Planktonic Invertebrates in the Assessment of Long-Term Change in Water Quality of the Sorbulak Wastewater Disposal System (Kazakhstan)

: The multicomponent composition of wastewater makes it challenging to assess its quality objectively, but the last one is a prerequisite for the safe re-use of wastewater. The solution to this problem should be aimed at ﬁnding criteria that make it possible to increase the objectivity of assessing the water quality of reservoirs with multicomponent pollution. This work analyzes the water quality of the Sorbulak wastewater disposal system in the summer of 2017, based on chemical variables and zooplankton structure and assess the long-term changes in the water quality of Sorbulak. According to the Kruskal–Wallis test, in 2017, the di ﬀ erences between the studied water bodies in the content of nutrients and heavy metals were mostly insigniﬁcant. From 2000–2002 to 2017, nitrate, nitrite nitrogen, and heavy metals in Sorbulak signiﬁcantly decreased. Zooplankton communities consisted of a relatively small number of eurybiontic species resistant to environmental factors. The variability of the quantitative variables of zooplankton was associated with the nutrients. Males dominated the population of the cyclopoid copepods Acanthocyclops trajani . In 2000–2002 individuals with morphological anomalies were found in cyclopoid copepods populations, but were absent in 2017. The appearance of individuals with morphological anomalies was associated with copper or lead. The chemical data and structure of zooplankton communities indicated that the toxic pollution of Sorbulak decreased by 2017 compared to 2000–2002. Our results demonstrate that the structural variables of zooplankton communities could be successfully used to assess the water quality of water bodies with mixed pollution. We recommend using not only the traditional set of biological variables (abundance, biomass, diversity indices, and the average mass of an individual), but also data on the structure of species dominance, the sex structure of copepod populations, and the presence of individuals with morphological anomalies for monitoring of water bodies with mixed pollution. total zooplankton, Shannon Ab, Shannon Bi, ∆ -Shannon, species number, average individual mass).


Introduction
Due to socio-economic, demographic, and climatic reasons, the share of re-use wastewater for irrigation increases from year to year in different parts of the world [1,2] and especially in arid areas. However, such a policy of using water resources poses a serious threat to public health, due to the accumulation of toxic compounds in food [3][4][5], the risk of contracting infectious diseases [6], and the Wastewater was planned to be used for irrigation of about 22.8 thousand hectares of land to prevent overflow and breakthrough of the barrier dam of Sorbulak; less than 3.0 thousand hectares remained by 2000. Irrigated fields are situated several kilometers northwest of the shoreline. The watering of fields is carried out, due to the canal, which originates in the western bay of Sorbulak. For an emergency, the Right-Bank Sorbulak Canal (RBSC) with a length of 63 km was additionally built in the 1990s. There are eight ponds on the canal referred to as RBSC ponds. Pond No. 7 is the largest. Pond No. 8 is the last in the system. In case of an emergency (for example, when the volume Wastewater was planned to be used for irrigation of about 22.8 thousand hectares of land to prevent overflow and breakthrough of the barrier dam of Sorbulak; less than 3.0 thousand hectares remained by 2000. Irrigated fields are situated several kilometers northwest of the shoreline. The watering of fields is carried out, due to the canal, which originates in the western bay of Sorbulak. For an emergency, the Right-Bank Sorbulak Canal (RBSC) with a length of 63 km was additionally built in the 1990s.
There are eight ponds on the canal referred to as RBSC ponds. Pond No. 7 is the largest. Pond No. 8 is the last in the system. In case of an emergency (for example, when the volume of water in Sorbulak approaches the maximum permissible level), part of the sewage is discharged through the canal into ponds, bypassing Sorbulak. Then, from the last pond, water is released into the Ili River through the RBS canal.
Sorbulak is shaped like an irregular triangle. RBSC ponds are with an indented shoreline. According to the average depth (Table 1), Sorbulak and ponds belong to the type of shallow water bodies [30]. The water area and volume of RBSC ponds vary significantly and depend on the discharged wastewater volume. Sorbulak is characterized by relatively high water transparency, while the values of this variable in ponds are low.

