The article presents an assessment of the quantitative and qualitative composition of zooplankton in the zoocenosis of the water bodies of Astrakhan. There have been analyzed bioindicators that allow to determine the structure of zooplankton during the research period. A comparative characteristic of the reservoirs has been carried out, according to the indicators of species density (species richness), complexity of the community structure and dominating species. Samples of zooplankton collected in the autumn of 2016 and 2017 in the water bodies within the city limits (the Tsarev river (stations Strelka, Bridge) and the Kutum river (station Strelka opposite the Civil Registry Office and nearby N. Ostrovsky Street) served as the research material. According to the analysis results of biodiversity indices, it has been found that the highest increase in quality indicators (almost doubled in 2017, compared to 2016) was observed in relation to the density of organisms in the Tsarev river. At the same time, the indicator of complexity of the plankton community structure, expressed in quantitative terms, showed an increase by 1.4 times at all stations in 2017, compared to 2016. This is a signal of improvement in the environmental component in urban water bodies. According to the Simpson index values in 2017, plankton species can be assumed to dominate in the zoocenosis. The population of the zoocenosis in the water bodies of Astrakhan is exposed to the anthropogenic impact, which causes changes in the quantitative and qualitative characteristics of communities and their structure. However, increasing values of all three considered biodiversity indices in 2017 indicate a gradual restoration of the biocenosis structure, and an increasing proportion of species in composition of the plankton community.
Today, biological indication methods are recognized as highly effective, since they allow to directly assess the state of the ecosystem and its individual components. Even casual examining of hydrobionts can provide fast and reliable information on water quality. The rivers Kutum and Tsarev located on the territory of the city of Astrakhan were selected as objects of research. Samples of zooplankton collected in the fall of 2016 and 2017 served as research material. Organisms belonging to different zones of saprobity were selected as an indicator group. Calculations of weighted average saprobity were made. Both reservoirs under study have been found to belong to beta-mesosaprobic zone. At station 1 of the Kutum River and at station 1 of the Tsarev River there were registered equal values of the indicator of weighted average saprobity that correspond to oligosaprobic and beta-mesosaprobic zones. This may be explained by the change of community and by the fact that the identified organisms at these stations had a very wide tolerance to polluting factors. At station 2 of the Kutum River in 2016 and 2017 there was stated the predominance and increase in the number of bioindicators of the oligosaprobic zone, while at the other stations the organisms gravitating towards beta-mesosaprobic zone did not seriously change their indicators. As it can be inferred from the results of the saprobiological analysis of the urban watercources (the Kutum River and the Tsarev River), they are undergoing an active process of self-purification, culminating in mineralization of water (beta-mesosaprobic zone) with transition to oligosaprobic zone (almost pure water). However, the presence of indicator organisms and their predominance in plankton samples give grounds to regard the studied water bodies as moderately polluted. That is why it is recommended to lower the recreational load on the urban water bodies by reducing wastewaters discharge from the urban storm sewage system.
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