water, phytodetritophages–filtrators + collectors dominated in quantity. This trophic group made the greatest contribution to the benthic biomass of gray and peaty silts, in all zones and parts, except the Volzhsky part. On silted-up shell, the largest share in the total biomass was made up of phyto-detritophages–filtrators + collectors and detritophages–swallowers. Detritophages–swallowers formed the main part of benthic biomass on silted sand and in the Volzhsky part. When conducting a comparative analysis with the results obtained in 1978, a decrease in the biomass of oligochaetes on gray silts was recorded by ~3 times. Likely, this is due to a decrease in the occurrence and abundance of a large oligochaete that previously dominated on gray silts, an indicator of β-mesosaprobic conditions, Tubifex newaensis. In 2019 and 2021, this oligochaete was not included in the dominant complex of gray silts. It began to be represented by polysaprobes Limnodrilus hoffmeisteri. This can serve as an indirect sign of an increase in the trophic status of a reservoir to a typically eutrophic one and an increase in the accumulation of organic matter in soils.