Field Sampling
The studies of Sorbulak were performed in the summer of 2000, 2001, 2002, and 2017. RBSC ponds were studied only in the summer of 2017. A total of 45 zooplankton samples were taken altogether with samples to determine nutrients and heavy metals. The measures of the temperature and pH of the surface water layers were taken in the field environment by using the Hanna HI 98129 instrument in parallel with sampling. The transparency of the water was measured using a Secchi disk. Coordinate referencing of the stations was done by Garmin eTrex GPS-navigator.
Zooplankton samples were collected using a small Juday plankton net (input diameter 12 cm, mesh size 64 µm) by pulling it from the bottom to the surface. Surface water samples were taken at the same stations to determine the total dissolved solids (TDS), total hardness, nitrites, nitrates, ammonia, phosphates, iron, manganese, fluorine, silicon, heavy metals, and permanganate index [31]. We collected chemical and biological samples according to the state water quality monitoring program, approved in the Republic of Kazakhstan [32] for shallow unstratified water bodies [33]. Samples for analysis of TDS were taken in 1-L plastic containers. Samples for determining the nutrients were taken in 0.5 L glass containers and fixed with 1 mL of chloroform. Samples for determining the amount of easily oxidizable organic matter (permanganate index PI) were taken in 0.3 L glass containers and fixed with two ml concentrated sulfuric acid. Water samples for analysis of heavy metals were taken in 0.5 L plastic containers and fixed with concentrated chemically pure nitric acid. From 2000 to 2002, only four heavy metals (Cd, Cu, Pb, and Zn) were determined, while in 2017, additionally Cr and Ni. All samples were stored in a refrigerator and delivered to the laboratory within 1-2 days after collection.

Laboratory Analysis
Conventional methods of chemical analysis of water samples were used [34,35]. All water samples were analyzed in triplicate or quadruplicate. Nitrite nitrogen, nitrate nitrogen, ammonium nitrogen, phosphates, silicon, manganese, and iron were determined photometrically. Depending on the type of analysis, Griss's or Nessler's reagents, metallic cadmium, ammonium molybdate in combination with ascorbic or sulfosalicylic acid were used. The permanganate index (PI) was determined in acidic conditions by the Kubel method. Method sensitivity is 0.002 mg/dm 3 ; method accuracy is ±4%. The total hardness was determined by the volumetric complexometric method with black eriochrome or black chromogen.
Analysis of samples for heavy metals was carried out in the analytical laboratory "KAZEKOANALIZ" (accreditation certificate No. KZ.I.02.1017) according to the Interstate standard [36]. Heavy metal measuring was performed by mass spectrometry with inductively coupled plasma using Agilent 7500 A manufactured by Agilent Technologies, Santa Clara, USA (National Standard RK ISO). Abundance Sensitivity of Agilent 7500 A: Low Mass < 5 × 10 −7 , High Mass < 1 × 10 −7 .
Zooplankton samples were processed with standard methods [37,38] using guides to the species identifications [39][40][41][42]. A sample was brought to a certain volume (150-500 cm 3 ) to calculate the quantitative variables of planktonic invertebrates. After thorough mixing, three sub-samples were taken from the sample using a 1 mL stamp-pipette. In this sub-sample, all encountered individuals and age stages of certain species (the most numerous) were counted in Bogorov's cell. Then the sample was concentrated to a volume of 125-150 cm 3 . Three sub-samples were retaken from it, where less abundant age stages or species were counted. The whole procedure was repeated once more, while the sample was concentrated to a volume of 50 cm 3 . In the end, the sample, with its volume of 20-25 mL, was viewed in its entirety for counting large and rare species of planktonic invertebrates. The results of counting individuals are recalculated per 1 m 3 using the formula (separately for each sample dilution): where N is the abundance (ind./m 3 ), n is the number of individuals in a portion (specimens), V1 is the dilution volume (cm 3 ), V2 is the subsample volume (cm 3 ), V3 is the filtered water volume (m 3 ). The filtered volume of water was calculated by the formula: where h is the length of the net pulling (water column height), and r is the radius of the inner ring of the Juday net. The total abundance was found for each species of planktonic invertebrates. For each crustacean species, the total abundance was calculated by summing the abundance of individual age and size stages. The total abundance of zooplankton was determined by summing up the abundance of all species found in the sample.
For all species of planktonic invertebrates encountered, the mass of an individual was calculated according to formulas specific to each species [38]. We counted the number of mature females, females with eggs, and males separately to describe the sex structure of copepod populations.

