Impact of mini-hydropower on the chemical composition of water and phytoplankton of the reservoirs of the Forest-Steppe of Ukraine

The peculiarities of the transformation of river ecosystems in the reservoir at the construction of mini-hydropower plants have been found out. It was also found that the large-scale hydraulic engineering in the second half of the XX century caused significant changes in environmental conditions in the waterbodies, which has led to negative dynamics of species richness of aquatic organisms in general and phytoplankton in particular. Reservoir building has a significant negative impact on river ecosystems: currents slow down and disappear, canals become silted up, harmful substances of household and industrial waste gradually accumulate. The transformation of rivers into lake-type reservoirs is also accompanied by changes in the species composition and structure of phytoplankton. Eutrophication of reservoirs becomes typical. All this leads to significant degradation in aquatic ecosystems.


Introduction
At the beginning and in the middle of the XX century, small hydropower plants (MHPPs) were built on a large number of small and medium-sized rivers in Ukraine.In the 1930-1980-s, there were estimated more than 1,500 of them, now there are only 98 ones [1].The building of reservoirs has caused radical changes in the ecological condition of rivers.Currents slowed down or disappeared completely on large sections of riverbeds, water exchange began to be determined by the mode of operation of hydroelectric power plants.This dramatically changed, first of all, the hydrological and hydrochemical regimes of rivers [2].Another consequence was siltation and accumulation of harmful substances from industrial, agricultural and domestic effluents, which also leads to changes in the hydrochemical regime, mainly due to increased concentrations of biogenic substances [3,4].
At the beginning of the 21st century, scientists around the world came to the conclusion that rivers need to be revitalized.In practice, it is necessary to study each individual case in detail, and only after that a conclusion can be made about the feasibility of existing hydraulic structures.In the United States, about 900 dams were dismantled between 1990 and 2015.France and Canada have also completed important dam removal projects.In Japan, the dismantling of dams began in 2012 [5][6][7].
Due to the plans for significant expansion of hydraulic engineering on small rivers of the Forest-Steppe of Ukraine, it is extremely important to find out the nature of the impact of MHPP on hydrochemical and hydrobiological regimes.An important aspect of the study is to identify patterns of composition, structure and functioning of hydrobionts under the conditions of transformation of river ecosystems into ponds and lakes.It appears extremely necessary to develop measures to minimize the harmful effects of HPPs for optimizing the state of reservoir ecosystems.This study is relevant in the framework of the European Framework Directive (WFD), adopted in 1992 in order to intercalibrate rivers and ensure the synchronization of research and the possibility of comparison and generalization of information [8,9].
The selection of material was carried out during expeditions in 2013-2020 in the warm period (from April to September).Data from hydrological yearbooks, kindly provided by the Geophysical Observatory of the Hydrometeorological Service of Ukraine in Kyiv, were used to study the dynamics of hydrological conditions of reservoirs in the region.Processing of hydrochemical water samples (64 samples) was carried out in a specialized chemical laboratory of the Ukrainian Hydrometeorological Institute UkrGMI.Temperature, hydrogen index (pH), and the amount of dissolved oxygen in water were determined directly on the reservoirs using the certified electronic device Ecotest-2000 (№ 19027-99 in the State Register).To examine phytoplankton, a Rutner bathometer was used (water samples were taken from a surface layer and a 1.5 m layer).Water was fixed with a 2% formalin solution.After settling, the sample volume was increased to 30-100 cubic using a siphon.Determination of the species and quantitative composition of algae was carried out jointly with the staff of the Institute of Hydrobiology of the National Academy of Sciences of Ukraine according to standard methods [10][11][12].Assessment of water quality of fishery reservoirs and reservoirs of complex purpose was carried out according to SOU 05.01-37-385: 2006 [13].Hygienic classification of bodies of water was carried out according to State Sanitary Rules and Regulations 2.2.4-171-10 2010 [14].Ecological assessment of surface water quality was carried out using generally accepted methods [10,15].

