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Species turnover or coherence in species co-occurrence as well as boundary clumping and nestedness in structural composition of ecological communities reflect the extent of determinancy in their organization (Leibold, Mikkelson, 2002). These phenomena may be a consequence of either interactions between species or heterogeneity in spatial distribution of populations density. We have examined statistical patterns of species structure variability using benthic communities of riverine ecosystems as an example. The ecosystems studied are characterized by strongly pronounced linear gradient of landscape features and environmental factors. The results of a long-term hydrobiological survey being conducted at 22 observational stations on the Sok River along with its tributary, the Baytugan River (Lower Volga basin, total watercourse length is 375 km) are involved into the analysis. A spreadsheet for statistical processing of the data included 375 macrozoobenthic taxa contained in 147 samples. An assessment of species structure nestedness in benthic communities at separate sites and along the watercourse as a whole has been carried out using various metrics such as nestedness "temperature" (Patterson, Atmar, 2000), discrepancy measure (Brualdi, Sanderson, 1999), nestedness based on overlap and decreasing fill (NODE--Almeida-Neto et al., 2008) and others. Statistical significance of ecosystems structural determinancy has been tested by means of randomization procedures and standard null models (Gotelli, 2000). The conclusions seem to be ambiguous and dependent on a level and scale of an ecosystem resolution into separate blocks, also on configuration and completion of initial bio-geographical tables. A searching for reliable and representative criteria of nestedness, invariant to various non-ecological modifications of the matrices but sensitive to estimation of analyzed ecological processes and suitable for comparisons of communities, is clearly needed. A quantitative estimation of species turnover and coherence in species cooccurrence has been performed using different indices of unique combinations and checkerboard score (Stone, Roberts, 1992) as well as Schluter's variance test. By means of empirical Bayesian approach (Gotelli, Ulrich, 2010) records of species pairwise combinations are formed where the frequency of species co-occurrence cannot be interpreted as a random value. Positive and negative relationships between taxa in macrozoobenthic communities, which are found out to be statistically significant, in most cases can be explained as being not the consequence of competition for resources but of spatial heterogeneity of biotopical conditions along the whole length of the watercourse.

Models for relationship between sampling effort and estimates of species number and other characteristics of species diversity are considered and evaluated. In the analysis, different randomization algorithms and other statistical methods of monitoring data processing are used including jackknife and bootstrap procedures, algorithms ICE and Chao2, Colwell-Mao interpolation model, Mikhaelis-Menten curves, and others. A comparative analysis of overall species richness in macrozoobenthic communities using streams of the Lower Volga basin as a case study is performed with the aid of different extrapolation models, and the resulting estimates are discussed. The relationships are analyzed between sampling effort (number of hydrobiological samples) and cumulative estimates of species richness and basic indices of species diversity. The means towards improvement of conclusions substantiation when ranking riverine communities by species diversity are considered.

Karyotype and morphology of Chironomus sp. larvae from the Caspian Sea have been described. 2n = 8. Chromosomal arm combination is AB, CD, EF, G (cytocomplex thummi). All chromosomes display conjugation of homologues. Centromere areas are of s-type. The nucleolar organizer and two Balbiani rings are disposed in arm C, and another Balbiani ring is in arm B. Half larvae have heterozygotic paracentric inversions in arm D. The larva belongs to salinarius form. One scleritis is colorless, an occipital scleritis is light and irregularly colored. Premandible has 4-5 cogs. Epipharingeal ridge has 18-23 dens. This new species is close to salinarius group, especially to Ch. albidus Konst., but differs from it at the larva stage in the structure of non-massive basal antenna segment, in the presence of large quantity of premandible denses, in light color of occipital scleritis, and in a series of morphometric rates. A similarity berween Ch. paraalbidus and Ch. albidus, on the one hand, and Boeotendipes, on the other one, has been found out.