Statistical Analysis
We applied statistical methods to describe the structure and assess the similarity of the species composition of zooplankton communities in the studied water bodies. Statistical analysis of the relationship between ecological and biological variables makes it possible to assess the indicator role of planktonic invertebrates in assessing the water quality of aquatic ecosystems.
The number of species per sample, an average individual mass of an organism, and ∆-Snannon were calculated to describe the zooplankton structure. An average individual mass of an organism (mg) was calculated as the total biomass divided by the total abundance of zooplankton for each sample. Shannon index was calculated both based on the abundance and the biomass of species in the sample [43,44] using Primer 6 Software (https://primer.software.informer.com/6.0/) [45]. The first version of the index is designated as Shannon Ab (bit/ind.), the second one as Shannon Bi (bit/mg) for the convenience of distinguishing them. We calculated the values of ∆-Shannon as an arithmetical difference between Shannon Bi and Shannon Ab. ∆-Shannon characterizes the structure of the dominance of species. In large-sized communities, the distribution of species by abundance is more uniform than by biomass. Accordingly, the Shannon Ab values are higher than the Shannon Bi, and ∆-Shannon is positive [46][47][48][49][50][51][52]. In disturbed communities, small species are dominated [53]; the Shannon Bi is higher than the Shannon Ab, and the values of ∆-Shannon are negative.
The calculation of community composition similarity was doing as the network analysis in JASP (Jeffrey's Amazing Statistics Program) 0.9.0.0 (https://jasp-stats.org/) on the botnet package in R-Statistica (https://www.r-project.org/) [54]. JASP plot was created as a calculation result of the percentage of an abundance of species on the level similarity 50% and as significant only when p < 0.05. The line thickness between stations reflect the correlation value; blue is positive, red is negative.
Statistical data analysis was performed using the Statistica 10 Software [55]. We calculated mean values with standard errors for all variables (pH value, Total Dissolved Solids, hardness, PI, Si, Fe, Mn, Cd, Cr, Cu, Ni, Pb, N-NO 2 , N-NO 3 , N-NH 4 , PO 4 , abundance, and biomass of Rotifera, Cladocera, Copepoda, total zooplankton, Shannon Ab, Shannon Bi, ∆-Shannon, species number, average individual mass), excluding the sex ratio, because females or males of copepods were absent in some samples. The sex ratio was calculated for each water body for each sampling date by dividing the average abundance of males by the average abundance of females. Differences in the mean values of chemical and some biological variables between the periods 2000-2002 and 2017 were performed using the Kruskal-Wallis test.
Principal component analysis (PCA) was performed using CANOCO 4.5 Software (https://canoco. software.informer.com/4.5/) based on 2000-2002 and 2017 [56]. We used PCA as a linear indirect gradient analysis because our data is represented by short lists of biological and environmental variables. PCA is also used to identify critical environmental gradients from environmental datasets. The identified gradients became the basis for a concise description and visualization of different taxa preferences of the habitat (niches) using the ordination diagram. For multivariate analysis, we took the external factors (temperature, depth, TDS, PI, N-NO 2 , N-NO 3 , N-NH 4 , PO 4 , PI, Si, Fe, Mn, Cd, Cr, Cu, Ni, Pb) and the structural variables of zooplankton communities (quantitative variables of Rotifera, Cladocera, Copepoda, total zooplankton, Shannon Ab, Shannon Bi, ∆-Shannon, species number, average individual mass). Moreover, we used the abundance of dominant planktonic invertebrates (Daphnia galeata, D. magna, D. pulex, Bosmina longirostris, Acanthocyclops trajani, Cyclops vicinus) and the sex ratio in the populations of cyclopoid copepods A. trajni and C. vicinus.

Hydrochemical and Toxicological Characteristics of the Sorbulak Wastewater Disposal System
According to the chemical data (Table 2), in the summer of 2017, the water was alkaline, soft, and fresh in ponds, brackish and medium hardness in Sorbulak. The maximum concentrations of easily oxidizable organic matter (PI), silicon, nitrite nitrogen, nitrate nitrogen, phosphates, copper, and zinc were recorded in pond No. 7; iron in pond No. 8; ammonium nitrogen in Sorbulak. The content of manganese, cadmium, chromium, nickel, and lead in all water bodies was at a very low level. According to the Kruskal-Wallis test, statistically significant differences were found between the average content of nitrite nitrogen in Sorbulak and pond RBSC No. 7 (p = 0.002), and between pond RBSC No. 7 and No. 8 (p = 0.036).  (Table 3). According to the Kruskal-Wallis test, the content of easily oxidizable organic matter (p = 0.0001) and ammonium nitrogen (p = 0.001) increased and the content of Pb (p = 0.01), Zn (p = 0.006), Cu (p = 0.0009), Cd (p < 0.001), nitrite nitrogen (p < 0.02), and nitrate nitrogen (p < 0.002) decreased significantly from 2000-2002 to 2017.

