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The broad concept of "homeostasis" in relation not only to organisms, but also to populations, ecosystems, and socio–ecological and economic systems (in the context of sustainable development) is discussed. Sustainability is the major quality of systems. The following mathematical formalizations and examples of various definitions of sustainability are discussed: according to Lyapunov (stability), Lagrange (steadiness), Poisson (periodic stability), Svirezhev (hierarchical stability), Holling (elasticity), Fleishman (vitality), and Gnedenko (reliability). [ABSTRACT FROM AUTHOR]

Abstract—The authors discuss the use of the Greenell concept of ecological niches to analyze the taxonomic structure of communities and their relationship with environmental factors. The modeling of ecological niches was carried out using the results of 30-year studies of macrozoobenthos communities on 90 small and 12 medium-sized plain rivers in the region of the Kuibyshev, Saratov, and Volgograd reservoirs. Geoclimatic indicators, relief characteristics, and hydrochemical assessments of the water quality at sampling points were considered as abiotic factors, where 11 variables with the least collinearity were selected. The multidimensional space of the initial abiotic factors was projected onto the orthogonal axes of the first two principal components; ordination diagrams with plotted points of the presence of species were constructed. The model of the potential ecological niche of each species was represented as an area in the space of latent variables, in which the habitat suitability index Z estimated by the probability density of occurrence corresponds to the given constraints. The authors used the Schoener and Hellinger indices to quantify the proportion of overlapping niches belonging to two different benthos taxa. The matrix of distances between overlapping niches was formed for all possible paired combinations of the 40 most ecologically significant macrozoobenthos species. Cluster analysis of the obtained distance matrix was carried out by the methods of hierarchical agglomeration and fuzzy k-means. In the general multidimensional space of abiotic variables of the region studied, four areas of collective niches for groups of species with maximum distances between their centroids were identified. The analysis of intergroup variation of environmental factors is given, and the characteristic features of each group are discussed: ecological strategy, biological traits, and tolerance level. [ABSTRACT FROM AUTHOR]

For the first time, an assessment of the taxonomic and quantitative structure of the phytobenthos of seven saline rivers in the basin of the hypersaline Lake Elton (Volgograd region) is presented. The main types of algal communities developing on substrates were revealed: diatom phytobenthos, communities of diatoms with cyanoprokaryotes, cyanobacterial films and mats, and epiphyton. The phytobenthos consists of local, mosaic-distributed communities with a small similarity in their species composition. The structure of algal communities is determined by Bacillariophyta (with the predominance of species of g. Navicula) and Cyanoprokaryota (of the gg. Phormidium, Leptolyngbya, and Geitlerinema). The abundance and biomass values vary widely, reaching their highest values (109 834–417 472 million cells/m2 and 523.6–567.6 g/m2) in cyanobacterial communities of mesohaline rivers at a water salinity reaching 16–19 g/L. Mineralization as a factor influencing the structure of communities most rigidly acts in the mouth sections of rivers, by controlling the composition of the dominant species. For critical salinity values (>70 g/L), characteristic structural transformations of communities by the type of replacement of planktonic and benthic Bacillariophyta and Cyanoprokaryota by planktonic Chlorophyta (of the g. Dunaliella) and picoplankton were revealed. [ABSTRACT FROM AUTHOR]

For the first time, an assessment of the taxonomic and quantitative structure of the phytobenthos of seven saline rivers in the basin of the hypersaline Lake Elton (Volgograd region) is presented. The main types of algal communities developing on substrates were revealed: diatom phytobenthos, communities of diatoms with cyanoprokaryotes, cyanobacterial films and mats, and epiphyton. The phytobenthos consists of local, mosaic-distributed communities with a small similarity in their species composition. The structure of algal communities is determined by Bacillariophyta (with the predominance of species of g. Navicula) and Cyanoprokaryota (of the gg. Phormidium, Leptolyngbya, and Geitlerinema). The abundance and biomass values vary widely, reaching their highest values (109 834–417 472 million cells/m2 and 523.6–567.6 g/m2) in cyanobacterial communities of mesohaline rivers at a water salinity reaching 16–19 g/L. Mineralization as a factor influencing the structure of communities most rigidly acts in the mouth sections of rivers, by controlling the composition of the dominant species. For critical salinity values (>70 g/L), characteristic structural transformations of communities by the type of replacement of planktonic and benthic Bacillariophyta and Cyanoprokaryota by planktonic Chlorophyta (of the g. Dunaliella) and picoplankton were revealed. [ABSTRACT FROM AUTHOR]