Results and discussion
Hydrochemical characteristics are important markers of the ecological status of waterbodies.In terms of the content of basic chemicals, the water of the reservoirs of the Forest-Steppe MHPP meets the state fishery standards (MPC) in most indices.A slight excess of the MPC for magnesium (in 1.09-1.15times) was observed in the Yurpil and Korsun-Shevchenkivsky reservoirs.Exceeding the MPC for this element together with calcium leads to increased water hardness.However, the increasing change of water hardness has a positive effect on the state of aquatic organisms, and the decrease -on the contrary.In terms of salt composition, the water of the studied reservoirs belongs to the bicarbonate-calcium type.Bicarbonates predominate in all investigated reservoirs (table 1).
According to the criteria of pollution by components of a salt composition, water in reservoirs, on average, was determined as having an index -2.0,which applies to class II of category 2 (good, clean).According to ecological and sanitary indices ( State Sanitary Rules and Regulations 2.2.4-171-10), the average value of the water quality category of reservoirs was determined as 3.66 (4.0), which applies to class IV of category 4 (satisfactory, slightly polluted).
Considering the biogenic substances, there was detected a significant excess of concentrations of nitrites and phosphates.Exceeding the MPC was observed for the following indices: in the reservoirs Koropetske (nitrite by 3.5 times), Steblivske (nitrite by 5.0 times) and Korsun-Shevchenkivske (nitrite by 20.0 times and ammonium nitrogen by 1.4 times).All this indicates significant decay processes or significant industrial and domestic effluents.Excess MPCs for phosphates are also related to this.The worst situation is observed in Steblivske (phosphates by 10.6 times) and Korsun-Shevchenkivske (phosphates by 10.2 times) reservoirs.The high levels  (by 3.4-8.6times) of phosphates were also detected in the rest of the reservoirs (table 2).
Phosphates are known to be primarily associated with detergents, so they are most abundant in reservoirs located in densely populated areas.The situation with biogenic substances is IOP Publishing doi:10.1088/1755-1315/1254/1/0120205 complicated by the regulation of river runoff arising as a result of reservoirs creation.Slowing down and disappearing of currents leads to the accumulation of harmful substances from runoffs.The lower parts of reservoirs and the reservoirs in the lower part of rivers are the most polluted.
The upper reaches of reservoirs are characterized by minimal indices of biogenic substances (nitrites -0.054 ± 0.001; phosphates -0.233 ± 0.02), middle parts (0.129 ± 0.01; 0.366 ± 0.03), lower parts (0.157 ± 0.015; 0.417 ± 0.04).These data clearly reveal the consequences of river regulation by the dams.The current is maintained in the upper reaches, and the nearest territory has a minimum amount of settlements.The middle and lower parts contain a predominant number of settlements, which are related to industrial, agricultural and domestic effluents.The decomposition of organic matter during eutrophication, which is the source of biogenic substance accumulation, is also the most significant there.
The above-mentioned hydrochemical parameters play an important role in the formation of groups of aquatic organisms, primarily phytoplankton.The phytoplankton excessive development (eutrophication) correlating with a significant excess of the MPC for biogenic substances, especially phosphates, was observed.A rectilinear dependence of the number of cyanophytes from the number of biogenic substances has been proved.On the other hand, an exaggeration of the amount of phosphates leads to inhibition of the activity of algae, especially diatoms.As shown above, the amount of phosphates depends on a number of reasons, among The amount of phosphates is not significant in the small Koropetske reservoir, but cyanophytes predominate there (figure 1).
The quantitative ratio of different taxonomic groups of algae in general and cyanophytes in particular differs in the reservoirs of the Gnily Tikich River.There is observed an another pattern.Thus, one of the two studied reservoirs on the Gnily Tikich River -Zvenigorodske one is located within the settlements and Lotashivske reservoir is located in sparsely populated areas.In addition, the Lotashivsky reservoir is upstream, and Zvenigorodske one is in the middle part.Having almost equal amounts of phosphates in these reservoirs, Zvenigorodske reservoir has 372 ± 23 thousand cells/dm 3 of cyanophytes while Lotashivske reservoir shows the minimum number of cyanophytes (56 ≥ ±11 thousand cells/dm 3 ) and up to their total absence in the upper and middle parts (figure 2).There are much more diatoms in the studied reservoirs: 5960 ± 35 thousand cells/dm 3 in Lotashivsky reservoir and 7400 ± 43 thousand cells/dm 3 in Zvenigorodske reservoir (figure 2).
Similar results were obtained on the Hirsky Tikich River.Yurpilske reservoir is located in the upper reaches in sparsely populated areas.There is a minimal amount of biogenic substances and no cyanophytes.The number of diatoms is 4586 ± 41 thousand cells/dm 3 .The Gordashivske reservoir is surrounded on two sides by settlements.Exceeding the MPC of phosphates was observed there by 4 times.The cyanophytes (1529 ± 16 thousand cells/dm 3 ) were observed throughout the reservoir.The number of diatoms was observed by 6-9 times lower (730 ± 23 thousand cells/dm 3 ) comparing the previous reservoirs (figure 3).
The Steblivske and Korsun-Shevchenkivske reservoirs are located in the densely populated lower part of the Ros River.As shown above, there was observed a significant exceedances of the MPC for biogenic substances in these reservoirs.This significantly affected the species composition and abundance of phytoplankton: the number of cyanophytes is 3272±45 thousand cells/dm 3 (Steblivske reservoir) and 5173 ± 52 thousand cells/dm 3 (Korsun-Shevchenkivske reservoir).The number of diatoms is much smaller: 1232 ± 21 thousand cells/dm 3 (Steblivsky reservoir) and 1366 ± 27 thousand cells/dm 3 (Korsun-Shevchenkivske reservoir) (figure 4).
The relation between the concentration of nitrites and phosphates and the amount of cyanophytes according to the results of correlation analysis is characterized as close (R = 0.861) (figure 5).The observed reservoirs were classified as β-mesosaprobic and β -mesosaprobic in terms of saprobity and as mesotrophic (predominant type) in terms of trophicity.