The Structure of Zooplankton Communities
Forty-six species were recorded in the zooplankton, including twenty-three rotifers, fifteen cladocerans, and eight copepods (Table 4). In Sorbulak, the species richness of planktonic invertebrates in 2017 and 2000-2002 was the same, and higher than in the RBSC ponds. Rotifers Hexarthra mira, Keratella quadrata dispersa, Polyarthra sp., cladocerans Bosmina longirostris, Chydorus sphaericus, Daphnia galeata, Daphnia magna, Daphnia pulex, copepods Acanthocyclops trajani, Cyclops vicinus, and Sinodiaptomus sarsi were found in zooplankton during all study periods. According to the JASP network plot (Figure 2), the highest similarity was found between planktonic invertebrates from ponds No. in the species composition were mostly recorded between the community of pond No. 7 and other water bodies. Accordingly, pond No. 7 can be marked as a critical point for all calculated relationships, which shows some specific features of this water body. planktonic invertebrates from ponds No. 7 and 8 in 2017 and between pond No. 8 in 2017 and Sorbulak in 2000 (blue lines). A significant similarity was also established between the species composition of zooplankton in Sorbulak in 2001 and 2002. In 2017, the zooplankton species composition in Sorbulak was somewhat similar to all other sampling sites. The most significant differences were revealed between the species composition in pond No. 7 in 2017 and Sorbulak in 2000 (red line). Differences in the species composition were mostly recorded between the community of pond No. 7 and other water bodies. Accordingly, pond No. 7 can be marked as a critical point for all calculated relationships, which shows some specific features of this water body. The abundance of zooplankton had more than a fourfold range of fluctuations depending on the water body and research period ( Table 5). The highest zooplankton abundance was in Sorbulak in 2001 and pond No. 7 in 2017. The average zooplankton biomass varied by 13.0 times. Its maximum values were recorded in 2017 in pond No. 7. Copepoda mostly dominated. Rotifera or Cladocera were subdominants. Cladocera dominated in biomass, with Copepoda playing a minor role.
The composition of the dominant species in the zooplankton slightly varied over the years (Table  6). Most often, cyclopoid copepods Acanthocyclops trajani, cladocerans of the genus Daphnia, and the rotifers Hexarthra mira dominated. The ratio of the above species in quantitative variables of zooplankton varied depending on the water body. In Sorbulak, the proportion of Hexarthra mira and Daphnia pulex was higher than in the zooplankton communities of RBSC ponds. Simultaneously, the The abundance of zooplankton had more than a fourfold range of fluctuations depending on the water body and research period ( Table 5). The highest zooplankton abundance was in Sorbulak in 2001 and pond No. 7 in 2017. The average zooplankton biomass varied by 13.0 times. Its maximum values were recorded in 2017 in pond No. 7. Copepoda mostly dominated. Rotifera or Cladocera were subdominants. Cladocera dominated in biomass, with Copepoda playing a minor role. The composition of the dominant species in the zooplankton slightly varied over the years (Table 6). Most often, cyclopoid copepods Acanthocyclops trajani, cladocerans of the genus Daphnia, and the rotifers Hexarthra mira dominated. The ratio of the above species in quantitative variables of zooplankton varied depending on the water body. In Sorbulak, the proportion of Hexarthra mira and Daphnia pulex was higher than in the zooplankton communities of RBSC ponds. Simultaneously, the dominance of Acanthocyclops trajani and Daphnia magna in ponds was more pronounced than in Sorbulak. In some years, rotifers Polyarthra, Asplanchna girodi, and Keratella quadrata were also included in the dominant complexes of the surveyed water bodies. The abundance of the dominant species Acanthocyclops trajani varied significantly over the years of research, with extremums in pond No. 7 in 2017 and Sorbulak in 2001 (Table 7). Males dominated the population of A. trajani, with the minimum values in the RBSC ponds in 2017. The abundance of Cyclops vicinus was relatively low. Cyclops was not found in the RBSC ponds in 2017. Except for 2002, the sex ratio in the population of this species was equal or female-dominated.                  Remarkably, that individuals with morphological anomalies were recorded in the populations of Cyclops vicinus and Acanthocyclops trajani only in 2000-2002. Most often, there was a shortening of one of the furcal rami, as well as shortening and deformation of the furcal setae ( Figure 3). The abundance of such individuals was low, no more than 10-40 ind./m 3 , but they were found in 50-60% of the samples during the indicated research period. In the summer of 2017, individuals with morphological abnormalities were absent in copepod populations.

