Estimation of eutrophication processes of water objects of Stryzhen river

Abstract

According to the results of hydrological, hydrochemical and hydrobiological studies (May-October 2017) the estimation of eutrophication processes in water bodies of the Stryzhen River within the city of Chernihiv is presented. The seasonal dynamics of abiotic and biotic factors which form the water quality and trophic level of water bodies has been analyzed. It was found that the features of the hydrological regime of the Stryzhen River in 2016–2017 were low water content, the offset of the passage and a decrease in the duration of the spring flood. The volume of spring runoff was very low and reached 65% of the total annual runoff. Due to the moderate and heavy rains the water level rose slightly to 5-10 cm. It was established that at this time the movement of water in the channel sections of the reservoirs was absent. The important hydrodynamic characteristics of the ponds from an environmental point of view are mixing and wind-generated waves which cause horizontal and vertical migration of substances and enhance the self-purification capacity of water in the ponds. In the spring and summer, when forming a direct temperature stratification, the thickness of the surface layer of water which took part in the mixing processes in the first, second and third ponds was 0.5, 1-2 and 1 m, respectively. The thermocline divided the water column vertically into two parts, which differed from each other in temperature values, as well as in dissolved oxygen content and saturation, pH values, concentrations of ammonium nitrogen, nitrate ions and inorganic phosphorus. The surface layer of water was characterized by enough concentration of dissolved oxygen, whereas in the water column below the thermocline its deficiency was observed. In spring and summer there was alkalinization of the water in the surface layer of the ponds, which was caused by displacement of the hydrocarbon-calcium balance as a result of photosynthetic activity of algae. Under anaerobic conditions the concentration of ammonium nitrogen and inorganic phosphorus in the bottom water layer increased to 1.29–9.99 mgN/dm³ and to 0.39–4.76 mgP/dm³ respectively, the maximum values of which were observed in the first pond. During studies of the ponds of the Stryzhen River, it was found that phytoplankton was characterized by a rather rich species diversity of algae and high quantitative development rates that reached the level of water “blooming” due to the massive development of blue-green algae in July and October 2017. The species composition of macrozoobenthos in the studied ponds of the Stryzhen River was very poor, in total 35 species were found. The most favorable conditions for the development of benthic organisms were found in the river section above the third pond and below the first pond before the Stryzhen River flows into the Desna River. Bioindication analysis showed that the zoobenthos in the studied ponds was in a depressed state (TBI = 1-2), but in flowing river sites it was in the best conditions (TBI = 8). According to most of the studied indicators, water quality corresponded to categories 2-5, but content of dissolved oxygen and an inorganic phosphorus belonged to category 6-7 environmental water quality classification. According to the trophic index the studied ponds belonged to mesotrophic, but during “blooming” of water - to hypertrophic.