Conclusions
In terms of the content of basic chemicals, the water of the reservoirs of the Forest-Steppe MHPP meets the state fishery standards (MPC) in most indices.A slight excess of the MPC for magnesium (by 1.09-1.15times) was observed in the Yurpilske and Korsun-Shevchenkivske reservoirs.According to the criteria of pollution by components of a salt composition, water in reservoirs, on average, was determined as having an index -2.0,which applies to class II of  Hydrochemical indices of some basic chemical parameters and most biogenic substances exceeded or reached the MPC.
Thus, in the Koropetske reservoir the excess of biogenic substances is by 1.5-2.2times; in the reservoirs of the Gnily Tikich River -by 1.2-12.0times; in the reservoirs of the Gorsky Tikich River -by 1.4-3.3times; in the reservoirs of the Ros River -by 1.4-40 times.This in turn causes excessive development of phytoplankton, leading to "blooming" of water with excessive development of cyanobacteria.
A close relation has been figured out between the concentration of cyanobacteria and the amount of biogenic substances in the water, which depends on the territorial location of the reservoir.The effect of cyanobacteria on certain groups of algae and other aquatic organisms has been elucidated.Four groups of algae (Euglenophyta, Chrysophyta, Chlorophyta, Bacillariophyta), the overall density of which reaches 7000 thousand cells/dm 3 , are found in case of absence or minimum number of cyanobacteria (up to 3000 thousand cells/dm 3 ) in the reservoirs.This was observed in the Yurpilske, Lotashivske and Zvenyhorodske reservoirs.On the contrary, if the density of Cyanophyta increases over 5000 thousand cells/dm 3 , it remains only two groups of algae (Chlorophyta and Bacillariophyta) with an overall density of 1000-3000 thousand cells/dm 3 .This was observed in the reservoirs Steblivske and Korsun-Shevchenkivske.During the intensive "blooming" of water in the studied reservoirs, the suffocations periodically occurred, leading to the death of fish, crustaceans and other aquatic organisms.

Figure 1 .
Figure 1.The ratio of the main groups of phytoplankton (a) and the amount of biogenic substances (b) in the Koropets reservoir.

Figure 2 .
Figure 2. The ratio of the main groups of phytoplankton (a) and the amount of biogenic substances (b) in the reservoirs of Hnily Tikich River.

Figure 3 .
Figure 3.The ratio of the main groups of phytoplankton (a) and the amount of biogenic substances (b) in the reservoirs of Hirsky Tikich River.

Figure 4 .
Figure 4.The ratio of the main groups of phytoplankton (a) and the amount of biogenic substances (b) in the reservoirs of Ros River.

Figure 5 .
Figure 5. Correlation between the density of cyanophyte groups and the amount of biogenic substances in the studied reservoirs.

Table 1 .
Main hydrochemical characteristics of the MHPP reservoirs in the Forest-Steppe zone of Ukraine.