Nonparametric Correlation Analysis
The values of Spearman's rank correlation coefficients (rS) between environmental and biological variables are presented in Table 9. For rotifers, positive relationships were recorded with nitrite nitrogen, nitrate nitrogen, and TDS; negative relationships with copper and zinc content. For cladocerans, positive associations were revealed with the nitrite, nitrate, ammonium nitrogen,

Nonparametric Correlation Analysis
The values of Spearman's rank correlation coefficients (r S ) between environmental and biological variables are presented in Table 9. For rotifers, positive relationships were recorded with nitrite nitrogen, nitrate nitrogen, and TDS; negative relationships with copper and zinc content. For cladocerans, positive associations were revealed with the nitrite, nitrate, ammonium nitrogen, phosphates; negative relationships with TDS, manganese, cadmium, and lead content. For copepods, positive associations were established with nutrients, copper, zinc, and lead. The total quantitative variables of zooplankton showed a positive relationship with the content of nutrients and silicon; the relationship with the cadmium concentration was negative. The average mass of an individual increased with depth, as the r S values show. Positive relationships were recorded between the ∆-Shannon and Shannon Ab indices and depth, transparency, and TDS; the relationship between the Shannon Ab index and silicon was negative.

PCA Analysis
The PCA biplot (Figure 4) shows that total zooplankton biomass and Cladocera biomass were mainly associated with nitrite nitrogen, phosphates, to a lesser extent with depth, and PI; relationship with TDS was negative. The content of nitrate nitrogen partially determined Copepoda biomass. The values of the average mass of an individual and ∆-Shannon index were most closely related to ammonium nitrogen content and PI. Rotifera biomass and Shannon Bi values were negatively correlated with ammonium nitrogen, depth, and partially PI. The distribution of zooplankton abundance in the gradient of external factors was similar to the distribution of biomass.
Concerning toxic pollution ( Figure 5), Copepoda and Rotifera biomass was most closely associated with copper and zinc and negatively correlated with cadmium. Cladocera biomass and the average mass of an individual in zooplankton communities were in a negative correlation with the content of lead, cadmium, and TDS. On the contrary, Shannon Bi was positively associated with lead and partially with depth.    As Figure 6 demonstrates, the abundance of Bosmina longirostris, Daphnia pulex, D. magna, D. galeata, and Cyclops vicinus were negatively associated with ammonium nitrogen. The relationship between Acanthocyclops trajani and nitrites as well as nitrates was positive, and the negative one was with depth and TDS. The abundance of D. magna was predominantly associated with phosphates and partly with ammonium nitrogen. abundance of D. magna showed a negative dependence on cadmium and lead; A.trajani-on cadmium content. Figure 8 demonstrates that the abundance of mature individuals, individuals with morphological anomalies, and the dominance of males in the populations of A. trajani was associated with lead, and partially copper; the connection with TDS was negative. The total abundance and abundance of mature individuals in C. vicinus populations were associated with lead and zinc, but the abundance of individuals with morphological abnormalities was influenced by copper. The PCA biplot shows (Figure 7) that the abundance of Daphnia longispina, D. galeata, and Cyclops vicinus was predominantly associated with lead; there was a negative connection between the last one and zinc. The abundance of D. pulex and Bosmina longirostris was correlated with copper. The abundance of D. magna showed a negative dependence on cadmium and lead; A.trajani-on cadmium content.  Figure 8 demonstrates that the abundance of mature individuals, individuals with morphological anomalies, and the dominance of males in the populations of A. trajani was associated with lead, and partially copper; the connection with TDS was negative. The total abundance and abundance of mature individuals in C. vicinus populations were associated with lead and zinc, but the abundance of individuals with morphological abnormalities was influenced by copper.

