Current climate changes require special attention to the implementation of environmental activities in arid regions. The study of the biotic component of water bodies of such ecosystems and the patterns of their spatial distribution is an important area of scientific research. The river network of the semi-desert zone of the Russian Plain is one of the least studied lotic systems in the Lower Volga basin. In this river network, the plain Yeruslan River is of the greatest importance because it largely determines the environmental characteristics of this arid territory. Therefore, it is important to study the structural indicators and spatial dynamics of macrozoobenthos communities in the Yeruslan River. The aim of the work was to study species composition, the structural and quantitative indicators of macrozoobenthos from the source to the mouth of the Yeruslan River and to determine the conceptual belonging of the bottom communities of the plain river of the semidesert zone to a certain type of distribution. The Yeruslan River (51°18'3''N, 47°46'19''E) flows through the semi-desert zone of the Russian Plain (Volgograd region, Russia) and it is a tributary of Volgograd reservoir. The length of the Yeruslan River is 282 km, with a catchment area of 55700 km2. We collected samples of macrozoobenthos at 9 stations of the Yeruslan River (See Fig. 1) in June 2015 and July 2016. In the ripal zone, the integrated samples for quantitative macrozoobenthos analysis were taken using an Ekman-type grab sampler (surface area 25 cm2) in replicates (8X) and a handle blade trawl (0.2 м × 0.5 м). In the medial zone, samples were taken by an Ekman-type grab sampler (surface area 250 cm2) in replicates (2X). Samples were washed in the field using a mesh screen with 300-310-μm mesh size and preserved in 4% formaldehyde. At each station of the Yeruslan River, we used field analytical instruments for measuring pH and oxygen content. Water samples were taken for hydrochemical analysis at different sections of the river (See Table 1). We used the model of isolation by distance (Malécot, 1948), Monmonier’s maximum difference algorithm (Manni et al., 2004) and the Dickey-Fuller test (Dickey and Fuller, 1979) to perform statistical analysis of changes in the species structure of macrozoobenthos. The Yeruslan River flows within the geochemical province of continental salinity, which is characterized by an evaporative type of natural water regime, leading to progressive accumulation of salts. In this research, we found out that water was brackish at several stations of the river (1250-1420 mgl-1) due to water drainage of saline soils. We revealed that the Yeruslan River is polluted with nitrite nitrogen (at station 1) and phosphorus compounds (at stations 4 and 8) but concentrations of ammonium nitrogen, nitrate nitrogen, cadmium, copper, zinc and lead did not exceed the MPC. Comparative analysis has shown that from the source to the mouth of the river there are no significant changes in the speed of water velocity flow, and the physical and chemical conditions are specific for each station. In the river, we collected 132 species: 47 - Diptera, 20 - Oligochaeta, 11 - Mollusca, 11 - Grustacea, 11 - Coleoptera, 7 - Trichoptera, 7 - Heteroptera, 6 - Hirudinea, 4 - Odonata, 4 - Ephemeroptera, 1 - Lepidoptera, Hydracarina, Polychaeta and Megaloptera. The macrozoobenthos of the river is represented by limnophilic species in the upper, middle and lower reaches. This is due to the small slope of the Yeruslan River and the presence of permanent and temporary dams. In the mouth reaches, the macrozoobenthos communities included species of the Ponto-Caspian and Ponto-Azov zoogeographic complexes. At all stations of the river, Oligochaeta and Chironomidae were of high density. Also, in the river mouth, Mollusca were of high density (See Fig. 2). Statistical analysis of sequences of hydrobiological characteristics along the longitudinal gradient of the Yeruslan River using the Dickey- Fuller test showed that the presence of a stationary distribution trend with random “wandering” is typical of the series of total density and biomass of macrozoobenthos, the number of worms of the family Tubificidae, larvae of chironomids of the subfamily Tanytarsini and mayflies of the family Baetidae. For the other series of observations, the presence of a nonlinear trend is noted (See Table 3 and Fig. 3). The selection of a sequence of borders (barrier) between river communities within the ecosystem by Montmonier’s method using a matrix of species distances by the Bray-Curtis method made it possible to identify the source (station 1) with a high level of nitritic nitrogen in the water as one of the specific areas. The second most important border separates station 3 with a low content of dissolved oxygen, and the third one allocates the mouth reaches (station 9) as an independent area, where there is a cohabitation of river and reservoir species (See Fig. 4). Based on the analysis of fauna and using statistical methods, we found out that macrozoobenthos communities do not change from the source to the mouth of the river in accordance with the “the river continuum concept”. The habitat of taxa depends on local abiotic and biotic factors at each river station, therefore, we can assume that the distribution of macrozoobenthos communities, generally, corresponds to “the patch dynamics concept”. At the same time, stations 1, 3 and 9 form fairly isolated hydrogeomorphological areas, which is postulated by the concept of “the functional process zones”. It seems that the spatial distribution of macrozoobenthos communities in the Yeruslan River can be explained by a complex combination of two concepts: “the patch dynamics concept” and “the functional process zones”. This type of distribution seems to be typical of plain rivers with very low water velocity and the presence of dams.
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