DISTRIBUTION OF ALIEN SPECIES OF MACROZOOBENTHOS AND THE SPECIES COENOTIC COMPLEXES IN THE KAMA RESERVOIRS

The results of studies on the distribution of alien species in macrozoobenthic communities in the reservoirs of the Kama River cascade are provided. Twenty five alien species of the Ponto-Caspian and Ponto-Azov origins and 1 species - a representative of the Lake Baikal fauna - were registered. Differences in the species composition and structural characteristics of invaders of macrozoobenthos in the Kama and Volga reservoirs were shown. It was noted that mollusks of the genus Dreissena dominate in biomass in most of the studied biotopes. Three types of coenotic complexes of alien species have been identified: psammorelophilic profundal, characteristic for the Nizhnekamsk Reservoir and the Volgo-Kamsky and Kamsky reaches of the Kuibyshev Reservoir; the psammorelophilic coastal in Nizhnekamsk Reservoir; and the psammorelophilic coastal in Votkinsk and Kama reservoirs.

Virioplankton as an important component of plankton in the Volga Reservoirs

The distribution of virioplankton, abundance and production, frequency of visibly infected cells of heterotrophic bacteria and autotrophic picocyanobacteria and their virus-induced mortality have been studied in mesotrophic and eutrophic reservoirs of the Upper and Middle Volga (Ivankovo, Uglich, Rybinsk, Gorky, Cheboksary, and Sheksna reservoirs). The abundance of planktonic viruses (VA) is on average by 4.6 ± 1.2 times greater than the abundance of bacterioplankton (BA). The distribution of VA in the Volga reservoirs was largely determined by the distribution of BA and heterotrophic bacterioplankton production (PB). There was a positive correlation between VA and BA and between VA and PB. In addition, BA and VA were both positively correlated with primary production of phytoplankton. Viral particles of 60 to 100 µm in size dominated in the phytoplankton composition. A large number of bacteria and picocyanobacteria with viruses attached to the surface of their cells were found in the reservoirs. Viruses as the most numerous component of plankton make a significant contribution to the formation of the planktonic microbial community biomass. The number of phages inside infected cells of bacteria and picocyanobacteria reached 74‒109 phages/cell. Easily digestible organic matter, which entered the aquatic environment as a result of viral lysis of bacteria and picocyanobacteria, could be an additional source of carbon for living bacteria. The results of long-term studies indicate a significant role of viruses in functioning of planktonic microbial communities in the Volga reservoirs.

AULACOSEIRA PUSILLA (BACILLARIOPHYTA) – NEW SPECIES FOR THE FLORA OF THE VOLGA RESERVOIRS

In this paper, we restudy the morphology of A.subarctica low-frustule forms from the Volga reservoirs (Ivankovo, Gorky and Kuibyshev) using scanning electron microscopy. These forms occur as short colonies and one of them, by quantitative (colony size, valve diameter and mantle height, valve height/diameter ratio, number of areola rows and areolae in 10 µm) and qualitative (shape of spines, areolae arrangement of valve face and mantle) characteristics, corresponds to A.pusilla. The latter is a cosmopolitan species, new for the flora of Volga. A. Pusilla is very similar to A.subarctica by morphology and these two species are often found together that may cause problems with their identification.

Ecology and metabolic activity of cyanobacteria in large different types of lowland reservoirs in the European part of Russia

A comparative of the species composition and ecological-geographical characteristics of cyanobacteria plankton of 13 reservoirs of the Volga-Kama cascade and the Don (Sheksna, Ivankovo, Uglich, Rybinsk, Gorky, Cheboksary, Kuibyshev, Saratov, Volgograd, Kama, Votkinsk, Nizhnekamsk and Tsimlyansk reservoirs) is analysed. An assessment of the change in the distribution of cyanobacteria biomass in a latitudinal gradient along the Volga cascade is presented. The seasonal and long-term (1954–2016) dynamics of the abundance, biomass and the ratio of functional groups of cyanobacteria in the Rybinsk Reservoir, the second largest in the cascade of Volga reservoirs, are analyzed. A positive long-term trend of abundance and diversity of cyanobacteria and the leveling of their biomass between reservoirs along a cascade in recent years has been established. An increase in the abundance and diversity of nonheterocystous species was established in the direction from the Upper to the Lower Volga and during the long-term succession of phytoplankton of the Rybinsk Reservoir. This is associated with an increase of water mineralization. Based on current research in the 2010s of the Volga, Kama and Don cascade reservoirs, 14 variants of microcystin structures were identified, the diversity of which depended on the composition of the producing species. Using PCR analysis, the main producers of microcystins (Microcystis and Dolichospermum) were identified. Cyanobacteria Cuspidothrix issatschenkoi and/or Raphidiopsis mediterranea have been identified as possible producers of neurotoxic anatoxin-a in the Tsimlyansk Reservoir. It was shown that water temperature and nitrogen were the greatest importance for development of microcystin-producing species of cyanobacteria.