Chemical Variables
According to chemical data, in 2017, the content of pollutants in the water of pond No. 7 was higher than in Sorbulak, but the differences in mean values were mostly insignificant. With the same composition of wastewater entering all reservoirs, some chemical parameters variability can be associated with the natural processes of self-purification of water bodies. The large volume of the water mass of Sorbulak [21] contributes to the more substantial dilution of the incoming pollutants in comparison with shallow-water ponds. The accumulation of contaminants in the bottom sediments of Sorbulak [9] leads to a decrease in their concentrations in the water column, as was observed in other large water bodies [57].
In the long-term aspect, the toxic pollution of Sorbulak varied significantly, which is generally typical for such water bodies [58]. Therefore, the content of Pb, Cd, Zn, Ni, and Co in water in 1985 did not exceed the maximum permissible level [26]. In 1997-2000, the content of heavy metals increased: Zinc up to 0.017-0.056, copper up to 0.004-0.029, lead up to 0.034-0.082, cadmium up to 0.004-0.009 mg/dm 3 [59]. According to the Kruskal-Wallis test, from 2000-2002 to 2017, the content of nitrate, nitrite nitrogen, and heavy metals in Sorbulak water decreased significantly, which may be explained by socio-economic reasons. Despite the growth in Almaty and its environs, during the period under review, the volume of discharged wastewater decreased from 135.7 to 101.5 million m 3 [21].
The share of industrial wastewater also decreased from 35% in 1997-1998 up to 11% in 2004, due to the closure of many industrial enterprises in Almaty. Moreover, the high abundance and biomass of phytoplankton in wastewater reservoirs (5891.7-6287.3 million cells/m 3 and 4.8-5.6 g/m 3 ) in the summer of 2017 [51] were one of the significant factors contributing to the purification of the water column of Sorbulak and ponds from heavy metals. It is known that the absorption of heavy metals by algae can be more intense than the accumulation in bottom sediments [60,61].
The eurybiontic species listed above are resistant to the influence of various external factors. A. trajani is common species in the water bodies of Kazakhstan with a high level of organic and toxic pollution [28,52,59,66,67]. Species of the genus Bosmina are resistant to industrial pollution [68]. The sensitivity of Daphnia species to heavy metals is species-specific and inversely proportional to body size [69]. D. pulex can physiologically adapt to a wide range of external conditions [70], including toxic pollution [71]. D. magna is the most tolerant of toxic effects among the species of this genus [72]. A. trajani, D. pulex, and D. magna dominated zooplankton in different periods of research, starting from the construction of Sorbulak [26], which confirms their high ecological plasticity.
In addition to toxic pollution, Cyanobacteria have a pronounced effect on the species composition of zooplankton. Filamentous cyanobacteria disrupt food collection in Daphnia by mechanically acting on the filter apparatus with long trichomes [73]. The intensity of this intervention depends on the water temperature, and body size of Daphnia. D. longispina is able better to break apart cyanobacterial trichomes when the water temperature rises. D. magna morphologically adapts to high-trophic water bodies dominated by filamentous blue-green algae, due to the thickening of the filter apparatus bristles [74]. Species of the genus Microcystis affect both the phytoplankton community itself, due to the deterioration of illumination and the zooplankton and ichthyofauna, due to the release of toxins and changes in pH [75]. The reaction of Daphnia to toxins of blue-green algae is species-specific [76]. Experimental and field studies have shown that Daphnia thrives in the presence of green algae, cryptophytes, chrysophytes, or diatoms [77][78][79].
In 2017, cyanobacteria were the absolute dominants in Sorbulak with average biomass of 4.9 g/m 3 [51]. The biomass of green algae was almost fifteen times less (0.33 g/m 3 ). In pond No. 7, in addition to blue-green algae with biomass of 4.7 g/m 3 , dinophytic algae were also represented with biomass of 0.55 g/m 3 . In pond No. 8, phytoplankton contained only cyanobacteria, with biomass of 4.8 g/m 3 . These differences in the composition of microalgae can be the reason for the change in the proportion of species of the genus Daphnia in the zooplankton of the surveyed water bodies. The share of D. magna in the zooplankton biomass increased from 10.6% in Sorbulak to 67.6% in pond No. 8 and 76.0% in pond No. 7. The share of D. pulex decreased from 72.7% in Sorbulak to 21.2% and 11.7% in ponds No. 8 and No. 7, respectively. It can be assumed that D. pulex is more sensitive to the influence of cyanobacteria, as its role in zooplankton decreased in ponds, where the dominance of blue-green algae was almost absolute. A significant decrease in the proportion of D. galeata in zooplankton by 2017 may be associated with the high sensitivity of this species to blue-green algae [80].
Bosmina Bosmina was also absent [26]. D. magna is able to minimize the food stress caused by the filamentous cyanobacteria by setae thickening [74]. Although B. longirostris is common species in cyanobacterial blooms [81], it is probably unable to compete with such a large cladoceran species as D. magna.

