Spatial distribution features of bottom communities of a plain river in the Middle Volga river basin

Spatial distribution of hydrobiont communities depends on heterogeneous environmental conditions and is accompanied by formation of their specific determined patterns. For lotic ecosystems, various theories are devoted to their generalization: river continuum, flow neutrality, patch dynamics, ecological niches, landscape filters, etc. The aim of this work was to consider structural variability patterns of bottom communities at different sites of the water flow using the example of the Usa river - a typical plain river in the Middle Volga river basin discharging into the Kuybyshev water reservoir at the northwest extremity of the Zhiguli mountains (See Fig. 1). We used statistical methods to assess whether the listed theoretical constructions are applicable to explain the variation of the observed data. We carried out hydrobiological sampling during several years at 9 sites of the river, determined species composition, and estimated the number and biomass of 89 species (taxa) of macrozoobenthos. With the use of the Dickey-Fuller test, the hypothesis of the non-stationary character of the majority of population density series of the main taxonomic groups and generalized hydrobiological parameters and the existence of regular trends concerning the sequence of sites is confirmed (See Table 1). With the use of the Pielou's method of random skewers, statistical significance of the determined tendency in changing the structure of the entire community as a whole along the longitudinal gradient of the river is shown. We made a comparative analysis of rates of species richness change and estimated the beta-diversity for seasonal, long-term and spatial variants of combining observations (See Fig. 3). Using nonmetric multidimensional scaling, we constructed ordination diagrams of species and separated site groups corresponding to the characteristic zones of the watercourse (See Fig. 4). From the set of hydrological, hydrochemical and geomorphological environmental factors, we selected the most significant parameters defining the complex gradient of the river (oxygen saturation, flow velocity, and the phosphorus content (See Table 2). We showed that macrozoobenthos spatial distribution in the Usa River ecosystem is explained by the mechanisms of complex combination of all three concepts: "neutral pipe", "river continuum" and "patch mosaic". In the upper course, there is a high turnover of species and a considerable difference in taxonomic structure of bottom communities among sites, which is characteristic of neutral pipe. In the middle reaches, the balance of primary production is stabilised and there is an active exchange of species owing to direct and reverse drift. The taxonomic structure of biocenoses sequence in its peripheral part is strongly blocked and, consequently, a share of common species is great, which corresponds to the theory of the river continuum. In the lower reaches and estuary, the impact of external environmental factors becomes critical, and migration of species with waters from the Kuybyshev water reservoir is great, therefore a specific benthos structure is characteristic of "isolated zones". The article contains 4 Figures, 2 Tables and 30 References.

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