Spatial-temporal Transformation of the Volga Reservoirs’ Soil Complex. Communication 7. Sediment Formation in the Highly Running Reservoirs of the Volga Cascade

Large plain reservoirs are water bodies of a slow water exchange with Кw less than 10 year-1 contributing to multi-year (annual) flow regulation. All reservoirs of the Volga-Kama cascade belong to this type of reservoirs, except the Cheboksary and Saratov reservoirs with Кw about 20 year-1. Weekly and daily fluctuations of the runoff are observed in these reservoirs. This mode of operation of hydraulic constructions leads to an increase in the flow rate and fluctuations of the level, which determines the specific conditions for the formation and accumulation of bottom sediments characteristic of river systems. It is shown that the areas of coarse-grained sediments are 55 and 70%, respectively. The reservoirs under consideration are transit reservoirs for suspended substances and aerators for the oxidation of chemical pollutants and their neutralization, so they play a crucial role in the natural self-purification and management of natural water quality. Methods. The aim of the study is to characterize the spatial and temporal transformation of the lower layer of the ecosystem in reservoirs with high water exchange (by the example of the Cheboksary and Saratov reservoirs) and to identify its role in water self-purification taking into account biomarkers. It is known that river systems or systems similar to them are more efficient and are characterized as potentially saturated with food organic/mineral resources for hydrobionts. The assessment of water quality and bottom sediments of different hydro-morphometric characteristics, dynamics of water masses and operating conditions is presented using methods of bioassay and toxicology by the example of chironomid larvae and antioxidant defense system of benthic fish. Results. Based on the data of the integrated hydrobiological cruises during the growing season 2014–2020, it is difficult to say that the quality of water and bottom sediments, biotic and toxicological parameters depend on the flow rate, so further studies are required. The work is original, because there are no analogues in the world for monitoring of bottom sediments in reservoirs and quantitative assessment of sedimentation processes in space and time. The results of the biogeochemical analysis indicate, that in terms of the degree of deterioration of the eco/toxicological state, the Volga reservoirs are in a series: Volgograd = Saratov > Kuibyshev > Cheboksary > Gorky.

Assessment of the relationship between chlorophyll content and biogenic elements in large lowland resevoirs

