The SEASONAL DYNAMICS OF MACROZOOBENTOS AT GRADIENT MINERALIZATION IN THE TUZLUKKOL RIVER
Highly mineralized rivers are unique hydroecosystems. In the mineralization gradient terms, the bottom communities of the Tuzlukkol River polygalin section (Belyaevsky District, Orenburg Region) are characterized by seasonal dynamics of taxonomic, structural and quantitative indicators. Studies were conducted on the Tuzlukkol River within the nature monument «The Tuzlukkol Mud» in 2017-2018. Samples of benthic organisms were collected at 7 key stations. In total, 42 samples were collected and processed. In the composition of benthic invertebrates, 13 species and taxa are identified, which belong to the systematic groups Branchiopoda (1 species) and Insecta (12 species). The Chironomidae family is represented by 4 species: Culicoides sp., Cricotopus salinophilus , Chironomus salinarius , Paratanytarsus sp. In the studied part of the Tuzlukkol river, halophilic chironomid larvae Cricotopus salinophilus and Chironomus salinarius dominate in numbers and biomass. Microchironomus deribae , Paratanytarsus sp. (Diptera: Chironomidae), ceratopogonone Culicoides sp. (Diptera: Ceratopogonidae), S. aurata , D. caudata , Helophilus sp. (Ephydridae), Artemia salina (Branchiopoda), Sigara lateralis , Sigara assimilis , Sigara sp. (Insecta: Heteroptera), Berosus (Enoplurus) spinosus (Insecta: Coleoptera) are also found on selected sites. Within the natural monument of regional importance «The Tuzlukkol Mud» the level of mineralization, which provides the species with a sustainable existence in mineralized rivers, ranges from 4.09 to 50.13 g/l. However, some species ( Artemia salina , Cricotopus salinophilus , S. aurata , D. caudata ) are found with mineralization up to 80 g/l. The number of macrozoobenthos species at stations varies from 1 to 10. The number of benthic communities' representatives at river stations varies from 12 to 9438 ind./m<sup>2</sup> and the biomass varies from 4.4·10<sup>-4</sup> to 39.81 g/m<sup>2</sup>. The average biomass of bottom fauna representatives was 6.99 g/m<sup>2</sup> in May, 2.59 g/m<sup>2</sup> in July, 6.21 g/m<sup>2</sup> in October. The benthic communities structure was dominated by the Chironomidae larvae, constituting 95.11 % and 72.23 % of the aquatic organisms total number and total biomass, respectively. In all seasons, the chironomids Chironomus salinarius larvae belong to the dominants category in terms of abundance and biomass within «The Tuzlukkol Mud», the natural monument of regional importance. As a result of the water mineralization level's increase, a general decrease in species diversity and simplification of the benthic communities structure occurs. Zoobenthos is represented by halobiont chironomids Chironomus salinarius and Cricotopus salinophilus , which received mass development together with ceratopogonides Culicoides sp.