Structural Variables
According to the Shannon index values (1.74-2.41 bit/ind. and 1.70-2.09 bit/mg in Sorbulak and 1.19-1.26 and 1.16-1.17 in ponds), the diversity of zooplankton of the surveyed water bodies varied from low to moderate levels [17]. The Shannon index values were similar to those established for zooplankton communities in water bodies with a pronounced bloom of cyanobacteria [82].
Disturbances in the structure of species dominance can be additional indicators of stress factors for zooplankton. The structure of species dominance in communities can be described using Clarke's W-statistics [83][84][85] or ∆-Shannon index [46][47][48][49][50][51][52]. In communities with large-sized species, the distribution of species by abundance is more uniform than by biomass. Accordingly, the Shannon Ab values are higher than the Shannon Bi, and ∆-Shannon is positive [46][47][48][49][50][51][52]. In disturbed communities, small species are dominated [53,[83][84][85]; the Shannon Bi is higher than the Shannon Ab, and the values of ∆-Shannon are negative. Positive values of the ∆-Shannon index (Table 8) indicated the absence of pronounced stress for zooplankton communities in the surveyed wastewater water bodies. This is due to the predominance of planktonic invertebrates resistant to a wide range of organic and toxic pollution. The convergence of the biomass and abundance curves, with a value of the ∆-Shannon index of 0.03 (Table 8), was only recorded in the zooplankton of pond No. 7 in 2017. It can be associated with a very high copper content in the water in this period ( Table 2).

The Influence of External Factors on the Quantitative Variables and Structure of Zooplankton Communities
Correlation and PCA analysis showed that the main factors of the spatiotemporal variability of the quantitative variables of zooplankton communities were the content of nutrients. This is because nutrients affect the composition and quantity of planktonic algae [86,87]; the latter, in turn, are food for planktonic invertebrates, especially Daphnia [88,89]. Copepoda was associated with copper and zinc content, which generally indicated their resistance to these metals. However, as a more toxic element [90], cadmium had a negative effect on copepods.
The response of planktonic invertebrates to toxic pollution is species-specific. Acanthocyclops trajani and Cyclops vicinus showed resistance to heavy metals. Our results confirm the literature data on the more excellent resistance of cyclopoid copepods to heavy metals compared to Daphnia [91]. In experiments on the assessment of chronic toxicity [92], cadmium suppressed the development of the cladocera Moina monogolica Daday at a concentration of 0.003 mg/dm 3 . In the period up to 2002, the cadmium content in Sorbulak was higher than the given value. However, cadmium, like lead, did not adversely affect Daphnia longispina, Daphnia pulex, and Daphnia galeata. The absence of a pronounced toxic effect of heavy metals on cladocerans in the wastewater reservoirs of the RBSC can be explained by various reasons, including the multidirectional interaction of pollutants with each other [12], the protective effect of the chemical composition of water [93] and food availability [94].
The influence of external factors on the sex structure of planktonic invertebrates is poorly studied. Typically, females are slightly more numerous than males in populations of cyclopoid copepods [95][96][97]. This is explained by the fact that one male can fertilize several females, and there is no need for an equal sex ratio [39,98]. Deviations from the normal sex ratio in crustacean populations are induced by stress factors, including changes in water temperature, TDS, and pH [99][100][101]. We have found that males' dominance in populations of cyclopoid copepods reaches an extremely high level (from 3-5 to 50 males per female) under conditions of intense organic and toxic pollution of the environment [28,59,66,67,86,102]. Considering the reference data, the recorded dominance of males in the Acanthocylops trajani population indicated the presence of stress factors for this species during all periods of the study of the wastewater reservoirs. According to the PCA biplot, disturbances in the sex structure in the A. trajani populations were predominantly associated with heavy metals concentrations. According to sex theory [100], it can be considered an adaptive response of A. trajani to stressful environmental conditions. Individuals with morphological anomalies were recorded continuously in Acanthocylops trajani and Cyclops vicinus populations from Sorbulak and ponds in 2000-2002 ( Figure 3) and 1998 [62]. They were completely absent in the summer of 2017. According to the PCA biplot, the appearance of individuals with morphological abnormalities in the cyclopoid populations was associated with copper or lead. Copper is highly toxic to most living organisms [12,103,104]. For cyclopoid copepods, copper is more harmful than zinc, chromium, and nickel [105].

