Assessment of the water quality of the Don River downstream from its confluence with the Temernik River based on the saprobity index and bioassay data

The Don River is one of the largest rivers in the East European Plain. The right tributary of the Don River—the Temernik River—flows through the metropolis of Rostov-on-Don and affects the water quality of the main watercourse. This study was aimed at the assessment of the organic pollution and toxicity of water in two horizons (0.3 and 9.0 m) of the Don River downstream from its confluence with the Temernik River using the saprobic index S and biotesting. Observations were carried out for 7 months in 2019. The data on the quality of the Don River water have been obtained from the combined results of two biological methods for the first time. The dynamics of changes in the quality of the river water in the course of 7 months is presented. The samples for the investigation were collected сoncurrently. Comparison of the water quality in two horizons (surface and deep) of the river section based on the data obtained using two biological methods has been conducted for the first time. The saprobity results for the phytoplankton indicated the presence of organic water pollution at both levels of the Don River during all 7 months of the study. The saprobity index values ranged from 2.0 to 3.6. Water quality was characterized as ranging from moderately polluted to polluted. This corresponds to α-β-mesosaprobic level. The maximum level of organic pollution according to the saprobity index was recorded in September for both horizons. Saprobity increases with an increase in the proportion of blue-green algae in the phytocenosis. The relative abundance of blue-green algae ranged from 45.83 to 77.77 %. The surface horizon had higher organic pollution during the warm period (May–September), and for the deep horizon it was higher during the cold period (April, October, and November). The toxic effect of the river water was manifested in the inhibition of the growth of Chlorella vulgaris. Water toxicity over the investigated period (7 months) was inconsistent. In each season, it only appeared for one month. Water toxicity at the both horizons was recorded in spring (April), summer (August), and autumn (November). High saprobity and toxic effect of the water did not always coincide. The concurrent presence of these characteristics is typical for the deep horizon. Toxicity fluctuations are apparently associated with the temporal pattern of the Temernik River pollution by the metropolis waste waters and with the measures for the Temernik River improvement taken by the municipal authorities.

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