In the second half of the 20th century, with increased rates of eutrophication, universal quantitative relationships between the content of biogenic elements and chlorophyll (Chl) were obtained, which serves as a marker of algae biomass, as well as an indicator of the trophic status of water bodies. Most of these studies were performed on lakes, however, the patterns and ratios found for lakes were not always observed in reservoirs, young ecosystems with high development dynamics. Long-term studies of phytoplankton productivity are carried out in the reservoirs of the Upper Volga. These studies have shown that a direct relationship of Chl with total nitrogen (Ntot) and phosphorus (Ptot) is rarely found and is at low correlation coefficients. A more successful attempt was to indirectly assess this relationship using the ratios of Chl/Ptot and Chl/Ntot, which was named “phytoplankton response” (Vinberg, 1987) corresponding to the term “efficiency” (Kalf, Knoechel, 1978). Our first data on Chl/Ptot and Chl/Ntot in the Upper Volga reservoirs refer to 1980-1990 (Mineeva and Razgulin, 1995; Mineeva, 1993; Mineeva, 2004). At present, new data have been obtained, which have made it possible to follow long-term trends in the development of phytoplankton in the reservoirs of the Upper Volga in connection with the availability of biogenic elements under variations in the trophic status of the reservoirs. Samples were collected in August 2015-2018 at 26 stations in three large reservoirs of the Upper Volga (56°51'-58°22'N, 35°55'-38°25'E): Ivankovo, Uglich and Rybinsk reservoirs. Chlorophyll content was determined by the fluorescence method (Gol’d et al., 1986) in integral water samples taken from the surface to the bottom. Data on biogenic elements obtained at the same time by Dr Irina Stepanova are given in our joint publication (Mineeva et al., 2021). The standard software packages for a personal computer were used for statistical processing (calculation of mean values and standard error of the mean (M±mM), correlation and variation coefficients, regression equations and graphing). In this research, we revealed that nutrient content in the Upper Volga reservoirs varied within similar limits, the average Ntot/Ptot ratio was the same, and the average values of Ptot and Ntot decreased in Rybinsk reservoir. Chl content was typical of the summer phytoplankton maximum in the Upper Volga reservoirs (See Table 1). Ptot content did not change much in Ivankovo and Uglich reservoirs, but in Rybinsk reservoir it increased in 2015 and decreased in 2016. The more stable content of Ntot decreased a little in the cool 2017. Chl concentrations of 20-30 μg/L obtained in Rybinsk reservoir in 2015 and 2018, in Uglich reservoir in 2017 and in Ivankovo reservoir in 2015 correspond to the eutrophic category; concentrations of 35-52 μg/L (Rybinsk reservoir in 2016, Uglich reservoir in 2015, 2016 and 2018, Ivankovo reservoir in 2016-2018) - to hypertrophic category, and only in 2017 in Rybinsk reservoir did they decrease to a moderately eutrophic level of 13 μg/L (See Fig. 1). It was found out that under these conditions Chl content per unit of biogenic elements varied in a wide range: Chl/Ptot from 0,03 to 2,13, Chl/Ntot from 2,55 to 123, with maximum values in the highly eutrophic Shosha reach in Ivankovo reservoir (August 2018). Chl/Ntot slightly differed in Ivankovo and Uglich reservoirs, but increased in Rybinsk reservoir; Chl/Ptot decreased in the range of Ivankovo, Uglich and Rybinsk reservoirs (See Table 1). Most of both ratios belong to narrower limits: 0.10-0.50 for Chl/Ptot (67% of all values) and 10–40 for Chl/Ntot (51%) (See Table 2). The average Chl/Ptot ratio for the entire data set is 0.43±0.03, and Chl/Ntot is 29.8±1.9 with variation coefficients of 77.0 and 63.9%. The interannual changes (See Fig. 1) show that all the maximum ratios were obtained in 2016, while the minimum in Ivankovo reservoir was in 2015, in Rybinsk reservoir in 2015 and 2017 and in Uglich reservoir in 2017 and 2018. Our results demonstrate that both ratios correlate with each other (See Fig. 2), as well as with the Chl content (See Fig. 3). The correlation of both ratios with Chl weakens in Ivankovo reservoir, probably due to the abundant development of macrophytes. We can observe the same in all reservoirs in the cool 2017 with a decrease in ChlCyan (the abundance of cyanoprokaryotes) and an increase in the proportion of ChlBac (the abundance of diatoms). Both ratios are insignificantly dependent on hydrological factors as well as on Ptot and Ntot content, but the situation can change in individual reservoirs and in different years of observation (See Table 3). As it was demonstrated, both ratios increase with the growth of trophy estimated by Chl. They do not significantly differ in mesotrophic and moderately eutrophic waters, but become much higher in eutrophic and highly eutrophic waters, indicating a more efficient consumption of nutrients in the latter. At the same time, Ptot and Ntot change little along the Chl gradient (See Table 4). Chl/Ptot decreases with the increase in Ptot and grows slightly with the increase in Ntot; Chl/Ntot does not change over the entire range of Ptot and Ntot; but both ratios vary along the gradient of Ntot/Ptot (See Table 5) that serves as an indicator of the biogenic limitation of phytoplankton and a factor regulating the development of algae. A retrospective analysis shows significant interannual fluctuations of both ratios in Rybinsk reservoir and a tendency to increase them in Uglich and Ivankovo reservoirs (See Fig. 1). Thus, Chl/Ptot and Chl/Ntot ratios are useful for analyzing the relationship between the development of phytoplankton and nutrient supply that is necessary in order to assess the efficiency of their use and their availability to algae cells. The results obtained allow us to conclude that phytoplankton in the Upper Volga reservoirs is less sensitive to the presence of nitrogen and more dependent on phosphorus.

Rate of sexual maturation and fecundity of the roach Rutilus rutilus (L.) of the Volga reach of the Rybinsk reservoir

Climate warming and the introduction of a new mass species of clam Dreissena bugensis can potentially lead to changes in the reproductive capacity of benthic fish species living in the upper Volga reservoirs. The age of puberty, the structure of the spawning stock, and fecundity are the main indicators of reproduction that determine, together with indicators such as the survival of young fish and the degree of fishing load, the relative number of species in the ichthyofauna. The purpose of this work was to study the size and age of the onset of puberty and the dynamics of fecundity of roach Rutilus rutilus in the Volga ples of the Rybinsk reservoir. Determination of individual absolute fecundity was performed by counting eggs in fresh gonads, without the use of fixation. The age of fish was determined by scales, and vertebrae and operculum were used as additional recording structures. As a result, it is shown that the beginning of sexual maturation of female roach occurs when reaching the age of 3+ and a length of more than 9 cm. Late maturing females were 4+ years old and over 18 cm long. The beginning of sexual maturation of males could not be determined. The smallest males in the catches, 7–8 cm long, were all sexually Mature. The maximum age of fish reached 12+ years with a length of no more than 32 cm. Individual absolute fertility ranged from 1,75 to 178,46 thousand eggs. The value of relative fertility ranged from 79 to 308, with an average of 171 eggs/g. The result concluded that high absolute and relative fecundity of roach and dynamic changes of fertility in terms of increase in average water temperature and absolute improvement of fodder after the invasion of Dreissena bugensis, may indicate the attainment of specific limit reproductive capacity of this species.