This paper presents results of comprehensive studies of saline rivers in the arid Elton Lake region based on hydrobiological surveys of planktonic and bottom communities. Two tables representing the species structure of communities and produced on the basis of survey data collected on 13 river sites in 2013 and 2018, including data on the abundance of hydrobionts belonging to 94 different taxonomic groups of macrozoobenthos, meiobenthos, and zooplankton, have been compared. The co-inertia analysis has identified a high statistical consistency of the data matrices determined by objective spatial distribution patterns of aquatic organisms. A randomization test was applied to the Procrustean correlation coefficient, and it showed that the conjugacy of the two species structures in the space of latent variables is statistically significant (p = 0.00026). Concurrently, a change trend was noted for the taxonomic composition of some surveyed communities affected by dynamic abiotic factors. The relationship between the taxonomic structure of hydrobionts and the combination of 30 abiotic environmental factors measured in the course of the hydrobiological and hydrochemical monitoring of the studied river sites was analyzed. Using canonical correlation analysis and the technique of projection to latent structures, a complex of ordination diagrams has been produced permitting the identification of specific "environmental niches" for each species group, including certain sets of biotope characteristics. A significant mutual correlation between the planktonic and bottom communities has been established. This correlation indicates close interrelations between these communities determined both by biotic interactions and by mutually consistent reactions to changes in aquatic environmental factors. [ABSTRACT FROM AUTHOR]

Abstract—Data on the status of summer phytoplankton of the Usa River and its tributaries are given. In total 184 taxa of algae with a rank lower than the genus from eight divisions were recorded in algal flora, with Bacillariophyta (43–57% of the composition) and Chlorophyta (23–47%) predominating in the rivers. The abundance range in algocenoses was 0.02–5.2 million cells/L; of biomass, 0.01–1.8 mg/L; the content of chlorophyll-a, was 0.01–3.1 μg/L. The ecological heterogeneity along the watercourse causes significant dynamics in the taxonomic composition, which is characterized by mosaic distribution in different areas, and changes in the quantitative structure of plankton communities. An increase in the species diversity, abundance, biomass, and chlorophyll-a concentration from the source to the mouth of rivers is noted. The ecotone effect in the zone of mixing and transformation of the waters of the lower reaches of the watercourses and Usinskii Bay of the Kuibyshev Reservoir is expressed in an increase in the diversity and amount of algae, an increase in the coenotic role of Cyanoprokaryota and planktonic green algae of the Chlorococcales order, which are dominant in Usinskii Bay. The influence of hydrological conditions, the content of nutrients, and the anthropogenic impact on the formation of algocenoses is discussed. The taxonomic composition, distribution, structural characteristics of algocenoses in the absence of nutrient limiting depend on hydrological factors, and biotopic heterogeneity of natural and anthropogenic origin have been studied, whereas their relation to the content of nutrients in eutrophic conditions has not been revealed. The Shannon species diversity index varied from 0.5 to 2.7 bits/specimen in the river algocenoses. The minimum values were noted on the upstream sections of the rivers; the complexity of the structure is typical for the algocenoses of the mixing zones of the waters of the upper part of Usinskii Bay and the mouths of the Muranka and Tisherek rivers, as well as for the communities of the sections of rivers with anthropogenic impact. [ABSTRACT FROM AUTHOR]

The statistical background for the relationship between the taxonomic diversity of macrozoobenthos and water mineralization is presented in the example of the benthic communities of plain rivers in the Lower Volga River basin. Significant differences have been found for a complex of 35 indices of species and phylogenetic α- and β-diversity in the tributaries of the Saratov and Volgograd reservoirs compared to the saline rivers of the Elton Lake basin. The hypothesis of phylogenetic niche conservatism [5] is confirmed by a significant effect of phylogenetic overdispersion at low and medium salinity of watercourses and phylogenetic clustering at high water salinity. Mean salinity tolerance has been assessed for each of the 356 species considered; this index was interpreted as the statistical probability of inhabiting the waters of certain mineralization by particular species. A statistically significant phylogenetic signal corresponding to the species traits was revealed by Pagel's lambda and Bloomberg's K tests. Mean salinity tolerance for taxa of various systematic levels has been presented as a cladistic tree. [ABSTRACT FROM AUTHOR]

Depending on the season, the primary production of planktonic communities determined by radiotracer analysis varied within a broad range, from 6 to 314 µg C/(L h). Primary production in cyanobacterial mats was 4.2‒10.9 × 103 µg C/(dm3 h), and Chl a content varied from 6‒13 to 132‒140 mg Chl a/m2. For the plankton, the highest values were revealed in summer (25‒46 mg Chl a/m3), with the maximum in August (223 mg Chl a/m3). High abundance of bacterioplankton (0.3‒7.4 × 106 cells/mL) and massive growth of diatoms (0.15 × 106 cells/mL) with predominance of the genus Chaetoceros were found. Sulfate reduction rates varied from 0.037 µmol S/(dm3 h) in the upper reach to 61.87 µmol S/(dm3 h) in the river mouth. [ABSTRACT FROM AUTHOR]

Abstract—A comparative analysis of the variability of the species structure of macrozoobenthos communities in small and medium rivers of the Middle and Lower Volga river basin has been carried out. The statistically significant influence of the natural climatic zone, where the watercourse is located, and manifestation of the latitudinal gradient of biodiversity are shown. A list of indicator species has been made for each geographical zone identified using different criteria of biotopical distribution. Based on the analysis, the indicator value (IndVal) index, which takes into account the zonal distribution of both the occurrence frequencies of a species and its abundance ratios, was recognized as the most adequate. Association rules that contain combinations of species that most frequently occur together in hydrobiological tests have been formulated using the Apriory algorithm. [ABSTRACT FROM AUTHOR]