Indicator Role of Zooplankton in Assessing the Water Quality of Water Bodies with Multicomponent Pollution
Abundance, biomass, diversity indices, and size structure of zooplankton are traditionally used to evaluate organic pollution of water bodies [17]. A positive relationship between the quantitative variables of zooplankton communities and nutrients was also recorded in our studies. Heavy metals or other toxicants contained in wastewater partially neutralize the stimulating effect of nutrients on primary producers [106], and as a consequence, on planktonic invertebrates. The ratio of taxonomic groups, for example, cladocerans and copepods, can be used to assess the ecological state of water bodies with mixed pollution. We established a positive relationship between cyclopoid copepods and the content of heavy metals in water bodies of the Sorbulak system. The relationship between cladocerans and heavy metals was negative. Thus, the dominance of cyclops, which are the most resistant to toxicants [91], is one indicator of stress for zooplankton communities.
We can also use the structure of species dominance, the sex structure of copepod populations, and the presence of individuals with morphological anomalies to monitor water bodies with mixed pollution. We have previously shown that the structure of species dominance is a more sensitive indicator compared to the species composition [46,107]. This is because external factors first cause intrapopulation rearrangements (for example, changes in the ratio of age stages in crustacean populations and the size of adults). Then the percentage of species in the quantitative variables of communities changes, i.e., the structure of their dominance. The species composition changes when the gradient of external factors goes beyond a particular species tolerance limits.
The composition of species dominating in the zooplankton of the surveyed water bodies has been relatively constant, since the formation of Sorbulak. The positive values of the ∆-Shannon index in the zooplankton of the studied water bodies were due to the dominance of species resistant to multicomponent pollution.
The cyclopoid copepods with morphological anomalies were presented in Sorbulak in 2000-2002, and they were absent in 2017. A revealed association between these individuals and heavy metals can indicate the toxic pollution of water bodies investigated, as was shown earlier [67,86,105,106]. These results corresponded to the chemical analysis data on the decrease in the level of toxic pollution of wastewater reservoirs of the Right-Bank Sorbulak Canal in 2017 and demonstrated the interannual increase of water quality in it.
Thus, the comprehensive studies carried out allow us to conclude that the level of toxic pollution of the Sorbulak wastewater disposal system has decreased over the past fifteen years. In 2017, Sorbulak and ponds were mainly characterized by organic pollution. Sorbulak water can be used for irrigation as previously planned. Given the significant variability of all parameters, constant monitoring of the Sorbulak wastewater disposal system is required.

Conclusions
Our results demonstrated that zooplankton communities of water bodies with multicomponent pollution consist of a small number of eurybiontic species. The variability of the quantitative variables of zooplankton is associated with nutrients, and as a consequence, algal communities. In particular, Cyanobacteria can affect the species composition of cladocerans. The most resistant species to cyanobacterial bloom is, obviously, Daphnia magna. It can be assumed that the ratio of species of the genus Daphnia may be an indicator of cyanobacterial bloom impact, although additional studies are required for the conclusion. The dominance of cyclopoid copepods as the most resistant to a wide range of external factors is one of the indicators of stress for zooplankton, and in particular, the indicator of toxic pollution. Moreover, the dominance of males and the appearance of individuals with morphological anomalies in copepod populations is an indicator of chronic stress associated, among other things, with toxic environmental pollution. Hence, our results demonstrate that the zooplankton structure can be successfully applied to assess the water quality of reservoirs with mixed pollution. We recommend using not only the traditional set of biological variables (abundance, biomass, diversity indices, and the average mass of an individual), but also data on the structure of species dominance, the sex structure of copepod populations, and the presence of individuals with morphological anomalies.